Forecasting

Various items about forecasting the weather

Models

This section provides a brief explanation of some of the output seen on the most popular sites available on the WWW that store, or otherwise make available NWP output.

Information on the various models

(NB: information presented with no warranty!)

Although a nominal data initialisation time is shown (00, 06 UTC etc.), modern NWP analysis routines 'assimilate' data over a period either side of these times. With integration of asynoptic data (i.e., not at standard times), plus the use of previous operational (or special pre-analysis) 'runs' to provide background fields, the use of such DT (data time) indicators is more a convenience for us mortals!

A computer completes its calculations quite quickly with the modern 'super numbercrunchers' in use, and the computations are in any case performed using code that is written to optimise speed: however, always bear in mind that there will be a delay in availability to us on the Internet due to the post-processing of the raw NWP data into a format that can be squirted onto data-networks.

 Model  UKMO- GLOBAL
 Data times: (UTC)
(i.e. the nominal time for initialisation, raw data ingest etc.)
 00 & 12 (but other runs are performed to 'update' the analysis)
 Number of full 'runs' per 24hr:  2
 Out to T + ..... (hr)  144
 Usually available:
(on the Internet .. the output will be available in a more timely fashion on 'closed' networks. )
 0500 - 0800; 1800 - 2100
 Other information:  Currently (winter 2005 / 06) the UK Global model has the following specifications:
~40 km horizontal resolution & 50 levels vertical resolution: a change in late 2005 put several additional levels into the stratosphere (and extended the 'top' of the model too) as well as reducing the grid-length from 60km: I understand that it is intended to go to 70 levels (twenty more in the troposphere) sometime in 2006.
 Note that a limited-area, finer-resolution (~12km) model [ North Atlantic European / NAE ] is run 4 times daily, with an earlier cut-off time, out to T+48. However, this is not (generally) available on the Internet. It currently takes it's boundary conditions from the previous main Global model run, updated by a 're-analysis' phase.
Other models, at 'mesoscale' resolution (~11km or less) are also run, using the above for 'boundary' conditions - these are also NOT available publicly. Of particular note is the future operational running of the UK 4KM model, expected late 2005 or early 2006. (see HERE for more on UKMO models.)

 

 Model  NOGAPS
 Data times: (UTC)
(i.e. the nominal time for initialisation, raw data ingest etc.)
 00, 06, 12 & 18
 Number of 'runs' per 24hr:  4 (but 2 via some Internet portals)
 Out to T + ..... (hr)  144
 Usually available:
(on the Internet .. the output will be available in a more timely fashion on 'closed' networks. )
 On the US Navy site .. 0500 - 0600 & 1700 - 1800; On other sites, circa 0800 & 2000.
 Other information:  On the US Navy site (if you get past the various security checks etc.), you can get intermediate hour runs as well.

 

 Model  NCEP - GFS (formerly: AVN)
 Data times: (UTC)
(i.e. the nominal time for initialisation, raw data ingest etc.)
 00, 06, 12 & 18 (not all 'runs' are carried by all Internet sites.)
 Number of 'runs' per 24hr:  4 (but 2 only to some Internet sites)
 Out to T + ..... (hr)  384 (high resolution to T+180/7.5 days; lesser resolution thereafter .. to 16 days) [ see note below re: resolution ]
 Usually available:
(on the Internet .. the output will be available in a more timely fashion on 'closed' networks. )
 about 4.5 to 5 hr after initial time (e.g. 00Z issue usually available by 05Z), but perhaps earlier on the US Navy site. Note that the first 180hr will be available fairly quickly; the remaining files (at courser spatial and temporal resolution), a little way behind.
 Other information:  NB: Now (since April 2002) the primary model run from NCEP - anything labelled 'MRF' are essentially 'look-alike' output based on the full Global AVN run as above.
[ Historical: Up to April, 2002, the 'MRF' (medium-range forecast) run was output once daily, to T+384, with the same physics, mathematics etc., as the (then) AVN, (now) GFS, but with a later cut-off time to allow extra data to be used - supposedly to achieve a better end result. Tests showed that it was no longer necessary to do this, hence the standardisation on the 4-times daily GFS (AVN) as above. (to emphasise, although these models run out for over 2 weeks, don't take each time-step slavishly; major errors can occur beyond even 3 or 4 days lead time.)
Resolution: Currently (Winter 2005/06), the GFS has an approximate horizontal resolution of ~40km up to T+180h, and ~80km thereafter; there are 64 vertical levels. However, bear in mind that model development may render these figures obsolete: visit this site for updates.

 

 Model  ECMWF
 Data times: (UTC)
(i.e. the nominal time for initialisation, raw data ingest etc.)
 00 & 12
 Number of 'runs' per 24hr:  2
 Out to T + ..... (hr)  240 (i.e. 10 days) ... but limited output to Internet users .. either 6 or 7 days according to site, fields etc.
 Usually available:
(on the Internet .. the output will be available in a more timely fashion on 'closed' networks. )
 On the EC site, and those carrying such data (e.g. Wetterzentrale), then about 6 to 7 hours after data-time (e.g., 00Z run available between 06 and 07Z). However, where additional processing is required, then a considerable delay from this time may be encountered.
 Other information:  This model runs out to 10 days, but only the first 6 days (7 days for mslp fields) are freely available via these sites.

 

 Other models  DWD: GME (Global Modell Ersatz) (or Global model replacement)
 Data times: (UTC)
(i.e. the nominal time for initialisation, raw data ingest etc.)
 00, 12 (& a short update run at 18)
 Number of global 'runs' per 24hr:  2 (for full output)
 Out to T + ..... (hr)  174 (i.e. just over 7 days) ... but limited output to Internet users.

 

 Other models  MétéoFrance: "ARPEGE" (Arpege is the acronym for Action de Recherche Petite Echelle Grande Echelle: Research Project on Small and Large Scales. This is the successor model to the EMERAUDE baseline model.)
 Data times: (UTC)
(i.e. the nominal time for initialisation, raw data ingest etc.)
 00, 12
 Number of global 'runs' per 24hr:  2
 Out to T + ..... (hr)  240 (i.e. up to 10 days) : NOTE: French model output to Internet users very limited.

 

NWP basics

NWP prediction is performed by solving mathematical equations which describe the behaviour of the atmosphere. The principal equations involved, and the purposes for which they are evaluated are:-

Three equations of motion:  Under Newtonian laws, the acceleration of a particle is proportionate to the forces acting upon that particle. These equations handle the 'balance' between the pressure gradient force (horizontal & vertical), the Coriolis 'force' (primarily horizontal) & acceleration due to gravity (principally in the vertical).  Fundamental to the analysis & forecast of wind flow, hence advection of air, convergence / divergence, vorticity development, vertical motion (in concert with the continuity equation) etc.
Continuity equation  A defined 'parcel' of air retains the same mass, regardless of how it moves about, or how it is deformed: 'conservation of mass'. Using this statement, a link is established between vertical and horizontal convergence.  Used to define vertical motion (not measured directly) from fields of horizontal motion ('the wind'), and hence key to describing formation / dispersal of clouds, precipitation etc. Indirectly involved with diagnosis of development due to vorticity changes.
Thermodynamic (or heat) equation  Relates heat transformation due to atmospheric processes to 'parcel' volume & internal energy changes as air ascends / descends: the 'bicycle pump' analogy.  Required to describe the the various adiabatic processes mathematically, key to understanding the degree of stability of an air mass.
Moisture equation  In similar fashion to the continuity equation (above), this describes the fact that moisture is not gained or lost, but can change phase (with consequent latent heat exchange), or is diffused via mixing actions (i.e. exchange with water / land surfaces).  Describes the changes in humidity content of an air mass (hence dew point, vapour pressure & indirectly to wet-bulb variants), which in turn are used implicitly to handle cloud / precipitation processes in the 'model' atmosphere.
Equation of state  Relates the density, pressure & temperature of a 'parcel' in a perfect gas; the atmosphere is not a perfect gas, but can be assumed to behave as such.  Used to describe mathematically changes in these fundamental properties of a parcel as they move vertically, and in concert with the thermodynamic equation, are used to describe the behaviour under adiabatic transformation.
Hydrostatic equation  In the vertical, there are two (main) forces which are roughly in balance: the upward-directed force ("buoyancy") due to the pressure gradient [ pressure falls with height ] & the downward force due to gravity. Any disturbance of this balance following atmospheric development leads to vertical accelerations.  Describes the relationship between height & pressure (the 'barometric equation') and is used in the algorithms to diagnose atmospheric stability; also important when considering the explicit modelling of small-scale vertical motion.

And the variables usually 'carried' within the model schemes are:-

  • Three components of wind velocity: two horizontal & one vertical;
  • Air density;
  • Temperature (or Potential temperature);
  • Pressure;
  • Humidity (usually in terms of the humidity mixing ratio):
    [ In addition, some models (certainly small-scale models) will also 'hold' an explicit representation of liquid water and ice fraction, surface wetness / character (i.e. ice/snow cover), vegetation development etc. ]

We can't possibly analyse each field at the level the atmosphere produces: it is difficult enough to describe the broader (or synoptic) scale motions. This means that instabilities are already 'built-in' from the point of analysis - which if not 'damped' mathematically can feed through to the ensuing forecast in an unstable fashion. This used to be a common problem with early NWP, but is a rare occurrence now.

All analyses are approximate (human or model); the key is obtaining the most consistent such starting point. NWP 'runs' don't start with a 'blank chart' - they integrate short-period forecasts from a previous solution (either the 'main' run, or a special 'update' calculation) and by careful injection of new data, a "best solution" starting point (or initialisation stage) is achieved.

There are broadly two types of atmospheric computer model in use around the globe:

In a grid-point model, the variables above are held at intersections of a regular matrix, or at fixed points between the intersections, and it is at these points that the equations outlined above are applied. Finite-difference calculations are used to solve the equations, i.e. solutions for the variable concerned 'nudge' the calculations a small fraction of time (a "time-step"). A grid point model (such as the UKMO Global) is usually defined in terms of the number of vertical levels, and the horizontal (approximate) grid-length.

In a spectral model, the variables are represented by periodic functions which are the sum of several waves, the calculations being performed via various methods such as Fourier series, spherical harmonics etc. The fields are therefore represented by wave functions of differing wavelengths. To take a 'real world' example, it is possible to model the behaviour of a cork bobbing on the surface of a river by considering the individual energy strands carried within that flow, then integrating them at one point - the cork. Spectral models (such as the GFS, NGP & EC) are regarded as computationally more efficient, and the solutions are available for every point on the globe, rather than tied to a regular grid array. These models are usually defined by the numbers of vertical levels and the wave-number truncation: thus T382L64 would indicate 'triangular' truncation at wave-number 382, with 64 levels in the vertical. To convert the wave-number 'horizontal resolution' to an approximate grid-length, divide 360 by the wave-number, divide by 3 (it takes 3 grid-points to define a wave), then multiply by 111.1km (per deg. latitude).

Grid spacing (or wave numbers) will determine computational resources required - shorter grid-length (or more wave representation), then the greater the amount of calculating power required: also remember that the calculations are performed both horizontally and vertically. Vertical levels are not evenly spaced: there are more levels near the ground (the boundary layer) & around the tropopause, to capture the often high degree of variability in the altitude bands, but even then, there are never enough!

Some elements are not even attempted, nor can they be attempted e.g. CB/TS, turbulence, fine-detail of precipitation processes etc. To use computing time more efficiently, approximations and assumptions are used [ parameterization or parametrisation ], both when dealing with the application of the primary equations and the mathematical methods used.

The numerical simulation, both in it's analysis and forecast output can NEVER be a perfect representation of the real atmosphere - and it is difficult to conceive of a time when it might be. Users should be aware that small-scale processes (e.g. thunderstorms, sea-breeze circulation) cannot be explicitly handled in the modern generation of global models. However, these same important processes can have a significant effect on the broader-scale evolution; errors generated via the parameterization (or parametrisation) routines can have significant 'downstream' consequences.

The real atmosphere is highly complex; it is fluid-flow performing in four dimensions, which numerical models can only approximate to the processes involved. Some of the motions / processes (such as the micro-scale movement of the air itself) cannot be handled, and even fairly large 'blocks' of weather features, such as individual thunderstorms or local turbulent flow can only be 'parametrized' (or 'fudged' if you prefer).

But this introduces problems much as if you 'nudge' a supermarket trolley: it is not always possible to predict the outcome! It is no wonder that the trough disruption process for example, which is taking place throughout a large vertical slice of the atmosphere, and involves a skewing of the thermal profile at differing rates, is often poorly handled. And get the disruption wrong - everything following on is going to be in serious error.

You'll read old soaks like me bang on about how the basis of a good forecast is a good, detailed analysis. This adage applies particularly to NWP routines: a human can sometimes 'correct' or allow for a poor or inadequate analysis through experience & 'gut instinct': a computer will simply use the data it has and employ the routines written for it without mercy! Consider the analogy below:-

I sometimes think predicting the various weather variables numerically is like trying for work out what the EXACT time of arrival of a bus on a typical 'town-circular' route might be, as it is about to leave the terminus (T). You would need to model a large number of variables as below:-

bus_round_loopA To predict the time-of-arrival at stop 1 is fairly straightforward; the distance will only be a matter of a few hundred metres, and a knowledge of the average speed, instantaneous traffic flow etc., between T & 1 will yield a prediction within 60 seconds of the actual time, perhaps below 30 seconds. The short time-step, plus the restricted impact of 'external' variables allows a fairly accurate prediction. [ In meteorology, this is analogous to a 'nowcast' routine, with results over half-an-hour to 6 hour periods. ]
bus_round_loopB To predict the time at stop 2 is complicated by several factors: we need to predict the numbers of passengers getting on / off at the previous stop & whether they tender cash, the exact fare or use some form of pre-pay card / concession: we also need to predict the traffic flow between 1 & 2, and also make allowance for being held (or not) at the light-controlled junction. All these variables can be predicted in advance, but require a much greater amount of information than for the previous time-step. The accuracy of prediction at stop 2 could drop dramatically, though for the majority of cases, it will still be within 60-90 seconds. [ In meteorology, this would simulate the problem for the short-range forecast - say the first 48 hours of a run. ]
bus_round_loopC Predicting the time-of-arrival at 6 (and later stops) to any sort of accuracy (before it has left T remember) is now fraught with difficulty. Passenger loading (on/off), ticket types / difficulties, road-traffic density, timing of light-controlled crossings etc., all need to be 'built-in' to this model, either explicitly (by monitoring actual traffic / passenger behaviour) or implicitly by using 'notional' numbers based on an previous experience (analogous to parametrisation). To achieve an accuracy of +/- 3 minutes for stops 6, 7 and 8 on a typical town-circular bus route on any given trip is very difficult. [ In meteorology, this typifies the problems when trying to forecast 5 or more days out. ]

So it is with the weather: a computer model of the atmosphere must have information about a wide variety of variables that might impact upon the outcome - and the longer down-stream the forecast, the more information & mathematical processing is required.

. . . and there is the added problem that whereas the bus tends not to modify it's environment as it runs on it's course, in meteorology developments can / will significantly modify the broadscale pattern, which in turn will affect the way the forecast 'pans out'!


So, NWP models are trying to predict an outcome given the information they have (which can be limited) & using the current understanding of atmospheric physics / thermodynamics (which is developing all the time); at short 'lead times', they will do well in the main, but further 'down stream', errors can be large.

To try and handle the uncertainty almost inevitable in modern-day computer output, the concept of the 'ensemble' is used: a control run is performed (same model physics as the 'main run' but at lower spatial & temporal resolution), and then the initial field is 'perturbed' (or nudged) slightly, and a series of further forecast runs performed. Again, reference to a 'real-world' example might help. Here we have a ski slope, where the skier starts a downhill run from point 'A' and ends up at point 'D'. Each time he or she 'pushes-off' from the top, the starting point is very slightly different.

ski_slope_basic From the same starting point (analysis), and using the same physical structure (basic equations), we get a different result (forecast).
ski_slope_A At the top of the slope (B) such deviations are of little consequence - this might represent the first 6 to 12 hours of a weather forecast;
ski_slope_B . . . but at 'C' & 'D', the deviation becomes somewhat more significant. It may be that the upper height contours (or mean-sea level pressure) is the parameter to be forecast: it can be seen that early model deviation at B & perhaps C would have little impact upon the final forecast: at 'D' though, considerable variation can be found, which might make the difference between a roaring 'Storm-Force 10' and just a good 'Force 6/7 blow'.
ski_slope_C In an ensemble prediction system (EPS), several 'runs' are made using the 'control' as the core 'driver', and the results can be clustered together into like patterns. The human analyst will have high confidence in a solution where there are a large number of ensemble members (the individual ski-slope runs) giving a similar solution; the confidence would be poor where the results are peppered over a wide span of outcomes.

To see an example of ensemble output (with explanation) from the GFS EPS . . . click HERE.

The ECMWF ensemble prediction system (EPS) is regarded as one of the best in the world. It uses 51 members in its scheme: one 'control' run (a half-resolution horizontal analysis / two-thirds vertical resolution - based on the main operational output) & 50 members with an analysis slightly perturbed from the control member. Unfortunately, they aren't available on the Internet!


One question that is often asked is .. if all models use the same data, why do they come up with different results?

There are so many variables involved with atmospheric modelling, that I'm surprised when models do agree: some of the factors that distinguish one centre's output from another are:

  • Most importantly, the initial condition (the 'analysis') on which the subsequent forecast calculations are based, may be assessed in different ways. This is much as if you had two forecasters faced with the same set of observations, but who produce a slightly different analysis - location of front, depth of low, sharpness of trough etc.
  • The various centres will (usually) have available all the same raw data, but what they do with it may be different; they may not assimilate all the data, or may be selective about which datasets are used - this applies particularly to satellite-derived data.
  • A different 'weighting' may be applied to data: for example, an isolated ship in a data-sparse area might have greater influence in one centre's scheme than another.
  • The methods used in numerical analysis are now highly complex and as has been noted elsewhere, data are not just used at the 'primary' DT's (00, 06, 12 & 18 UTC), but are used asynoptically - but the methods of doing this are different from centre-to-centre, hence different analyses on any one occasion, and therefore a different forecast outcome.
  • Then of course there are different grid lengths (or wave numbers for spectral models), and a variation in the number of vertical levels used: these later will be spaced differently.
  • It is impossible for every nuance of the world's orography (depth / shape of valleys, height profiles of hills & mountains etc.) to be 'carried' within a numerical simulation: even coastlines & islands must be smoothed - think of the ins & outs of the Norwegian fjords or the clusters of small islands in the Aegean. So each centre will have a slightly different representation of the physical earth - it may not matter when output are averaged over lengthy periods, but for an individual situation, the interaction between land surface and the atmosphere may be crucial.
  • Some models are 'tuned' to produce results at higher definition for specific purposes - e.g. the tropical models, GCM's used for climate simulation work or mesoscale models used for short-range, high spatial resolution work.
  • The time-steps (or mathematical procedures used) will be different one from another, even if the actual equations [ see above ] are the same.
  • The way some processes are parameterised will be different - some may have an explicit (and 'near-real-time') representation of the surface type, whilst others will use climatology, or have a mixture of methods.
  • Models will have different ways of coping with 'polar convergence' - the physical distance for a given degree of longitude will vary from equator to pole - not only at the surface but aloft as well. Some centres will skew the grid they use to make sure that the primary area of interest is over them, so 'notional' poles will result which may affect, even if only slightly, the forecast over the domain further away.
  • Another variable that is not always obvious is that modellers may add 'fudge' factors (or correction factors) to offset know model biases. For example, no matter how many levels are squeezed in at the top of the troposphere / lower stratosphere, capturing the 'top-strength' of a jetstream (comparatively shallow vertically) is difficult. Once identified, a percentage correction may be added to the output at the post-processing stage, but in some situations this may not give an appropriate result.

All centres will have available the same raw data, but what they do with it will be different - hence the large variations on occasion, particularly at the longer lead-times.


Bibliography:

Dynamical meteorology: an introductory selection; Ed: B W Atkinson
(Methuen 1981 / The Royal Meteorological Society)

and visit the Met Office web site HERE for a lot more on numerical modelling, and in particular the way the UK national weather provider goes about it.

Ensemble: 'Meteogram'

This is an example of a 'Plume' or 'Meteogram' (or Meteogramme in German), that is available on the Wetterzentrale site. For each site, selected from the 'drop-down' menu, output from the Ensemble run of the NCEP GFS model can be viewed, specifically the temperature at 850 hPa ("850 hPa Temperatur") and 12hr accumulated precipitation ("Niederschlag"). [ Although not stated on the page, I assume the precipitation is rainfall-equivalent, as there is no allowance for explicit phase-change within the diagram. ]

Example Meteogram for Manchester

AT THE TOP OF THE DIAGRAM (TEMPERATURE AT 850 hPa):-
The thick red line shows the 30 year average value.
The thick blue line shows the output from the Control Run of the Ensemble model (lower resolution than the operational!)
The thick white line shows the ensemble mean (i.e., very roughly, an 'eyeball' guide to the trend over the period).
The thin differently coloured lines are individual ensemble members (11).

AT THE BASE OF THE DIAGRAM (PRECIPITATION TOTAL FOR PREVIOUS 12HR):-
The thick white line shows the ensemble mean.
The thin differently coloured lines are the individual ensemble members.

Cautionary notes:
+ ensembles are best used to detect future trends rather than to define the precise outcome on a particular day, particularly beyond about 3 days.
+ where the ensemble members cluster tightly together (as they should in the first couple of days of any output), then confidence in the outcome will be high.
+ where many ensemble members cluster tightly together, with only a single 'outlier' to either side, then confidence in the mean of the dominant cluster can be regarded as reasonably high (but not a 'perfect' solution).
+ the greater the 'spread of the fan' as forecast lead time increases, the more uncertain is the outcome. This will be the normal pattern in mid-latitudes.

[ For more on Ensembles, Operational runs, Control runs etc., see the Glossary ]

Sembach

Example of output

This section is simply describing what the various lines, coloured areas are, what interval/units the contours, isobars, colour-filled areas use etc. The use, or meteorological significance of the fields are dealt with in a separate section here.

The Sembach site is here.

IMPORTANT: I am not going to attempt a description of the full range of products, just the ones that might be most useful to those who want to 'dip-in' without much knowledge.


UKMO (with forecaster interpretation)

NB: The output viewed on this site is based on 'raw' Met Office model products, with forecasters at Sembach interpreting same by adding the fronts - the cloud representation is based upon model relative humidity fields. Bearing in mind that UKMO 'FAX' output is sometimes heavily modified as seen on other sites (e.g. Wetterzentrale, Westwind), there may be significant differences seen at times from (apparentley) the same centre's output.

 Example: Brief description of parameters (etc.) on charts shown.
 
sembach output example
Yellow lines: MSLP isobars (units = hPa / interval = 4 hPa)
Conventional frontal symbology: click HERE for examples of types of fronts etc.
Grey shading: Cloud-amounts (roughly for the layer 6000 to 14000 ft [ 1800 to 4300 metres ] /i.e. 800 to 600 hPa), inferred / integrated from upper level relative humidity fields. See caption at foot of each chart for explanation.
Blue/Green circles (varying size) or crosses (varying size & shapes), or triangles: precipitation types, intensity etc. See matrix at foot of each diagram for more details.

 


 

Unisys

The Unisys site is here. It is blessed with an excellent guide to the use of its products here.

Westwind

This section is simply describing what the various lines, coloured areas are, what interval/units the contours, isobars, colour-filled areas use etc. The use, or meteorological significance of the fields are dealt with in a separate section ... here

The Westwind site is here.

IMPORTANT: I am not going to attempt a description of the full range of products, just the ones that might be most useful to those who want to 'dip-in' without much knowledge. Also, the fields presented below are not necessarily from the same situation, run-time etc. They are there simply to aid a quick identification for each product. Many of these products 'mirror' those on the Top Karten site - so refer to the notes for that site to get the complete picture.

 


Georg Mueller (relay from Wetterzentrale site of GFS output (formerly AVN & MRF products)

 Example: Brief description of parameters (etc.) on charts shown.
 500 hPa, Bodendruck (MSLP), Re-Top (Thickness) (500/1000)
mslp_tthk_500
White lines: MSLP isobars (units = hPa / interval = 5 hPa)
Dark grey/dash & double-dot lines: thickness isopleths for the layer (500-1000) hPa. (units = dekametres [dam] / interval = 18 dam)
Colour shading: 500hPa height, colour coded as at scale by diagram. (units = dam / interval = 4 dam). Orange/Red high values - blue/indigo etc., low values.
Black line: the 552 dam contour (of the 500 hPa surface) - highlighted as the approx. median value to trace major troughs/ridges in pattern.
 Bodendruck (MSLP) und 850 Aeq.PT (Theta E)
mslp_850ThetaE
White lines: contours of 850hPa isobaric surface. (units = dekametres [dam]/ interval 4 dam)
Black solid/dashed lines: temperature at 850 hPa (solid >= 0degC; dashed < 0degC: units = degC / interval = 5 degC.)
Colour shading: value of Theta E (equivalent potential) temperature at 850 hPa as per shading chart at side/interval = 2 degC.) A better tracer than temperature alone, because it is based upon the dew-point as well as air temperature.
 Richtung (direction) & Betrag (strength/force) 10m Windes (winds)
10 metre wind fields
White 'arrows': 10m (i.e. 'surface') wind using conventional wind-barbs: e.g. arrow points in direction FROM which wind is blowing (NOT with the wind); a long feather = 10knots, a short feather = 5 knots, a triangle = 50 knots & speeds are built up using these combinations.
Colour shading: Wind speed (at 10m) colour shaded as scale at side of diagram (units = knots / interval = 2 knots). Blue/green light winds - yellow strong winds - red gales or stronger.
 2m Temperatur
2m temperature
Black/thin contours: 2m (i.e. 'screen' - level) temperature. (units = degC / interval = 10degC).
Colour shading: 2m temperature as scale at side of diagram. (units = degC / interval = 3 degC). Orange/Red high values - blue low values.
 6h - Niederschlag (Precipitation)
6 hrly ppn accum
Colour shading: 6 hourly precipitation (rain, snow etc.) in the period up to the time of the chart. See scale at side of chart - the deeper the blue the more precipitation - purple/violet etc., exceptional rainfall/snowfall accumulations. (units = mm): also displayed is a field labelled 'rot=Konvektion' (red dashed lines) which appears to indicate areas of PPN with a high convective / instability element.
 700hPa und Vertikalbew. (Vertical motion)
700 vertical velocity
White lines: contours of 700hPa isobaric surface. (units = dekametres [dam]/ interval 4 dam)
Black line: highlighted 700hPa contour at 300 dam. Useful to trace major changes in the pattern.
Colour shading: vertical motion as scale at side of diagram - yellow to red to violet: upward values; green to blue to violet: downward values. (units = hPa/h; interval 4 hPa/h)
NB: negative is upward motion!
 Bodendruck (MSLP), Wolken (Cloud)in %, ReTop (Thickness) 500/1000
mslp_cloud_tthk
White lines: MSLP isobars. (units = hPa / interval 5 hPa)
Coloured lines: 500-1000hPa thickness isopleths (units = dam; interval 18 dam)
Monochrome shading: % cloud cover based on medium level moisture (i.e. 'frontal') - see scale by side of diagram (black low % cloud values, light grey/white high % cloud values)
 850 hPa und Temperatur
850 tt etc.
White lines: contours of 850 hPa isobaric surface (units = dekametres [dam] / intervals = 4 dam).
Black lines: isotherms of 850 hPa temperature. (units = degC / interval = 5 degC).
Colour shading: temperature at 850 hPa- see scale at side of chart. (units = degC / interval = 2 degC, blue = cold, orange/red = warm).
 500 hPa, T (Temperature) und Bodendruck (MSLP)
mslp_500
White lines: MSLP isobars (units = hPa / interval = 5 hPa).
Dark grey/dash & double-dot lines: temperature at 500 hPa (units = degC / interval = 5 degC).
Colour shading: 500hPa height, colour coded as at scale by diagram. (units = dekametres [dam] / interval = 4 dam). Orange/Red high values - blue/indigo etc., low values.
Black line: the 552 dam contour (of the 500 hPa surface) - highlighted as the approx. median value to trace major troughs/ridges in pattern.
 200 (or 300, 500 etc.) hPa streamlines ("stromlinien") & wind strength ("windgeschwindigkeit").
200hPa output
 White lines (with arrows): streamlines following the wind flow at the level given. (Levels based on pressure levels - such as 500, 300, 200 hPa)
Colour shading: speed of wind - see the scale at the side of the chart; nearer to red / purple, high values; nearer to green/blue lower speeds.
NOTE: 300 or 200 hPa good approximation to jet level winds.

 


GME (German Weather Service, DWD), 'Global Modell' [ output via wetter.com ]

 Example: Brief description of parameters (etc.) on charts shown.
 10m Wind GME (kt) [ also isobaric pattern ]GME MSLP and Winds Blue lines: isobars (mslp) at 5 hPa intervals: (NB: as this is a German chart, High's are labelled 'H' (Hoch or Hochdruckgebeit - area of high pressure) and Low's are labelled 'T' (Tief or Tiefdruckgebeit - area of low pressure.)
Thick blue line: 1015 hPa isobar (nearest value to ISA standard.)
Black wind arrows: 10m wind forecast in knots (usual convention - see elsewhere).
Colour shading: Beaufort force groups - see scale at bottom of each chart.
 Tiefe (low) or Mittel (medium) Bewölkung (cloud cover)Low cloud and MSLP  Black lines: Isobaric pattern, every 5 hPa and 1015 hPa isobar highlighted in bold (see above).
Grey shading: from light to dark grey, according to scale (usually bottom/left), for 'FEW', 'SCT', 'BKN' & 'OVC' cloud amounts. (See the main FAQ here.)
 Example of an analysis with 'named' pressure systems (ex- Free University of Berlin)FUB analysis  Usual monochromatic chart symbology, but the major pressure centres are named: BLUE names for high centres, and RED names for low centres.

 


Wetter3 output based on GFS model (selection only .. many of the ideas are covered above).

 Example: Brief description of parameters (etc.) on charts shown.
 850hPa Fronten (objective frontal zones), Bodendruck (mslp) & 700hPa Vertikalbewegung (vertical velocity)
850Fr_mslp_700VV
White lines: MSLP isobars. (units = hPa / interval = 3 hPa (NOTE: unusual choice of standard isobars).
Colour-fill/shading: 700hPa vert. velocity - orange > red > purple implies upwards / negative values; green > blue implies downwards / positive values (see scale at right; units = hPa/hr.
Grey/thin lines: based on model parameters - close 'packing' indicates near-surface frontal zones using objective techniques.
 6 - stuendiger Niederschlag (6hr precipitation total in mm to hour given)6hr rainfall totals  Blue colour shading: light blue, low precipitation totals, dark blue (or purple), higher accumulation: see scale at right of diagram.
White lines (thin): contouring of same at 1, 5, 10, 25 and 50 mm.
 Niederschlagsstaerke (precipitation intensity) [mm/hr at hour given: 6hr time-steps] und Niederschlagsform (precipitation type)rain_snow output  Blue (Blau)/ Purple (Rosa) shading: blue shading for rain (regen) or freezing-rain (eisregen); purple shading for snow (schnee) or soft hail / snow pellets (Graupel). Intensity is given by the depth of shading - see scales at right-hand side (in mm/hr).
White (thin) lines: intervals of 0.1, 1, 2 (etc.) mm/h; Note that for the snow, the PPN is 'rainfall equivalent' not snow depth. (See notes HERE.)
 850hPa aequivalent potentielle Temperatur (ThetaE), Bodendruck (mslp)
850ThetaE_mslp
White lines: mean sea level isobars (see above) [hPa]/ interval = 5 hPa
Colour-fill/shading: 850hPa ThetaE field- see scale at right of chart. (blue = low, orange/red = high).
Light-grey lines: isotherms of 850 hPa ThetaE/interval=3 degC.
 2m Temperatur (2m temperature/degC)
[ similar conventions used for:
6-stuendige 2m Minimumtemperatur (min. temp), 6-stuendige 2m Maximumtemperatur (max. temp & 2m Taupunkt (dew point)].2m temperaturesW3_Tmax
Thin grey (with intermediate thicker white) lines: 2 metre (i.e. 'screen - level') temperatures. (units = degC / interval = varying 2 to 5 degC)
Colour-fill/shading: 2 m temperatures (actual, maximum in 6hr, minimum in 6hr or dew-point: see scale at right of chart. (purple/blue = low values, orange/red = high values). [NB: the dew point is a measure of the absolute humidity of the air - i.e. how much water vapour is carried at the particular level given - in this case 2m above ground level.]
 Tiefe (low), Grenzschicht (boundary-layer), Mittelhohe (medium) or Hohe (high) Bewoelkung (cloud) Isolinien (% probability)
LowCloud
Colour-fill/shading: shading (green) according to the legend at right-hand side of chart - deeper green, higher-probability of cloud. (I'm not sure of the cut-off for 'boundary layer' versus 'low' cloud.)
Blue/grey dashed (30,45), or solid (60+): lines of equal % probability of cloud cover at the appropriate level.
 10m wind (in knots/kn)
SFC wind
Standard wind barbs: where the barb points to the direction FROM WHICH the wind blows: with half-feather representing 5kn, a full-feather 10kn and a triangle 50kn.
The barbs are also colour-coded, where the sequence deep orange > red > violet indicates stronger to stormy winds: see the scale at right-hand side.
 850 (and 700, 500, 300, 200) hPa Geopotential (height fields) and Temperatur (temperature/degC)
850ht and T
 Black lines: contours of the pressure level given (i.e. 850, 700 etc.); intervals = 4 or 8 dam, with one contour highlighted as follows: 850hPa (144dam), 700hPa (300dam), 500hPa (552dam), 300hPa (912dam) and 200hPa (1176dam). The highlighting of one 'standard' contour allows broadscale pattern changes to be seen on image-loops.
Colour-fill/shading: 850 (or 700, 500 etc.) temperature fields - see scale at right-hand side of diagram.
White lines: on some charts, isotherms in degC, intervals usually 5degC .. see labels.
 850hPa FQn Divergenz von Q-normal (divergence of the component of Q, normal to the isotherms) und Fronten (objective frontal zones).
850FQnDiv
 Colour-fill/shading: deep red negative FQn, deep blue positive FQn: units = fractional degs. Kelvin per sec*m^2. (NB: some areas 'blank' data usually seen.)
Thin grey contours: frontal zones inferred from 850hPa model parameters.
 700hPa relative Feuchte (relative humidity %).
700RH
 Green shading: 60% or higher relative humidity at 700 hPa;
Lines (dashed low values, solid high values): 15, 30, 45, 60, 75 & 90% relative humidity at 700 hPa.
 500hPa Geopotential (height field) and Vertikalbewegung (vertical velocity in hPa/h).
500 and VV
 Black lines: contours of the 500hPa surface [ interval=8dam ]: 552dam contour thicker (used as a 'tracer').
Colour-fill/shading: see the legend at the right of the diagram. (yellow > orange > red > purple .. increasing vigour of upward motion [negative values]; green > cyan > blue .. increasing vigour of downward motion [positive values].
 500hPa Geopotential (height field) and absolute Vorticityadvektion (absolute vorticity advection/ per h^2)
500 and VortAdv
 Black lines: contours of 500hPa field, with 552dam highlighted - 8dam intervals.
Colour-fill/shading: absolute vorticity advection - for values see scale on right side of diagram. (Green > cyan > blue > purple: increasingly negative vorticity advection [ NVA / non - developmental ]; yellow > orange > red: increasingly positive vorticity advection [ PVA / developmental ]
 
500hPa Geopotential (height field) Bodendruck (mslp) & Relative Topographie (Total thickness).500 and TTHK
 White lines: mean sea level pressure isobars (in hPa or mbar); intervals=5 mbar.
Black lines: 500hPa contours in dam; intervals=8dam, with 552dam highlighted (thick black line).
Colour-fill/shading: 500-1000 hPa total thickness (TTHK) field (green > cyan > blue > purple: increasingly cold; yellow > orange > red: increasingly warm. See scale at right of diagram.
Light grey lines: TTHK contours at 4 dam intervals.
 500hPa Geopotential (height field) and Schichtdickenadvektion (layer-thickness advection)
500 and TTHKadv
 Black lines: 500hPa contours (see notes above: interval 8dam)
Colour-fill/shading: thickness (500-1000hPa layer) advection: - see scale at right-hand side, expressed as a fractional change in Kelvin (or degC) per hour.
(green > blue > purple: cold advection; yellow > orange > red > purple: warm advection: the deeper the colour, the stronger the advection. Crudely: strong WARM advection coupled to FALLING contour height=> strong SURFACE DEVELOPMENT.)
 500hPa Geopotential (height field) & Divergenz des Q-Vektors (divergence of Q-vectors).
500 and DivQ
 Black lines: 500hPa height field (see notes elsewhere - similar ideas with one contour 552dam highlighted, thick black).
Colour-fill/shading: FQ values as at the scale on right-hand side of diagram ... yellow > orange > red > purple: negative; green > blue > purple: positive. (See notes elsewhere).
 KO-Index & 500hPa Vertikalbewegung (vertical velocity).
KOIndexVV
 Varying width blue-black lines: KO-Index, every 2 degC (or K); one of many instability indices, based on Theta E at 1000, 850, 700 and 500. Broadly, higher negative values imply greater potential instability.
Colour shading: vertical velocity (or "V V" negative, upward only) - deeper red/purple, then more vigorous upward motion implied.
A combination of high V V (highly negative values) and high KO value=> strong risk of vigorous convection in frontal situations.

 

 UKMO products (modified output: 'FAX')

 Example: Brief description of parameters (etc.) on charts shown.
 FAX output (final product)
UKMO FAX product
These charts are simply standard 'FAX' charts which depict MSLP isobars (4 hPa intervals), fronts, pressure centres etc., to conventional symbology. For more on these, see the FAQ entry here) and for Conventional frontal symbology: click here for examples & further notes.

 

 ECMWF products.

 Example: Brief description of parameters (etc.) on charts shown.
 500 hPa & Bodendruck (MSLP)
EC image
White lines: MSLP isobars (units = hPa / interval = 5 hPa)
Colour shading: 500hPa height, colour coded as at scale by diagram. (units = dekametres [dam] / interval = 4 dam). Orange/Red high values - blue/indigo etc., low values.
Black line: the 552 dam contour (of the 500 hPa surface) - highlighted as the approx. median value to trace major troughs/ridges in pattern.

 

Wetterzentrale

This section is simply describing what the various lines, coloured areas are, what interval/units the contours, isobars, colour-filled areas use etc. The use, org meteorological significance of the fields are dealt with in a separate section ... here.

The Wetterzentrale site is here.

IMPORTANT: I am not going to attempt a description of the full range of products, just the ones that might be most useful to those who want to 'dip-in' without much knowledge. Also, the fields presented below are not necessarily from the same situation, run-time etc. They are there simply to aid a quick identification of each product. You will note that the products from this site are mirrored on the 'Westwind' site, so many of the notes are applicable to each.


 

WETTERZENTRALE: 'TOPKARTEN'

 

UK Met Office (UKMO)

NB: scales referred to in the table below only appear on this site for individual frames, NOT on composite or looped imagery.

 Example: Brief description of parameters (etc.) on charts shown.
 500 hPa & Bodendruck (MSLP)EC 500 and mslp White lines: MSLP isobars (units = hPa / interval = 5 hPa)
Colour shading: 500hPa height, colour coded as at scale by diagram. (units = dekametres [dam] / interval = 4 dam). Orange/Red high values - blue/indigo etc., low values.
Black line: the 552 dam contour (of the 500 hPa surface) - highlighted as the approx. median value to trace major troughs/ridges in pattern.

 

UKMO modified (FAX) product

 Example: Brief description of parameters (etc.) on charts shown.
 FAX output (final product)
UKMO FAX output
These charts are simply standard 'FAX' charts which depict MSLP isobars (4 hPa intervals), fronts, pressure centres etc., to conventional symbology. For more on these, see the FAQ entry here) and for Conventional frontal symbology: click here for examples & further notes.

 

ECMWF products (ex EC site)

 Example: Brief description of parameters (etc.) on charts shown.
 MSLP isobars and 850hPa predicted wind speed
EC mslp pattern
Black lines: MSLP isobars. (units = hPa / interval = 5 hPa: every 20 hPa, e.g. 980, 1000, 1020, 1040 hPa the lines are bolder - this aids detection of major changes in the pattern.)
Yellow/Green infill: 850hPa wind speed - see scale on right-hand side.
 500 hPa contours
EC 500 hPa pattern
Blue lines: 500 hPa Contours. (units = dekametres [ dam ] / interval = 6 dam.)

 

 ECMWF products (on the Wetterzentrale site)

 Example: Brief description of parameters (etc.) on charts shown.
 500 hPa & Bodendruck (MSLP)EC 500 and mslp White lines: MSLP isobars (units = hPa / interval = 5 hPa)
Colour shading: 500hPa height, colour coded as at scale by diagram. (units = dekametres [dam] / interval = 4 dam). Orange/Red high values - blue/indigo etc., low values.
Black line: the 552 dam contour (of the 500 hPa surface) - highlighted as the approx. median value to trace major troughs/ridges in pattern.

 

GFS (formerly known as AVN / MRF) products

(if any products are missing from this list, see the display at the 'Westwind' site ..... here

 Example: Brief description of parameters (etc.) on charts shown.
 500 hPa height, T (Temperature) & Bodendruck (MSLP)
500 hPa HTB
White lines: MSLPisobars (units = hPa / interval = 5 hPa).
Dark grey/dash & double-dot lines: temperature at 500 hPa (units = degC / interval = 5 degC).
Colour shading: 500hPa height, colour coded as at scale by diagram. (units = dekametres [dam] / interval = 4 dam). Orange/Red high values - blue/indigo etc., low values.
Black line: the 552 dam contour (of the 500 hPa surface) - highlighted as the approx. median value to trace major troughs/ridges in pattern.
 850 hPa height & Temperatur
850 ht & T
White lines: contours of 850 hPa isobaric surface (units = dam / intervals = 4 dam).
Black lines: isotherms of 850 hPa temperature. (units = degC / interval = 5 degC).
Colour shading: temperature at 850 hPa - see scale at side of chart. (units = degC / interval = 2 degC, blue = cold, orange/red = warm).
 Bodendruck (MSLP), Wolken (Cloud) in %, ReTop (Thickness) 500/1000
Wolken,BodenD
White lines: MSLP isobars. (units = hPa / interval =  5 hPa)
Coloured lines: 500-1000hPa thickness isopleths (units = dekametres [dam]; interval 18 dam)
Monochrome shading: % cloud cover based on medium level moisture (i.e. 'frontal') - see scale by side of diagram (black low % cloud values, light grey/white high % cloud values)
 6h - Niederschlag (Precipitation)
6hr Niederschlag
Colour shading: 6 hourly precipitation (rain, snow etc.) in the period up to the time of the chart. See scale at side of chart - the deeper the blue the more precipitation - purple / violet etc., exceptional rainfall / snowfall accumulations. (units = mm): note that on this display there is NO discrimination between rain and snow.
Red dashed-lines: labelled as 'Konvektion', which I interpret as the model detects a high degree of instability in the area.
 2m Temperatur
2m temperature
Black/thin contours: 2m (i.e. 'screen' - level) temperature. (units = degC / interval = 10degC).
Colour shading: 2m temperature as scale at side of diagram. (units = degC / interval = 3 degC). Orange/Red high values - blue low values.
 700hPa und Vertikalbew. (Vertical motion)
700 ht&VB
White lines: contours of 700hPa isobaric surface. (units = dekametres [dam]/ interval 4 dam)
Black line: highlighted 700hPa contour at 300 dam. Useful to trace major changes in the pattern.
Colour shading: vertical motion as scale at side of diagram - yellow to red to violet: upward values; green to blue to violet: downward values. (units = hPa/h; interval 4 hPa/h)
NB: negative is upward motion!
 Bodendruck (MSLP) und 850 Aeq.PT (Theta-E)
Bodendruck & 850TE
White lines: contours of 850hPa isobaric surface. (units = dekametres [dam]/ interval 4 dam)
Black solid/dashed lines: temperature at 850 hPa (solid >= 0degC; dashed < 0degC: units = degC / interval = 5 degC.)
Colour shading: value of Theta E (equivalent potential) temperature at 850 hPa as per shading chart at side/interval = 2 degC.) A better tracer than temperature alone, because it is based upon the dew-point as well as air temperature.
 2m Taupunkt (Dew Point temperature)
2m dewpoint temperature
Black lines (thin): dew point temperature at 'screen' level. (units = degC / interval = 10 degC, but selected values only.)
Colour-shading: See scale at right of diagram: blue, cold & orange/red, warm. ( units = degC / interval = 2 degC.)
 CAPE und Lifted Index
GFS_CAPE and LI output
Grey or White-lines: Lifted Index (units =  degC / interval = 2 degC for values above zero, 1 degC values below zero); solid grey lines > 0 (poor LI values), dashed white lines<=0degC (moderate or 'good' LI values)
Colour-shading: See scale at right of diagram: blue/green - low values of CAPE; orange/red - high values of CAPE (units  = J/kg / interval  = variable, but generally 100 J/kg over much of range.)
 Nds in mm (rot=konvektiv) + 0° - Grenze in m
Freezing Level example
Brown lines: height of freezing level (in metres, above mean sea level).
Colour shading: indication of rainfall (or equivalent rainfall) accumulation per 3 hr.
Individual figures: PPN accumulation in mm (remember, if snow, then it is the equivalent water).

 

NCEP (GFS) Ensemble products (ENS)

 Example: Brief description of parameters (etc.) on charts shown.
 500 hPa mean contour pattern
500 hPa ensemble mean
Colour-fill: Mean 500 hPa pattern for the entire ensemble - see scale at right. Blue, low values to orange/red high values. (units = dam / interval  = 4 dam)
Black line: the mean position of the 552 dekametre [dam] contour at this time step - useful when comparing with other products with same highlight, and to check run-to-run consistency.
 500 hPa contour Spaghetti plot
500 hPa spaghetti plot
Blue, red, yellow, green etc. lines: Individual members of a particular ensemble run for 500 hPa contours at 516, 552 and 576 dekametre [dam]. (Click HERE for a brief explanation of Spaghetti plots)
 850 hPa mean temperature pattern
850 ensemble mean temperature
Grey lines: the ensemble meanpattern of 850 hPa temperature. Dashed below zero degC, solid at and above freezing. (units = degC / interval = 5 degC)
Colour-fill: see scale at right. Blue, cold to orange/red warm. (units = degC / interval  = 2 degC)
 850 hPa isotherm Spaghetti plot
850 spaghetti plot of temperature
Blue, red, yellow, green etc. lines: Individual members of a particular ensemble run for 850 hPa isotherms at -15, 0, +15 degC. (Click HERE for a brief explanation of Spaghetti plots)
 Meteogramme
Meteogram
 For explanation of these diagrams (also known as 'plumes'), see HERE

WXMAP

This section is simply describing what the various lines, coloured areas are, what interval/units the contours, isobars, colour-filled areas use etc. The use, or meteorological significance of the fields are dealt with in a separate section ... HERE.

The WXMAP site is here.

IMPORTANT: I am not going to attempt a description of the full range of products, just the ones that might be most useful to those who want to 'dip-in' without much knowledge. Also, the fields presented below are not necessarily from the same situation, run-time etc. They are there simply to aid a quick identification for each product.

 


 NOGAPS products (US Navy model)

 Example: Brief description of parameters (etc.) on charts shown.
 MSLP, thickness & 12hr precipitation
MSLPtthkPPN
Gold lines: MSLP isobars. (units = hPa / interval = 4 hPa)
Alternate blue & purple lines: thickness (500-1000 hPa) blue 528 & 552, purple 540 & 564 dekametres [dam])
Colour-fill regions: 12 hour precipitation rate (units=mm/12hr / interval=variable, see colour quadrant top/right of diagram) [ It's not entirely clear if this is the accumulated rainfall in the previous 12hr, or the instantaneous rate at some point in the last 12hr - my thoughts would be the former. ]
 300 hPa height & isotachs
300 ht and isotachs
Green lines: 300 hPa contours. (units = metres / interval = 120 m)
Black (thin) lines: isotachs of the 300 hPa wind. (units = knots [kn]/ interval = 10 kn)
Wind arrows: conventional plots of wind-barbs at 300 hPa (units = kn)
Colour-fill: strongest winds at this level (>= 50kn), according to scale at top/right of diagram. (Use to pick out approximate alignment of jet core).
 500 hPa height & relative vorticity
500 height and rvort
White lines: 500 hPa contours. (units = metres / interval = 60 m).
Colour-fill: Relative Vorticity (at 500 hPa .. roughly the level of non-divergence)
(units=10-5s -1/ interval=2 : see colour quadrant at top-right of diagram - blue implies negative (non-developmental) values & red, orange positive (or developmental) values.
 850 hPa temperature, relative humidity & wind
850Trhwind
Purple/solid lines: 850 hPa temperatures above 0degC. (units=degC / interval=3 degC)
Blue line: the Zero degree C isotherm at 850 hPa (labelled 'FR')
Indigo/dash lines: 850 hPa temperatures below 0degC. (units=degC / interval=3 degC)
Wind arrows: conventional plot for wind barbs at 850 hPa.
Grey-fill: 850 hPa relative humidity. (units = % / interval = 20%: see grey-scale quadrant at top/right of diagram).
 Wave height & surface wind
wave forecasts
Wind arrows: conventional plots of the 'over-sea' wind-barbs (units = knots [kn]).
Colour-fill: Wave height. (units=feet / interval = 3 ft: see colour chart at top/right of diagram - blue - low values, orange-to-red high values.)
[ This would appear to be the 'total' wave height, i.e. wind + swell wave. ]

 

AVN/MRF products (NCEP models)

(These output are in fact in identical format to that above for the NOGAPS model, which makes comparison easy. Only the wave / sea-surface wind field is not included in this set.)

 Example: Brief description of parameters (etc.) on charts shown.
 MSLP, thickness & 12hr precipitation
MSLPtthkPPN
Gold lines: MSLP isobars. (units = hPa / interval = 4 hPa)
Alternate blue & purple lines: thickness (500-1000 hPa) blue 528 & 552, purple 540 & 564 dekametres [dam])
Colour-fill regions: 12 hour precipitation rate (units=mm/12hr / interval=variable, see colour quadrant top/right of diagram) [ It's not entirely clear if this is the accumulated rainfall in the previous 12hr, or the instantaneous rate at some point in the last 12hr - my thoughts would be the former. ]
 300 hPa height & isotachs
300 ht and isotachs
Green lines: 300 hPa contours. (units = metres / interval = 120 m)
Black (thin) lines: isotachs of the 300 hPa wind. (units = knots [kn]/ interval=10 kn)
Wind arrows: conventional plots of wind-barbs at 300 hPa (units = kn)
Colour-fill: strongest winds at this level (>= 50kn), according to scale at top/right of diagram. (Use to pick out approximate alignment of jet core).
 500 hPa height & relative vorticity
500 height and rvort
White lines: 500 hPa contours. (units = metres / interval = 60 m).
Colour-fill: Relative Vorticity (at 500 hPa .. roughly the level of non-divergence)
(units=10-5s -1 / interval=2 : see colour quadrant at top-right of diagram - blue implies negative (non-developmental) values & red, orange positive (or developmental) values.
 850 hPa temperature, relative humidity & wind
850Trhwind
Purple/solid lines: 850 hPa temperatures above 0degC. (units = degC / interval = 3 degC)
Blue line: the Zero degree C isotherm at 850 hPa (labelled 'FR')
Indigo/dash lines: 850 hPa temperatures below 0degC. (units = degC / interval = 3 degC)
Wind arrows: conventional plot of wind-barbs at 850 hPa.
Grey-fill: 850 hPa relative humidity. (units=% / interval=20%: see grey-scale quadrant at top/right of diagram).

Explanations

MSLP patterns

Isobars join points of equal atmospheric pressure. On one side of the isobar pressure is smaller than the isobar value, on the other side greater. A common metaphor is a topographic map with it's lines denoting height: mountains are equal to areas of high pressure, dales equal to lows. Similarly, we can talk about ridges and troughs (or valleys). Wind follows roughly the direction of the isobars so that if you walk along an isobar and the wind is blowing onto the back of your neck, the lower pressure is at your left hand side (in the Northern Hemisphere .. reverse the rule for the Southern Hemisphere) *. Due to friction, the wind is not exactly parallel to isobars, but turned about 10-30 degrees towards lower pressure.

Furthermore, there are several local effects (e.g. the sea breeze) which affect wind and make this general rule not valid in such cases.

The closer the isobars are to each other, the higher the wind speed. (Meteorologists talk about steep or strong pressure gradient.) There are rules and even scales of how to determine the wind speed from distance between the isobars. When using them, be sure to check that the isobars are between the intended steps (usually 5 hPa but sometimes 4, 8 or 10 hPa) and the scale and projection of the underlying map.

Between tropical and arctic areas, at so called mid-latitudes, the isobars tend to form closed circles round the lowest pressure. These are called frontal depressions, and most of meteorological interest at our areas is concentrated in them. Tracking the frontal depressions is the main reason to look at MSLP chart.

(* this was first defined & explained by a Dutch meteorologist, Buys Ballot in 1857, and the 'Law' is named in his honour.)


500 hPa patterns

Level of 500 hPa is roughly dividing the mass of the atmosphere in two. It lies near 5 km, and it's height is typically analysed at intervals of 40 or 80 m, corresponding to MSLP isobars at intervals of 5 or 10 hPa.

The shapes of 500 hPa isohypses are similar to those at the MSLP isobars: even here you can find lows, highs, ridges and troughs, though the latter two are more usual than closed circles. Formations are much smoother because the underlying surface (except in the high mountain areas of the world) doesn't have an effect at these high altitudes (and because observations are sparse).

If you compare 500 hPa and MSLP charts, you often find a family of small surface lows below an upper level low. Also, the weather under a 500 hPa low or trough tends to have more precipitation, at least showers, even is there are no fronts present.

For numerical models, 500 hPa height (and especially the stream pattern it reflects) is much easier to get right than the exact location of individual surface lows and fronts. So we look at it to get a general outlook of weather, especially to talk about 5-10 days ahead.


Spaghetti plots

So named because of their resemblance to a certain Italian food. Spaghetti plots are another way of viewing Ensemble forecasts. Each of the individual forecasts are merged into one image. To reduce the confusion a bit, only two or three contours are drawn. The purpose of a spaghetti plot is to give the user some idea of the uncertainty in the forecast. When the contours are drawn in the same place, a high level of confidence can be used in conjunction with the Ensemble forecasts; when the plot resembles it's namesake (a plate of spaghetti) then the forecast has a very low confidence level, and it is probably best not to make any plans based on this forecast. Generally the ensemble members diverge with time, making forecasts further into the future much less certain.


Cloud forecasts

The Cloud cover forecasts give an output relating to the percentage of sky covered with cloud. Hence 100% would be overcast, 50% (equivalent to 4 oktas) predicts a half cloudy, half blue sky. This is useful for picking out the frontal systems and determining where they are most likely to be the most intense from the higher percentages. Low cloud (roughly below 6000 ft / 1800 m) is notoriously difficult to forecast and global NWP models do not usually attempt this output. Therefore forecasts on these sites should only be used to infer cloudiness at medium and high levels. There are other representations of cloud forecast, which may show overlays of cloud at different levels, and some centres output 'boundary layer' cloud forecasts - not sure how accurate these are though, or how defined.


Precipitation (PPN) forecasts

[ NB: Precipitation is a collective term which includes rain, snow, sleet, hail etc. ] These charts are usually of rainfall accumulation (or the amount of rain that would have fallen if dealing with snowfall). That is, the amount of rain predicted to have fallen in the preceding 12 hours. Hence a value of 6 on a forecast chart for 1200 means that 6 mm of rain should fall between midnight and midday of the day in question. [NB: 1 mm of rain depth equates to 1 litre of rain per square metre - some centres will in fact output rainfall forecasts in this manner, i.e. 40 litres/m2.]

NOTE: Some of the forecasts are of 6 hour accumulations, 24 hour accumulations, or even of 1 hours accumulation. The charts are usually superimposed on another field, usually surface pressure, although the COLA-MRF images (in Wetterzentrale site) are shown with vertical velocity.


850hPa temperature fields

850 hPa level is roughly at 1.5 km, usually above the atmospheric boundary layer. That means there is no diurnal temperature variation, and the underlying surface such as cool sea doesn't affect it's temperature. That is why 850 hpa temperature is used to distinguish air masses and thus to locate cold and warm fronts.

Because the models have had several problem in surface parametrisation, 850 hPa temperature forecasts have been more accurate than those for lower levels. So we used to bring the 850 hPa to surface by adding 15 (or 10) degrees, and use it instead of surface maximum temperature. [***]
WARNING: all this works only at low altitudes, not in the mountains. It works only when the sun is heating the ground: not at the sea or windward coast, not at night-time, not at winter. 15 is the continental value (for dry adiabatic lapse rate); in Ireland they use 10 (for moist adiabatic lapse rate) and I guess at the Alps the 850 hPa temperature is more or less equal to the surface temperature.

You can see from all these warnings, that using the 850 hPa temperature to infer the screen temperature is problematic at best!

[ *** To a first approximation, you can also use T850 to assess the surface temperature in persistent precipitation, provided the layer 850 - surface is near-saturated: in this case, add ~7 degC (i.e. obeying the Saturated Adiabatic Lapse Rate) to the forecast temperature at 850 hPa. ]


Vertical motion fields

The magnitude and sign (i.e. whether "upward" or "downward") of motion in the vertical is obviously important in operational meteorology. All NWP models output such guidance - usually expressed in terms of the variable 'omega' (w), which is defined as the rate of change of pressure with time. Pressure is used, rather than something more intuitive like vertical velocity, because upward/downward motion in the atmosphere due to dynamic forcing (e.g. frontal ascent), comes about as a result of a disturbance of the "hydrostatic equilibrium" - or the balance of forces in the vertical between gravity trying to collapse the atmosphere & the upward, opposite - directed pressure gradient force.

The 'Omega equation' which is used to calculate vertical motion is made up of two terms: an element due to the rate of change of vorticity advection with height, and an element due to horizontal thermal advection. It turns out that solving these terms gives upward motion (ascent) a NEGATIVE value, and downward motion (subsidence) a POSITIVE value: this can be confusing when first looking at these charts, but it might be easier to remember that 'minus' values are tied to falling pressure values or '-ve' pressure tendency (i.e. potential development) & 'positive' values with rising pressure values or '+ve' pressure tendency (i.e. weakening systems, or building anticyclones). (A little more on this HERE).


Vorticity fields

We live on a spinning planet and air in motion tends to pick up a 'turning' moment due to this rotation at every point, except right on the equator (where the local horizontal component of the earth's angular rotation is zero). This natural 'twisting' motion, or local vorticity, is due to the Coriolis acceleration at any particular latitude (f).

Any developments in the atmosphere which act to enhance this effect will increase the likelihood (other factors being right - e.g. enough humidity, sufficient thermal contrast) of development leading to wind, rain etc. Any developments which act to reduce this action negate development. Vorticity due to atmospheric developments, or flow patterns, is termed relative vorticity(z) [ relative to earth 'normal' ].

The sum total of vorticity at any one point (Absolute Vorticity or zA) = f + z. For the vast majority of cases, Absolute Vorticity is positive, but Relative Vorticity can be positive or negative, and it is usually (though not always) this latter quantity that is seen on WWW sites.

... and why are these changes in vorticity important? Well, anything that encourages air to spin more quickly (Advection of Positive [relative] Vorticity, or PVA) will lead to a narrowing of an atmospheric column, and it's expansion upwards, leading to formation of cloud & precipitation .. if of course there is sufficient humidity. The opposite mechanism (Advection of Negative [relative] Vorticity, or NVA) will lead to a broadening of a column, allowing it to contract vertically, with the air subsiding - a non-developmental state.


Thickness **

Thickness (as used on many of these sites) is a useful tracer of warm and cold air in the lower 'half' of the troposphere (surface to about 5.5km), where a large part of the 'real weather' is to be found. Bulk movement of warm air (high values) and cold air (low values) can be followed by looping the fields. In particular, note carefully areas where the thermal contrast is increasing as these are regions of potential major baroclinic development.

However, be aware that working out things like maximum temperatures, snow-level etc., from this rather crude measure is fraught with difficulty and best avoided.
For more, you can follow the links in the FAQ entry for Thickness ... here

** NB: Some will know this quantity at 'Relative Topography', hence the 'Re Top' seen on sites using the German language.


Ensembles

More and more you will read / hear the term 'ensemble' in modern-day weather forecasting. Beyond about 3 days (and in some circumstances less than that), it is not very useful to treat a particular model frame as the answer to the forecast for that particular time-step, but much better to adopt the ensemble techniques as briefly described in the FAQ Glossary, the relevant entry for which you can find ... here. You might also like to read the entry on the 'Poor Man's Ensemble Technique' ... here which explains how YOU can effectively use the various models here described in this fashion.


Jet Streams

On the broad-scale, these are 'key' to deciding the overall complexion of the weather-type in any one geographical region in the mid-latitudes, i.e. roughly between 35 and 75 degrees N/S. The various entries in the FAQ & Glossary are useful starters ... for a basic definition, follow the link: here, and to delve even more deeply into the mysteries of upper air patterns etc., try this page .... here


Equivalent Potential Temperature ('Theta - E')

A useful quantity because it is calculated taking the humidity content of the air into account - and as most 'interesting' meteorology is to do with atmospheric moisture, Theta E (QE) is a better 'tracer' of air-mass property (at 850 hPa**) than temperature alone. It is a conservative property (doesn't change much) during both dry and moist adiabatic processes.

> The actual value of Theta-E can be useful: it can be used in algorithms to calculate daytime maximum temperature and snow-risk for example (as for Theta-W#). Remember though that this variable couples temperature AND absolute humidity: so high values show areas of warm, potentially more humid air (e.g. tropical maritime or modified tropical continental air-masses); lower values pick out colder, lower-humidity content air (e.g. polar maritime air-mass).
> Where there is a marked or 'sharp' discontinuity in values (best seen on colour-enhanced output), then some attempt at frontal placement can be made.
> The location of axes of plumes of high (orange/red) Theta-E, act as a "focus" for significant thundery activity: a plume of such air running northwards over NW Europe ahead of an upper trough coming eastwards may herald the initiation of a ' Spanish Plume' (q.v.); event.
> Although not strictly appropriate to these charts, it is pertinent to note here that the vertical distribution of Theta-E (and Theta-W) with height is a useful diagnostic of whether an air-mass possesses Potential (or Convective) Instability (q.v). If a column of air is lifted bodily, the temperature decrease varies from level to level, particularly because some layers become cloudy (and acquire the released latent heat) sooner than others. Therefore, certain layers can become unstable simply because they have been 'mechanically' lifted - these can be found by inspecting a thermodynamic diagram (or tabulated list of data), and finding layers [roughly between 900 and 400 hPa] where Theta-E (or Theta-W) decreases with height.
#[ in the UK service, the Wet-bulb potential temperature (Theta-W) is more usually used for these purposes, but is not readily available on external web sites - Theta E will do the same job though.]

** [ Beware of using these parameters at 850 hPa as tracers of air-mass change slavishly. Provided the air in the lowest 2 km of atmosphere is well-mixed, then they are indeed excellent for this purpose. However, in stagnant situations, or where there is marked 'de-coupling' between the quasi-frictionless flow above about 900 hPa and the surface, (or boundary-level) air, changes at 1500 m (i.e. 850 hPa) may not tell the whole story. ]


CAPE & Lifted Index

I've 'bracketed' these two parameters, because they are often considered together:

CAPE: it would be better if this parameter was always referred to by it's full expansion: "Convectively Available Potential Energy"; unfortunately, this doesn't happen, so we have the situation where newcomers to the world of 'severe convection' are bemused by all the talk of a quantity 'CAPE' which is not adequately defined. I'll have a go here:.....

When a parcel is lifted by whatever means from the surface (or another level), it will cool according to the basic laws of thermodynamics. The rate of cooling is dependent upon whether the parcel is 'dry' (i.e. unsaturated) or 'moist' (saturated). The rate of cooling is known as the Dry Adiabatic Lapse Rate (DALR) in the former case, and the Saturated Adiabatic Lapse Rate (SALR) in the latter. By inspecting the temperature of the parcel at any point in its upward travels, and comparing with the 'environment' through which it is travelling (best done on a thermodynamic diagram), it will either be cooler (denser) than the environment (stable), neutral (the same temperature), or warmer (less dense) than the environment (unstable). In this latter case (unstable/parcel warmer than environment), then the parcel (packet, bubble whatever), has Potential Energy, manifest as continued upward motion, which was initiated, and made Available due to Convective activity: C-onvectively A-vailable P-otential E-nergy - CAPE. [ For more on stability, lapse rates etc., see the main FAQ.]

The greater the excess in temperature (parcel warmer than environment), then the greater the energy available - and all other things being equal (e.g. no compensating descending currents due to broadscale dynamic effects), this should result in stronger upward motion, potentially more 'severe' conditions (downdraught gusts, heavy precipitation, larger hail, higher tornadic threat etc., etc.).

CAPE is simply the integration (sum) of the energy that a parcel would have throughout its vertical 'life', once convection is released. It is calculated on a thermodynamic diagram by assessing the area between the Environmental Lapse Rate (ELR/actual temperature trace) and the Parcel trace, provided the temperature of the latter exceeds the former. [ See a crude example HERE ]
The units of CAPE are J/kg (energy per unit mass of atmosphere), and some typical values are:
> ~ 150 - 300: heavy/intense shower-rainfall with/without thunderstorm - mainly 'slight' TS
> ~ up to 1000: 'moderate/severe' thunderstorm
> ~ up to 2500: 'severe/intense' thunderstorm.

(Note carefully: many so-called 'critical' values for CAPE & LI - see below, have been defined based on spring/summer US experience and may not be applicable to European/'all-season' storms - do NOT treat these figures as a 'threshold' or a 'requirement'!)
Some other figures for CAPE are outlined in the main FAQ.

Lifted Index (LI): In many respects, this can be viewed as a very crude measure of the above - i.e. CAPE. It is very easy to assess (probably why it was devised in the first place), and so has gained widespread currency in the operational / severe weather world. It is defined simply as:-
LI=T(500) - T(P);
where LI=Lifted Index (degC), T(500) is the temperature at 500 hPa as found from an actual radio-sonde ascent (or computer/model output) and T(P) is the temperature that a parcel would have, if encouraged to reach the 500hPa level (about 18000 ft or 5600m) by whatever means. [ See a crude example, based upon a Skew T, logP diagram: HERE ]

It will be readily seen that when the parcel is cooler (denser) than the environment (i.e. a 'stable' state), LI will be +ve; when the parcel is warmer (lighter) than same, (i.e. the 'unstable') state, LI will be -ve. The more 'negative' the number, the greater the excess of energy the parcel has at this point, hence it's crude likeness to CAPE (above).

Over the years, some rough figures have been developed which relate the value of LI to expected conditions: note that they must NOT be used slavishly in this respect:
LI +ve: stable, non-convective.
LI=0: neutral - non-severe convection may be possible.
LI -ve: unstable .... and the following are offered as a guide ....IF CONVECTION IS INITIATED!
LI -1 to -4: moderate thunderstorms/mainly small hail,
LI <-4: possible severe thunderstorms/large hail,
LI <-10: possible severe/intense thunderstorm, whirlwind phenomena (e.g. tornadoes), 'giant' hail etc. (see cautionary note above re: applicability to European conditions)


Precipitation type forecasts

(i.e. rain or snow phase)
These charts show precipitation intensity (Niederschlagsstaerke) and precipitation type (Niederschlagsform). For all meteorological NWP output, it is important not to "follow the dots" slavishly, that is, not to believe every twist and turn of the output at every time-step. This is particularly so for this product.

The output is decidedly 'blocky', and the output resolution (i.e. what you actually see), is certainly rather course; a single 'block' appears to cover one or two medium-sized English counties. Whether the model resolution is equally course is not known: indeed, if orography is used (and it would be unusual if it isn't), then it must be very crude.

The output is in mm per hr (mm / h); this is fine for liquid precipitation (e.g. rain), but snow output (rosa / purple / schnee) is also shown as mm / h .. these aren't mm of snow on the ground: this is the rainfall equivalent. As a very rough conversion, multiply the intensity by 10, so 1.5mm / h rainfall-equivalent=> 1.5 cm / h. But remember, that not only is this a highly variable quantity, it assumes that the model has got a perfect representation of the lower-tropospheric temperature / humidity profile, and the precipitation intensity is correct, and the model 'knows' about the surface (snow covered, frozen etc.), and that all the snow settles!

In my view, it would be best to regard the 'snow' phase colours as scales of probability in any one situation: the deeper the shade of purple, the higher the probability of snow reaching the surface .. be very suspicious of the lower-decimal (roughly < 0.8 mm/h) figures for snow rate.


Freezing levels

(or 0°C isotherm levels)
On the Topkarten site, there is a section where you can choose from various fields with 3 hr time-steps (though over a limited domain); in particular, and most useful currently, the field labelled:- " 3h Niederschlag " will show precipitation amounts in millimetres. (I assume where the phase is snow then it is 'liquid water equivalent', though I can't see any legend to that effect), and also shown are contours of the height of the zero degC level above mean sea level (in metres): remember to make an allowance for your location - so if you are 200 m altitude, and the forecast is 500 m amsl, the zero deg.C level will be 300 m above you.

This latter shouldn't be taken as the 'snow-level' as such, because of course snow can descend (depending upon the relative humidity in the boundary layer & intensity) well below this level, but it is a useful guide & provided it doesn't change too much with time, it can be used to infer the 'usable' (for winter sport purposes) snow-line in hilly / mountain areas.

If the freezing level is less than ~300 m above your elevation (remember that the output heights are amsl - adjust for your location), then it is reasonable to assume that there is a >50% probability of PPN falling as snow inland. If it is <100m then there is a >=90% probability of snow. (However, in coastal areas with an onshore wind off a relatively warmer ocean, then these figures will not apply so well if at all). [ Ref: Boyden Met.Mag 1964 ]

The horizontal resolution of the GFS is approx. 40 km out to 180hr (~80 km beyond that), and the output resolution of these charts, according to the site information, is 50km, so for this display you are seeing near-full model resolution, as opposed to the main charts, which are 'thinned' to ~100 km. The GFS is still a crude model to use for 'local' work though, bearing in mind that modern mesoscale models are working down to the ~10km or better level.


The Omega Equation

There is an equation in atmospheric physics known as "the Omega equation": it allows synoptic-scale vertical motion to be accurately estimated by analysing temperature and wind fields above the boundary layer, leading to a value for vertical velocity (allocated the greek letter 'omega'), in pressure co-ordinates (dp/dt).
[ By the way, because pressure decreases with increasing height in the atmosphere, ascending air gives rise to a negative value: therefore on computer output, -ve values imply ascending air, +ve values descending air. ]

In it's broadest exposition, the Omega equation brings together the forcing due to vertical variation of absolute vorticity advection, the thermal advection terms (various) and components due to diabatic warming. These factors can be viewed subjectively (e.g. a PVA area engaging a focus of strong warm advection will lead to strong upward motion and vigorous surface development).

Q-Vectors

A numerical model though needs to have all this in terms of rigorous mathematical formulae (albeit with some approximations) that can be solved. On the way to calculating all this development, the models spew out all sorts of diagnostics that can be used by forecasters: one such are 'Q-vectors'. These are found by using a 'cut-down' form of the Omega equation developed by Hoskins, Draghici & Davies in 1978.

The vector "Q" is dependent upon the rate of change (with time) of the potential temperature gradient brought about solely by horizontal change in the wind-field (derived using model contour fields, geostrophic.)

Q-vectors are not a measureable quantity like the actual wind or the temperature - they are more akin to the 'thermal wind', with which they are in fact allied.
They are derived, as indicated above, by manipulation of the Omega equation. In brief, the concept of the Q-vector is a mathematical representation of the atmosphere's attempt to restore the thermal field once it has been disturbed from its 'equilibrium' state of both hydrostatic (1) and geostrophic (2) balance. This 'restoring force' is deemed to be via ageostrophic motions, which are most important in determining where development is occurring.

[Notes:
(1): hydrostatic balance: the 'struggle' between the pressure gradient acting upwards, due to decreasing pressure with height, and the force of earth's gravity acting in the opposite direction - the hydrostatic equation represents this process.
(2): geostrophic balance: the 'struggle' between the pressure gradient (all planes, though usually taken as horizontal) and the deflection due to the earth's rotation - the equations of motion represent these.]

Where Q-vectors are convergent (i.e. negative divergence), the forcing is normally associated with tropospheric ascent (or low-level development / falling pressure.
Where Q-vectors are divergent, the forcing is normally associated with tropospheric descent (or low-level decay / rising pressure ).

Also, where Q-vectors are directed from cold to warm air, (at 850 hPa) this implies frontogenesis (a tightening-up of the thermal field); the opposite implies frontolysis (or frontal decay).

On various charts available via the Westwind site:-

FQ :=Forcing due to Q-vector convergence / divergence.

FQ: +ve divergence==> descent / non-developmental
FQ: -ve divergence==> ascent / developmental (or convergence)
The higher the number (+ve or -ve), the deeper the colour and the greater the forcing term. These data are usually shown in association with the 500 hPa field, roughly at the level of non-divergence (or maximum vertical velocity).

FQn

:=the component of Q 'normal' (i.e. at right-angles) to the isotherms and for most work, the isotherms used are those at 850 hPa; for mountainous terrain though, 700 hPa should be used.

 

FQn: strongly -ve in cold air=> frontogenetic

 

FQn: strongly +ve in warm air=> frontogenetic

 

Best used to find areas of 'strong blue' (positive) immediately adjacent to 'strong red' (negative); this implies synoptic-scale processes in place enhancing frontal development / ascent etc (or frontogenesis).

 

 

Example of CAPE & LI

 Example CAPE & LI on SkewT, LogP  On this example (of a Skew T, log P diagram), CAPE is the area enclosed by: X - A - TOP - B - Y - X (shaded green); Lifted Index is given by the difference (observing the sign) between the temperature (B-A): see explanation below:....
 A  The point (and associated temperature) where a saturated parcel given this particular ascent crosses the 500 hPa isobar=T(P)
 B  The actual temperature, T(500), from the radio-sonde ascent in this particular situation. The difference B-A [ or T(500) - T(P) gives the Lifted Index.
 X  The temperature at the surface (SFC) which initiated this particular parcel's ascent. From X to LFC, ascent-cooling will be at the Dry Adiabatic Lapse Rate (DALR).
 Y  The lowest 100 m or so of the ascent as given in the radio-sonde ascent; this will become heavily modified due to afternoon heating.
 LFC  The 'Level of Free Convection', above which the parcel, rising from the surface, will be saturated, and cool at the Saturated Adiabatic Lapse Rate (SALR), in this case from LFC to TOP.
 TOP  The point at which the temperature of the parcel=temperature of the environment, and buoyancy is neutral; the parcel theoretically ceases to rise of it's own volition - but given enough positive energy below this point, tops may (& often do) 'overshoot'. (also known as the 'Equilibrium Level')

 

ThetaE, ThetaW & derived parameters

The relationship between Theta E, Theta W and theoretical maximum temperature and snow probabilities (latter based on Bradbury, 1970).

 

Notes:

1. The relationship for day maximum temperature was developed specifically for SE England. In mid-winter in particular, at latitudes further north, the expected maxima will be lower. Also note that the figures are not necessarily based on a plentiful data-set over the whole range of expectations; for example, at the very high and very low end of the scale, the numbers of events used to find the theoretical 'base' maxima will have been much smaller than for the 'middle', or most-likely area of the data-set. With experience, you may indeed find your own figures are better than these, so keep a record of what happens in each occasion.
2. The figures will not apply over a full snow-cover, as insolation gives a different heating response over such surfaces.
3. The techniques use the data from 850 hPa (or mbar), which is very roughly at 1500m (or 5000 ft) amsl. Under conditions of strong anticyclonic subsidence, this level may not represent the air mass in the lowest 50 hPa of the atmosphere. Cold-undercutting (and indeed warming over sea surfaces) should be allowed for. Also, in cases of very high pressure (over 1035 hPa / mbar), the 850 hPa / mbar level will be even higher up, and will represent the near-surface even less.
4. Once you have found the basic working figure for maximum temperature, 'correct' for cloud cover using the table given. Note that basic figures & corrections are only given to the nearest 1degC and the results should not be used slavishly - arguments involving the odd degree between one situation and another are pointless given the approximations involved & the accuracy of the raw input data (whether model or ascent-based).
5. In the case of the snow risk, remember it is for mean sea level. You need to adjust for altitude using the adjusting table given.

Method of use:
(a): Find expected ThetaE (Equivalent Potential Temperature) over the area in question at around the middle of the day - from latest NWP output, or from radio-sonde ascents.
(b): Against the appropriate value of ThetaE in the table below, look up the value for that month - interpolating / adjusting as necessary.
(c): Look at the list of corrections given underneath and choose appropriate value: apply.
(d): For snow probabilities, find the base (i.e. msl) figure from the extreme right-hand column.
(e): Work out the altitude of the site in question, and using the table of adjustments, find the % risk of snow for that altitude. (IMPORTANT NOTE: In my experience, these figures are too pessimistic and should be used as a very rough guide only. I find that you need persistent, moderate or heavy precipitation before these figures are relevant. What I do is use the 50% equivalent isopleth at msl ... 3degC ThetaW / 16 degC ThetaE ... as a 'tracer' for 'snow-worthy' air at lower levels, then use other means to decide on the precise risk.)

 850 thetaE
(degC)
 850 thetaW
(degC)
 Jan.  Feb.  Mar.  Apr.  May  Jun.  Jul.  Aug.  Sep.  Oct.  Nov.  Dec.  Snow
Prob. (at msl)
 54  18  19  20  21  23  25  26  26  25  24  22  20  19  
 46  16  17  19  19  21  23  25  25  23  23  21  19  17  
 40  14  16  17  18  20  22  23  23  22  21  19  17  16  
 36  12  15  15  17  19  21  21  21  21  19  17  15  15  -
 32  11  13  15  15  18  20  21  21  20  19  17  15  14  -
 30  10  13  14  15  17  19  20  20  19  18  16  14  13  -
 26  8  11  13  13  15  17  19  19  17  17  15  13  11  1%
 22  6  9  10  11  13  15  16  16  15  14  12  10  9  14%
 18  4  8  9  10  12  14  15  15  14  13  11  9  8  34%
 14  2  6  7  8  10  12  13  13  12  11  9  7  6  78%
 10  0  4  5  6  8  10  11  11  10  9  7  5  4  96%
 8  -1  3  4  5  7  9  10  11  10  8  6  4  3  100%
 6  -3  1  3  4  6  8  9  10  9  7  5  3  2  100%
 4  -4  -1  1  1  4  6  7  7  6  5  3  1  1  100%
 2  -5  -3  -1  0  2  4  5  5  4  3  1  0  -1  100%
 0  -6  -5  -3  -1  0  2  3  3  2  1  -1  -2  -3  

Corrections to be applied (for the maximum temperature figures):

A day of heavy overcast with precipitation: apply correction to above of at least -2 to -3degC, and for air-masses laden with cloud and having high low-level humidity (spring / summer), then the correction may be as high as -5degC.
A day of bright, virtually uninterrupted sunshine (but no snow cover) apply + 1degC. On days of strong sunshine over a dry ground (in a relatively warm air-mass for the season), then correction is at least +2degC, and may be up to +3degC.

Use the following table to adjust the % snow risk (at mean sea level) in the above to the required elevation:

 Elevation (in metres)  % PROB at msl >>>>  20%  40%  60%  80%
 50m    28%  50%  68%  84%
 100m    39%  59%  73%  89%
 150m    50%  68%  78%  93%
 200m    59%  73%  83%  98%
 250m    66%  78%  88%  100%
 300m    72%  83%  92%  100%
 350m    77%  88%  98%  100%

 

Forecasting 'rules of thumb'

Some techniques, largely empirical, for use by 'single observer forecasters'. Often rough & ready, but over time have been proven to have some value. As with many things in meteorology though, don't follow too slavishly!

Empirical cloud forecasts

EMPIRICAL 'RULES' FOR LOW CLOUD

General note:

When computing cloud heights from surface (i.e. screen) data, don't try and be too clever: the nearest 200ft or so is reasonable - the higher the cloud base, the more likely the error, because the methods below take no account of the humidity structure above the surface, and subsequent mixing (i.e. entrainment / detrainment) with the cloud environment as convective over-turning will alter the character / base / thickness of the cloud.

CLOUD HEIGHT: [ Cumuliform only ]

If T = screen (air) temperature and Td = air-mass dewpoint (degC),
then
base of Cu (in feet) = [ T - Td ] * 410
... sufficiently accurate to use 400: i.e. (difference T-Td) * (4) * (100) [ which is easier to do in your head!]
base of Cu (in metres) = [ T - Td ] * 125
... sufficiently accurate to use 120: i.e. (difference T-Td) * (12) * (10) [again, easier to do in the head.]
base of Cu (in kilometres)  =  [ T - Td ] / 8
... this gives approximately the same result as that for the 'metres' version above.

(remember, if using the screen dew point at the start of the day, this will change: often it will be lower during the afternoon as the low-level air is thoroughly mixed with air that has been dragged down from aloft, but during the post-dawn period, and for a little while afterwards, the dew-point could increase; this is particularly the case when there has been mist, fog or a heavy dew formation at the end of the night. Also, precipitating cloud bases tend to be lower, as the rain / snow will moisten up the below-cloud environment.)

CLOUD HEIGHT: [ Stratus / low-base Stratocumulus only ]

If T = screen (air) temperature and Td = air-mass dewpoint (degC),
then
base of St {or low Sc} (in feet) = [ T - Td ] * 400
(as above, use [difference] * [4] * [100])
base of St {or low Sc} (in metres} = [ T - Td ] * 122
(as above, use [difference] * [12] * [10])
(Precipitation such as drizzle or fine rain will moisten up the below-cloud environment and therefore lower the cloud base - perhaps significantly.)

For Stratus forming over a cooling land mass (i.e. inland over Eastern England on an east or northeasterly airflow), the approximate (very crude in my opinion) cloud base as the cloud forms is given by [ 75 * 10m wind speed ] (base in feet, wind in knots).

CLOUD AMOUNT: [ Cumuliform cloud ]

As a very rough guide, and given no tendency to thick Sc formation, then if the air-mass RH IN THE CLOUD ENVIRONMENT is around 50%, amounts will average out at half-cover (i.e. FEW/SCT, TEMPO BKN)

If the RH is around 75% , the amounts will average out at more than half-cover (i.e. BKN).

[ Stratocumulus formed by the spreading out of Cumulus ]

A reasonably accurate estimate can be made by dividing the relative humidity (U) of the air WHEN THE CLOUD FIRST FORMS by 6, and subtracting 6 from the answer. The resulting value will signify the number of tenths of sky likely to be obscured when convection is at a maximum. (This method dates from pre-WWII days, hence the reference to 'tenths')

So % cover = 10 * ( U / 6 ) - 6

(this implies that for U < 36%, any Sc Cugen should clear; U >~ 80%, and near-full cover can be expected to be maintained, perhaps only clearing late in the afternoon / early evening - depending upon the thickness of the cloud.)

As with methods above, this only uses the screen values and the cloud environment will dictate the exact evolution.

Temperature Forecasting

Night Minima

1.McKenzie

If the day maximum screen temperature is denoted by (Tx) and the air-mass dew point temperature (Td), and the latter is assumed not to change appreciably overnight, then

T(min)=1/2 (Tx + Td) - K

where K varies according to the expected surface wind speed and average cloud amountovernight.

McKenzie originally did his work for Dyce (Aberdeen) and it was presented (though not published) in 1944 in an internal memorandum within the Meteorological Office. Since that time, constants for many Met Office site have been calculated (Claude Kensett), but as a rough guide:-

 Wind / cloud overnight

 K

 Dead calm / clear skies throughout night  ~ 8 or 9 (an extreme case)
 Light wind / small amounts cloud  ~ 6 to 8
 Light wind / cloud coming and going  ~ 4 to 7
 Light wind / generally cloudy  ~ 2 or 3
 Moderate wind / small amounts cloud  ~ 3 to 5
 Moderate wind / cloud coming and going  ~ 2 to 4
 Moderate wind / generally cloudy  ~1 or 2
 Brisk winds (most cloud classes)  ~ 1 or 2

There are other corrections you can apply given the time of year etc. The beauty of the method, is that it is easy to apply .. add together a couple of commonly known variables, divide by 2, and then fudge the 'correction' K.

The initial figure (i.e. before applying 'K' above), gives an approximation to the dusk temperature, following (empirical) work performed by Boyden & Saunders. However, when there is an inversion of temperature in the lowest few hundred metres of the atmosphere, then take off 2degC from this value.

2.Craddock & Pritchard

If the midday (12 GMT) screen temperature is denoted by (T) and the midday (12 GMT) dew point temperature (Td), then

T(min)=0.316T + 0.548Td - 1.24 + K

where K varies between -2.2 for near calm, clear skies or near clear overnight to + 2 or 3 for windy, mostly cloudy situations. This method is only applicable for inland stations, and was developed over Eastern England.


Day Maxima

Methods for forecasting day maximum temperatures are listed elsewhere on this site:-

  1. Based upon the 850 hPa actual temperature 
  2. Based upon the 850 - 1000 hPa partial thickness 
  3. Based upon the 850 hPa Theta E (or Theta W) method 
  4. Based upon the 500 - 1000 hPa total thickness (or Relative Topography)

General notes: solar (short-wave) radiation [ or insolation ] absorbed at the earth's surface is converted into heat and warms the ground (raises the temperature) - the warm ground in turn radiates a good deal of heat back to space at long wavelengths (relative to the shorter solar radiation) - much of this is absorbed in various ways by the atmosphere - water vapour - water droplets in clouds - and CO2 - subsequently returned to earth as LW radiation (back, or counter-radiation). A very shallow zone on / near the surface is warmed directly (by conduction) - but the bulk of the heat gained is directly re-distributed by convection or through turbulent mixing of air in contact with the surface or through radiation, and indirectly through latent heat processes.

The standard model of how the temperature varies throughout the 24 hours is neatly illustrated in the diagram (below), which is here reproduced from T. R. Oke ( 1978: 'Boundary Layer Climates', Methuen, London, pp. 31-34. The times are for illustrative purposes only. )

Day_night_radiation_balance

During the daylight hours (except at either end), when the sun is appreciably high in the sky, solar energy is received faster at the surface than it can be distributed upwards as net outgoing terrestrial (long-wave) radiation. This is indicated on the diagram between the points 'A' and 'B'. This leads to a rise in temperature (T) from the point of minimum (Tmin) to the time of day maximum (Tmax). Note carefully how Tmax occurs after the insolation peak - in summer this can be many hours later. During the periods when the sun is very low in the sky (early morning & late afternoon or evening, depending upon season), and also overnight, the rate of energy loss from the surface dominates and therefore the ground and air immediately above it undergo cooling - i.e. between 'B' and 'C', and between 'D' and 'A'. The difference between the daily maximum and minimum surface air temperature is known as the daily range of temperature and the profile described above is an approximation to the theoretical diurnal temperature range.

Once the 'standard' model is set, there are a number of conditions producing modification to the profile.

Wind speed (turbulence): The stronger the wind within the first few 10's of metres of the ground, the 'flatter' will be the profile of temperature depicted above. This is because wind-generated turbulence will mix the air (in much the same way as bath water is mixed by agitation), and so day-time heating is mixed through a deeper layer (rather than concentrated in the first few metres), and night-time cooling offset by warmer air being dragged down from above.

Humidity content: One of the most important controls on the night-time temperature is the absolute humidity content of the over-lying air. This is usually represented in the various algorithms by the dew-point (or perhaps wet-bulb) temperature. Water vapour is a very efficient absorber of outgoing long-wave radiation, which it then re-radiates - some of which comes back to the surface and offsets the fall of temperature. Even under clear skies, if the air is humid then the nocturnal fall of temperature is not as great as with low-humidity air. Also, if other factors are favourable then fog and / or low cloud will result, further arresting the fall of temperature.

Cloud cover (or fog): cloud cover (and fog) will reduce the amount of incoming solar radiation during the day, with maxima reduced from the theoretical potential as a result. At night, the cloud / fog will absorb, reflect and re-emit radiation back to the surface, acting like a 'blanket' and subsequent minima will be higher than if there was no cloud. Even irregular / fitful cloud banks will have an effect, and although layer low cloud types (stratocumulus, stratus etc.) are the most effective, even relatively thin & well-broken altocumulus will have a marked impact upon the final minimum.

State of ground: Day and night-time temperatures are lower over snow surfaces than than those without snow-cover. There are three reasons for this . . .

  1. Snow, particularly fresh snow, is a good reflector of incoming solar radiation, thus reducing the effectiveness of daytime sunshine.
  2. Snow (again, fresh cover in particular) is a good insulator due to the potentially large amount of air trapped within the snow. This prevents heat reaching the snow surface from the underlying ground.
  3. Snow surfaces radiates strongly, and at night this produces a strong radiative cooling effect.

[ The combined effect of (2) & (3) above is that night-time minima under clear skies and with light winds are likely to be some 2 to 4 degC lower than predicted by the equations above, and the time of minimum is much earlier in the night than for surfaces clear of snow. With cloud cover, then roughly 1 degC lower. ]

Temperatures tend to be higher by day and lower by night over dry, bare soil (or sandy) surfaces, away from urban influences.

  1. By day, little or no radiant energy is required to evaporate moisture - all energy is therefore available to heat the ground (and therefore the overlying air).
  2. With loose soil or sand in particular, the air trapped between the particles, prevents deep distribution of the heat energy, and it is most effective at the surface - again allowing a strong influence on daytime maxima.
  3. Overnight, dry, bare surfaces radiate strongly - and have little or no moisture available to raise the overlying humidity, which would offset any heat loss.
  4. Loosely bound surfaces (light soils, fine desert sand etc.), act like fresh snow - little or no heat can be conducted upwards from deeper down, and thus strong radiational cooling from the very top of the surface layers takes place. [This is something often forgotten by people visiting desert areas in winter - just how cold it can get.]

 

Wind direction and barometer tendency

Table relating wind direction to change in atmospheric pressure.

Ideally, these 'rules' should be used with other 'evidence'; for example, noting how the character of the sky changes over time, whether the wind is veering or backing, how fast (or otherwise) the pressure is changing. Neither does it (always) discuss the differences between on the coast, the coastal plains, or someway inland - these will make a difference in many situations. This table covers non of these points and therefore should be used with much caution.

BAROMETER RISING

 WIND  SPRING  SUMMER  AUTUMN  WINTER
 N  Mainly dry but cold weather. Night frosts.  Cold & dry except near east coasts.  First autumn frost is not far away; dry, sunny cold weather in the southwest.  Snow showers decreasing, followed by hard frost.
 NE  Cold, cloudy weather, with sunshine on western coasts.  Cool & cloudy in the southeast but fine & sunny in west & northwest.  A fine spell except near east coast.  A spell of severe weather in the north and Midlands.
 E  As NE, but more likely to persist.  Fine spell, breaking down later with rain from the south.  As NE  Cold or very cold, dry weather with serious risk of snow to follow.
 SE  A fine dry spell, relatively mild.  Two or three days of hot weather; thunder later.  'Indian Summer' weather.  As E, especially serious in late Winters. Persistent fog inland.
 S  A short warm spell.  A short, very warm spell.  As S, but less likely to persist.  Thick and persistent fog inland.
 SW  Good growing & sowing weather. Mild & mainly dry.  A warm spell with little rain, except on western coasts & hills, where drizzle likely.  Coastal & hill fogs in the west, but fine & mild inland.  Short spell of mild, cloudy weather, much hill fog.
 W  As SW  As SW  Temporary fine, mild spell.  As SW.
 NW  Showers decreasing. Short cool, dry spell.  As in spring, but cold (for summer).  Temporary cool spell with scattered showers.  Showers of sleet or hail, decreasing in frequency.

 

BAROMETER FALLING

 WIND  SPRING  SUMMER  AUTUMN  WINTER
 N  Snow/sleet showers, possibly thunder.  Cold thundery weather with hail.  Showers followed by night frosts.  Snow or snow showers, heavy on hills.
 NE  Rain in eastern areas, remaining dry in the west.  As in spring  Little immediate change.  Snow in eastern areas but not heavy.
 E  As NE  As NE  As NE  Thaw, possibly preceded by snow.
 SE  Rain approaching southwestern areas.  Thunderstorms.  Rain in the south, possibly heavy locally.  Heavy snow in the southeast.
 S  Rain imminent.  Rain & possibly thunder.  Rain imminent, probably heavy and prolonged.  As autumn. Very mild.
 SW  Rain imminent.  Rain imminent.  Rain & gales.  As autumn. Mild.
 W  Rain or showers.  As in spring.  Rain or showers. Mild weather continuing.  As autumn.
 NW  Showers & colder weather with hail and thunder.  As in spring.  Showery weather; risk of hail or thunder: cool.  Heavy showers of sleet or snow; risk of hail & thunder.

From: 'Experiments in Meteorology'

Some other rules based on observation of the barometer - published in Rinne, J., Koistinen, J., & Saltikoff, E. : Suomalainen sääkirja - etanasta El Niñoon. (In Finnish) Otava, Keuruu 2001. ISBN 951-9435-93-X
(With thanks to Elena Saltikoff for passing this on . . .)

Very basic barometer forecast rules

(NB ... assumes the unit is working properly!)

Look at the change in 3 hours.
If the pressure is descending, there is a low pressure coming.
If it's ascending (or rising), the low is passing or a high pressure is coming.
When the pressure is changing rapidly (> 6 hPa/3 hours), it's windy (or potentially windy).

More detailed:
Sinking (falling) slowly (0.5 - 3 hPa in 3h): low is weak, dying or moving slowly. You might get some rain but typically no high winds.
Sinking (falling) moderately (3-6 hPa/3h): rapid movement or deepening low. Moderate winds and rain in warm front. The low is passing you fast so day after tomorrow will typically be fine.
Sinking (falling) 6-12 hPa/3h: Storm.

And rise is connected to gradually drier weather.

[ N.B.
hPa (hectopascal) are the same thing as mbar (millibars), and in older publications and on older barometers, you may see the abbreviation 'mb', also meaning millibar. Typically the values (at sea level) will be within a range 50 hPa either side of 1000, but allow for some extremes at either end. An older barometer might have mmHg (i.e. millimetres of mercury, with numbers near 760) or even inHg (inches of mercury). ]

Fronts and Pressure Systems

The chart below is an attempt to place on one chart all the examples of frontal and pressure-centre type that you might see on some sites on the Internet. It is an unrealistic chart of course, though I have tried to keep the faith with meteorological theory!

map of different types of fronts etc.

 

Fronts:

Cold Front: Cold air replaces warm air at either the surface (surface cold front) or at some level aloft (upper cold front). Cold fronts are usually well-defined at the surface, and can have either ana- or kata-characteristics. (See the Glossary for more on this topic). Upper cold fronts (and occlusions) are best defined in terms of satellite imagery, particularly well picked out by the contrast between IR and VIS channels (when available) [ see Split-frontal type in the Glossary and also the article on Over-running Troughs].

Warm Front: Warm air replaces cold air at either the surface (surface warm front) or at some level aloft (upper warm front). Warm fronts are often ill-defined at the surface, particularly over land areas in the summer half-year, and careful analysis of dew points, changes in low-level cloud structure etc., is often required to pick out the front. Upper warm fronts can sometimes be located (on analysis) by reference to rainfall radar imagery.

Occluded Front: A classical Norwegian occlusion occurs where the surface cold front catches up with the warm front, and the warm-sector air is lifted off the surface in a wedge. Occlusions can be warm or cold, though on modern analysis charts, the distinction is not often preserved in the symbology. Cold occlusions mark (on the surface) a change to colder post-frontal air; warm occlusions mark a change to warm (or less cold) post-frontal air. The former is the more typical type, the latter a feature of winter and early spring, particularly where cold, continental anticyclonic blocks are slow to give way.

Upper Front: A front that has more significance above the surface - very roughly above 3km (or 10000 ft / 700hPa). Found by using parameters such as ThetaW, ThetaE, partial or total thickness etc. Broadly, can be regarded as occurring at some level between 850hPa and 500hPa, i.e. in the lower troposphere, below the level of Non-Divergence.

Convergence Line/zone: Most fronts have some form of convergence associated with them (except if they are very weak), but sometimes, due for example surface heating, convergence zones form which are not deeply baroclinic, but which have the potential to trigger intense convection. Best found by drawing streamlines, but remember that these latter only show confluence, NOT convergence (see the Glossary for explanation of all these terms). You need to inspect the wind speeds as well as directions to determine if there is true convergence of mass.

Trough: analogous to valleys on contour maps - with lower contour or pressure values along the axis of the trough relative to adjacent regions. (see "What is a trough?")

Frontal movement: except for the quasi-stationary front (which of course stays almost stationary), the 'spikes' or 'bumps' point in the direction that the front has been moving over the recent past (analysis) or is expected to be moving at verification time (forecast).

List of features shown on chart above

[ You will increasingly see on these charts occlusions with the same symbology as for frontolysing cold and warm fronts: I have not shown such above, as in my view, an occlusion is weakening anyway (from the standard Norwegian frontal theory), and to show such as weakening is a bit of 'over-egging the pudding'. Also, it is not included in the standard frontal types for use on monochromatic picture facsimile as authorised by the World Meteorological Organisation.]

 

Over-running Troughs

In view "A", the mid-tropospheric trough (nominally around 500 hPa) is a fairly sharp, easily identifiable feature, with the trough axis to the rear of the surface location of the Occlusion/cold-front. Under PVA-maxima conditions, vertical (upward) motion is focussed just forward of the trough-axis, leading to thick cloud, high precipitation intensity (other factors being right). Given the location of the upper forcing relative to the surface trough, the front appears to be a rearward sloping/ana-frontal type.


trough not yet overrun

Rearward of the trough axis, lies the zone of negative vorticity advection (NVA) associated with descending air and the area of relatively low relative humidity conditions (at mid-tropospheric levels) shown.


In view "B", the primary forcing trough has now 'relaxed' away and is losing some of its shape, thus the dynamics (vorticity advection >> strong upward motion etc.) are also weakening, and for this reason alone, the frontal activity will start to fragment.

However, there is an element of 'de-coupling' also in play, as the upper activity moves away from the lower-tropospheric humid zone, and in satellite imagery, the cold topped cloud (seen via IR channels) will move well ahead of the low level frontal break (seen in VIS channels), and will appear to be divorced from surface discontinuities such as wind-shift, dew-point drops etc.

trough now overrun

The upper trough will still have vorticity forcing associated with it of course (albeit weaker), and may manifest itself as the upper cold front shown - this feature is found to the rearward of the IR cloud mass, with the major mid-tropospheric dis-continuity being the change from high relative humidity shown, and the now advancing dry/descending air associated with the region of NVA. The surface front, by and large, loses considerable activity. However, care should be taken in this case, as in spring/early summer in particular, as the drier air moves into what is effectively the warm sector, and over-runs humid low level air, destabilisation can lead to marked convective activity.

Singularities developed for the British Isles

Climatologists have always been alive to the fact that similar weather patterns/types occur at certain times of the year with varying degrees of regularity - an annual 'singularity'. For a while, before dynamical methods of long-range forecasting were used, singularities were very popular, though controversial.

1. CEP Brooks
2. HH Lamb
3. Alexander Buchan
4. Barry and Perry (after Lamb as above)
5. Warmest days/periods
6. Coldest days/periods


IMPORTANT: Remember that many of the below were developed mainly using data for the latter half of the 19th, and first half of the 20th centuries. The climate has undoubtedly changed, whatever the reason! Use these with much caution.

[ The term 'singularity' was apparently coined (according to HH Lamb) by A. Schmauss in Berlin in 1938. In an article entitled "Synoptische Singularitäten", Schmauss demonstrated that the curves (in graphical terms) of meteorological elements (such as temperature), show singular points in the mathematical sense (dy/dx=0), and apparently the name came quickly into use for this reason.]


1. C.E.P. Brooks 

(ref: 1946: "Annual recurrences of weather; 'singularities'."; Weather, London, 1. pp. 107-130)

Period studied: 1889 - 1940
Principal groupings:
(1) October to early February, stormy periods with minor anticyclonic interludes.
(2) February to May, cold waves associated with northeasterly winds.
(3) The summer period of alternating cool fresh northwesterly and warm, sultry southwesterly winds.
(4) September and early October, spells of anticyclonic conditions and late "summers".

 Guide  Type  Overall period  Notional 'peak'  Frequency in 52 yr.
 Early January  "Stormy"  Jan 5th - Jan 17th  Jan 8th  45/52=87%
 Mid January  "Anticyclonic"  Jan 18th - Jan 24th  Jan 20th/21st  45/52=87%
 Late January  "Stormy"  Jan 24th - Feb 1st  Jan 31st  44/52=85%
 Early February  "Anticyclonic"  Feb 8th - Feb 16th  Feb 13th  29/52=56%
 Late February  "Cold spell"  Feb 21st - Feb 25th  Feb 22nd  22/52=42%
 Late February & early March  "Stormy"  Feb 26th - Mar 9th  Mar 1st  46/52=88%
 Mid-March  "Anticyclonic"  Mar 12th - Mar 19th  Mar 13th/14th  27/52=52%
 Late March  "Stormy"  Mar 24th - Mar 31st  Mar 28th  35/52=67%
 Mid April  "Stormy"  Apr 10th - Apr 15th  Apr 14th  37/52=71%
 Late April  "Unsettled"  Apr 23rd - Apr 26th  Apr 25th  27/52=52%
 June  "Summer monsoon"  Jun 1st - Jun 21st  n/a  40/52=77%
 July  "Warm period"  Jul 10th - Jul 24th  n/a  n/a
 Late August  "Stormy"  Aug 20th - Aug 30th  Aug 28th  35/52=67%
 Early September  "Anticyclonic"  Sep 1st - Sep 17th  Sep 10th  43/52=83%
 Mid September  "Stormy"  Sep 17th - Sep 24th  Sep 20th  31/52=60%
 Early October  "Stormy"  Oct 5th - Oct 12th  Oct 8th/9th  35/52=67%
 Mid October  "Anticyclonic"  Oct 16th - Oct 20th  Oct 19th  35/52=67%
 Late October & early November  "Stormy"  Oct 24th - Nov 13th  Oct 29th, Nov 9th, Nov 12th  52/52=100%
 Mid November  "Anticyclonic"  Nov 15th - Nov 21st  Nov 18th, 20th  34/52=65%
 Late November & early December  "Stormy"  Nov 24th - Dec 14th  Nov 25th, Dec 9th  51/52=98%
 Pre-Christmas  "Anticyclonic"  Dec 18th - Dec 24th  Dec 19th - 21st  29/52=56%
 Post-Christmas  "Stormy"  Dec 25th - Jan 1st  Dec 28th  43/52=83%

2. Singularities affecting the British Isles (after HH Lamb)


Hubert Lamb identified five 'natural' seasons, which don't perfectly correspond with calendar months. They are defined thus:-

 High summer:  18th June to 9th September
 Autumn:  10th September to 19th November
 Early winter:  20th November to 19th January
 Late winter:  20th January to 29th March
 Spring:  30th March to 17th June

According to Lamb, these are the important characteristics of each of the five 'seasons' as defined above.
HIGH SUMMER [18th June - 9th September]:
High frequency (using lengthy datasets) of similar weather types ... i.e. dry/warm or wet/cool ... in the period analysed by Lamb (late 19th & first-half 20th centuries), years with cyclonic/wet sequences are twice as likely as persistently anticyclonic (A) types ... in other words, in 'high summer', you are more likely to experience an 'unsettled' summer than a quiet, fine one; a common experience! ... depressions tend to be shallow, moving less rapidly than in winter (weak driving polar front jetstream [PFJ]) ... can remain slow-moving for many days ... the fine (A type) summers tend to result from 'offshoots' of the Azores high moving east across southern Britain / northern France ... frontal systems being weak and only of passing concern ... mainly affecting (in their most active phase) the north-west of these islands ... there is a tendency for high cells to move east (progression) and to end up over Germany or Denmark, introducing a warm or very warm S (southerly, Tc) type across central and SE Britain ... such periods are ended by troughs approaching from the Atlantic ... perhaps with a warm-plume advection type ahead ... thunderstorms & heavy rain etc. ... however, occasionally the PFJ is displaced much further south & somewhat stronger than 'summer-time' average (e.g. 2007) ... frequent depressions of an active nature ... plenty of rain ... temperatures near or below average.

AUTUMN [10th September - 19th November]:
First week (10th September onwards) driest of year on average, especially central & eastern areas ... but some years show tendency to a mid-September cyclonic / wet spell ... localised heavy (thundery) rainfall resulting from slow-moving areas of low pressure ... early / mid October can be 'two-faced': often 'unsettled' with rain/showers, but occasionally can give rise to anticyclonic / warm conditions, with southerly [S] weather types lifting temperatures to 'near-record' levels ... between 23rd October & 11th November, strong signal for wet/stormy weather with a sharp reduction in A-type weather; sharp reduction this type last week of October c.f. first week.

EARLY WINTER [20th November - 19th January]:
Lengthy spells of any weather type less likely than 'high summer' or 'autumn' ... any extended types tend to be westerly / zonal /mild in nature ... unusual for type established early in this period to persist to end (or into 'Late winter') ... tendency to a post-Christmas 'stormy' period [recent notable examples 1997, 1998 & 1999] ... in former times, if these were to the south, perhaps associated with significant snow [ but not so much in recent years ] ... circulation type around 'New Year' some guide to type later in winter ... significant correlation [Lamb] between cold weather late December / early January & cold winters overall.

LATE WINTER [20th January - 29th March]:
In 50% cases, 'lengthy spells' evident, but no preference to one type or the other (i.e. as between zonal-mild-windy & blocked/cold) ... coldest winters when persistent blocking highs Scandinavia/Iceland regions (Pc/Am types) ... cold, northerly types tend not to last much more than 5-7 days (and in recent years, not particularly cold anyway) ... mildest winters from high zonality [high NAOI] (Tm or rPm types) ... strong correlation between wet winters & mild winters ... dry/anticyclonic mid-March conditions often extend (persistence of type) for 2 weeks or more ... with right conditions, droughts can be prevalent & potentially severe, depending upon precipitation totals earlier.

SPRING [30th March - 17th June]:
Least likely to have extended 'runs' of similar weather ... changeability from day-to-day marked ... N & E types fairly frequent (high pressure anywhere from NW to NE), especially second-half April ... 'late' snowfalls not unusual (even in these days) due northerly outbreaks / polar lows or troughs ... May often brings quiet/dry with increasing chance of extended periods A-type.
[ Based on "The Climate of the British Isles", Longman, 1976 & Lamb's original work.]

  5th - 11th January
 RENEWED STORMINESS OF EARLY JANUARY
 Westerly type very frequent, esp on 8th. Anticyclonic type very infrequent. The mild/oceanic air masses do not penetrate into central Europe as often here as with earlier cyclonic singularities of 26th Nov, 28th Dec, and (later) 1st Feb.

 

  20th - 23rd January
 ANTICYCLONIC IN EUROPE AND S. & E. BRITAIN
 Dry/frosty in Europe. Anticyclonic, Southerly & Easterly types frequent in Britain.

 

  27th January - 3rd February
  RENEWED STORMINESS - GALES AND RAIN OR SNOW
  Lows pass into North & Central Europe from Atlantic. The first lows of the series commonly approach England from the South-West (N.B. liability for freezing rain with advance of mild air after some days of frost). Anticyclonic type rare.

 

  8th - 13th February
 FEBRUARY ANTICYCLONES
 Year's highest frequency of winter sports conditions in Alps & Southern Germany (a second peak follows in central & Eastern Europe 19th to 24th Feb.). Record frosts in cold winters. Fogs common inland in Britain when high forms in maritime air: Anticyclonic, Southerly & Easterly types common in Britain.

 

  26th February - 9th March
  COLD, STORMY PERIOD
  Northerly outbreak from Norwegian Sea and cyclonic weather over N. Sea and surrounding lands. Cyclonic type maximum 26th February - 2nd March; Northerly type maximum 28th February - 3rd March.

 

  12th - 22nd March
  EARLY SPRING ANTICYCLONES OVER BRITAIN & EUROPE
 Very quiet weather - large diurnal range of temperatures. Anticyclonic, Northerly & Easterly types common.

 

  28th March - 1st April
  COLD, STORMY PERIOD
  First of a series of northerly outbreaks from Norwegian Sea, with Cyclonic conditions in West, Central Europe and the Mediterranean. These outbreaks commonly occur about 4 times in April & early May, punctuated by warm, quiet anticyclonic intervals.

 

  12th - 19th April
  COLD, STORMY PERIOD
 The most regular of the successive cold stormy spells in April, (see above). Northerly type very common about 17th - 19th April. Atlantic highs liable to affect West & South-West Britain.

 

  29th April - 16th May
  NORTHERLY WEATHER - SOME ANTICYCLONIC INTERVALS
  Northerly type very common, but often anticyclonic in west. Easterly type common; and Westerly type less common than at any time of the year.

 

  21st - 31st May
  FORE-MONSOON FINE WEATHER PERIODS
  Anticyclonic & Southerly types very common. Fine, dry period.

 

  1st - 4th June & 12th - 14th June
  FIRST WAVES OF EUROPEAN SUMMER MONSOON: COOL, STORMY EPISODES
  Atlantic lows move across UK with Cyclonic type very frequent 1st to 4th June. Succesive lows travel rather further north and Westerly types become more common. The 2nd monsoonal wave reaches Germany around 12th - 14th June and the 3rd wave 18th - 22nd June (see below) brings westerlies in across the British Isles as the commonest type once more. These events are punctuated by recoveries of the anticyclonic tendency associated with advances of the Azores High. Thunder is very common over continent 3rd - 5th June as the cool oceanic air moves in over the hot land.

 

  5th - 11th June
  JUNE ANTICYCLONES OVER UK & W. EUROPE
 Anticyclonic intervals to June monsoon (see above). Anticyclonic type very common around 7th.

 

  18th - 22nd June (and following fortnight)
  RETURN OF THE WESTERLIES
 The most regular monsoonal invasion of the continent by cool, oceanic air from the west and NW leading to thunder and cyclonic activity. The S & SW of Britain often remaining anticyclonic: But over most of Britain, Westerly type is very common, especially around 20th. This 3rd wave of the monsoon reaches Germany about 24th - 26th June. Later waves are usually less pronounced and occur with varying regularity in early, mid & late July and mid-August.

 

  23rd - 30th July & following week
  THUNDERY, CYCLONIC WEATHER OVER EUROPE & BRITISH ISLES
  Stagnant lows common. Cyclonic type very frequent, especially around 4th - 8th August. Westerly & North-Westerly types also commonly occur. The mean temperature curve reaches its seasonal peak in most of the British Isles in the week 30th July - 6th August. (See also below: re "when do the warmest days occur?") Following the last singularity, late-summer anticyclones give Anticyclonic as common type around 15th August before the first storms of Autumn.

 

  16th - 30th August
  FIRST STORMS OF AUTUMN
  Depressions passing in high latitudes frequently produce cold N'ly outbreaks in the Norwegian Sea. In the British Isles, the weather is commonly Westerly or Cyclonic types, though Anticyclonic type may persist in the south.

 

  5th - 30th September
  "OLD-WIVES SUMMER" ANTICYCLONES
  Peak dates for Anticyclonic type in Britain are 7th - 10th, 16th - 21st and 30th Sep. The highs pass across the British Isles into Europe and Siberia. More cyclonic and Southerly type weather affects Britain after each 'High' system has passed away east into the continent. Cyclonic type is quite common around 24th Sep, and in cyclonic autumns, when the anticyclones keep further south, the period around 24th Sep is particularly liable to gales with vigorous lows passing over or close to the country. There is an increasing cyclonic trend during October in most years, but the frequency of anticyclones remains high early in the month, 1st to 10th Oct.

 

  24th October - 13th November
  LATE AUTUMN RAINS
  Stormy Cyclonic type frequent, especially in 2 peak periods 26th - 29th October and 9th - 12th Nov. In the interval there is a tendency to fair, mild Southerly type weather with high pressure over the continent. Anticyclonic type is very uncommon in Britain 26th - 28th Oct. Westerly types become increasingly frequent througout the period, as successive lows pass further north, reaching a peak on 8th - 12th Nov. Other common types in this period are: - Easterly type around 1st/2nd Nov; Northerly type around 25th/26th Oct. This last marks the first and most prominent autumn N'ly outbreak - which is followed in some years by rising pressure in Scandinavia with continental anticyclones common around 30th Oct - 6th Nov.

 

  15th - 24th November
 QUIET, FOGGY ANTICYCLONIC INTERLUDE
  Brief period, especially about 17th to 19th Nov. when Anticyclonic type is common, the anticyclones forming over Britain & W. Europe in maritime air. Westerly & Cyclonic types are uncommon at this time.

 

  25th November - 10th December
  EARLY-WINTER STORMS & RAINS
  Cyclonic period associated with progressive intensification of Atlantic westerlies and mobile lows, rather than with stagnant cyclonic situations over Europe. Waves of mild air spread east across Britain until blocked and lifted by the monsoonal development of stagnant cold air in the heart of the Eurasian continent. Cyclonic type is very common 25th - 29th November & 6th - 12th Dec. Westerly & North-Westerly types common throughout the period.

 

  19th - 23rd December
  CONTINENTAL & N. EUROPEAN ANTICYCLONES OF THE WINTER SOLSTICE
 Quiet, frosty weather on the European lowlands. Southerly type very common in Britain with Anticyclonic and Easterly types also quite frequent. Gales still frequent in Scotland. A preliminary, well-marked anticyclonic spell about 12th Dec commonly affects only East & Northern Europe.

 

  25th - 31st December
  CHRISTMAS-TIDE, THAW & STORMS OF THE END OF THE YEAR
  Cyclonic & Westerly types common in Britain, carried by a second surge into central Europe.

 


3. BUCHAN'S SPELLS.

(NB: strictly based on records across south-east Scotland, and specifically Edinburgh.)

Nine periods where put forward by Alexander Buchan in 1867 on the basis of 50 years of observations (though some texts quote only 1857-1866), constituting fairly reliable periods of unseasonal cold (6 cases) or warmth (3 cases).
Note that Buchan himself did not claim these as 'singularities' and it is widely accepted that they have little real predictive merit: they are included here for the sake of historical completeness. (**=shows some correspondence with Lamb etc.)
7th - 14th February: COLD**
11th - 14 April: COLD
9th - 14th May: COLD**
29th June - 4th July: COLD/COOL
12th - 15th July: WARM
6th - 11th August: COLD/COOL
12th - 15th August: WARM**
6th - 13th November: COLD
3rd - 14th December: WARM/MILD**


4. BARRY & PERRY (based on / updates by Lamb, as above & later)

( extracted from "The Climate of the British Isles", ed: Chandler & Gregory, chapter author A.H. Perry )
Where the 'type frequency' is=> 60%, then the characteristic is in bold type.

 Period:  Circulation type (Lamb):  Characteristics:  Type frequency (%)
(& significance level):
 Period:
 20 - 23 January  A,S, & E together  Generally dry and sunny in central & southern England.  50  1890-1950 (about 10 years)
 Year's lowest frequency of C type (10-12%) 24-26 January. (5% level - probably significant)
 12 - 23 March  A,N & E together  Notable rainfall (precipitation?) minimum in central & southern England.  70  1890-1950 (about 10 years)
 12-14 March peak of AC.  35 (1% level - significant)
 12 - 18 May  N type  Annual maximum about these dates;
14-20 May is sunniest week of the year in Ireland.
 30  1873-1961
 21 May -
10 June
 A type  Annual maximum frequency, 40% or more on some days during most of this period; driest weeks of year in Scotland, Ireland: more year-to-year variations in southern half of England.  (5% level - probably significant)  1873-1961
 18 - 22 June  W, NW & A together  Generally dry and sunny in southern England: cloudy & wet in Scotland & Ireland.  70  1890-1950 (about 10 years)
W type frequency 52% on 20 June  (1% level - significant)
 31 July -
4 August
 C type  Sharp peak (replaced by twin maxima around 20 July & mid-August).  35%+ (5% level - probably significant)  1873-1961 var.
 17 August -
2 September
 W & NW together  Wet in most areas.  70  1890-1950 (about 10 years)
 C type  Peaks 19 and 28 August.  30 (5% level - probably significant)
 6 - 19 September  A,N & NW together  Dry, especially east and central England.  80  1873-1961
 C type frequency, >20% between 6-12 September.  (5% level - probably significant)
 5 - 7 October  A type  Slight check to seasonal cooling.  40 (5% level - probably significant)  1890-1950 (about 10 years)
 24 - 31 October  C, E & N types  Great decline to year's minimum frequency of A type (<10%) about 28th - 31st October.  (1% level - significant)
(5% level - probably significant)
 1873-1961
 17 - 20 November  A type  Dry, foggy period in central and southern England.  30 (1% level - significant)  1873-1961
 3 - 11 December  W & NW together  Wet and stormy in most areas with 3 - 9 December generally wettest week of year on average.  70  1873-1961
 17 - 21 December  A type  Generally dry, foggy weather.  25  1873-1961

 


4. WHEN DO THE WARMEST DAYS OCCUR?


Although the longest day (maximum theoretical incoming solar radiation) occurs around 21st / 22nd June (the Summer Solstice), on average, the warmest days turn up about 4 or 5 weeks after this time, often occurring in early August.

> Using the mean 5-day temperature series for 1961-90 for three inland stations, the highest mean day maxima for Glasgow, Manchester and Heathrow fall in the period 23rd to 27th July.
In fact, with only 0.1degC or so difference, the period 13th July to 7th August is the 'high risk' period for the higher mean maximum temperatures.

This period also covers the warmest nights, which on average occur from 23rd July to 7th August.

The 'lag' beyond the date of the summer solstice is due to a combination of three effects:

1. The thermal inertia of the surfaces over which the air passes - particularly adjacent sea areas: the effect is not unlike the action of a 'night-storage' heater, offsetting the slightly weakening incoming solar radiation (slightly lower altitude of the sun / greater path-length through the atmosphere) and shorter day-lengths.
2. Warmer air can 'hold' a greater amount of water vapour (higher dew-points). In response to the lag (above), the warmest days will tend 'normally' to come after mid-June, and therefore the dew-points tend to be higher late June through to early September. The main effect of this is that night-time minima tend to be higher (water vapour acts as a 'blanket' absorbing / re-radiating outgoing earth-based radiation), so the 'day' starts off with higher temperatures, and given sufficient insolation, the day maxima will respond accordingly.
3. As 'High Summer' (July/August) is often associated with a weakened Polar Jet, cyclonic developments will be weaker, will find more difficulty in penetrating from the Atlantic, and the incidence of blocking is higher. Given the right circumstances, and the fact (from effects above), the land (and seas) will be warmer, this means that advection of warmed air from elsewhere, plus local stagnation of air-mass is much more likely.


>Using the series after JC Webb ('Weather' & 'Journal of Meteorology' - as updated) which lists the highest temperatures known by day (across the UK as a whole), the absolute high temperature of 38.1degC occurred on 10th August (2003) at Kew/Royal Botanic Gardens. [ Many stations across the southeast of England broke the old record (37.1, Cheltenham, 3rd August [1990]) on this day. In particular, 38.5degC was recorded at Faversham, Kent, but there are some doubts surrounding this figure.]
For Scotland only, the highest known/accepted temperature (32.9degC at Greycrook in the Scottish Borders) occurred on the 9th August (2003).

> Using the CET daily series, the period with the highest mean temperature runs from the 2nd week of July to the 2nd week of August. (roughly 10th July to 15th August). Inspecting the graph showing the progress of the mean daily CET record (1961-90), the peak occurs in late July, though with secondary, but slightly lower peaks, 2nd week of July & 2nd week of August.

> For several 'primary' synoptic-reporting stations around the UK, the values and dates of highest maxima are as below (but not updated due to station closure etc.):
GLASGOW: 4th August, 1975 31.2degC (Abbotsinch .. now closed)
MANCHESTER: 2nd August, 1990 33.7degC (Ringway .. now closed)
BIRMINGHAM: 3rd August, 1990 34.9degC (Elmdon .. now closed)
LONDON: 10th August, 2003 37.9degC (Heathrow)
CARDIFF: 3rd August, 1990 33.5degC (Rhoose .. now closed)

> The average maxima around the 'peak' period as noted above are circa 1.0degC higher than those around the 'longest' day.


5. WHEN DO THE COLDEST DAYS/PERIODS OCCUR?


> For the whole of the UK, using the series after JC Webb (JMet), the lowest absolute values in that series are:
(minus) 27.2degC, which occurred on the 30th December, 1995 at Altnaharra (Highland) and on 10th January, 1982 at Braemar (Grampian).

> > Using the mean daily Central England Temperature (CET) series [ 1961-1990 ], the curve dips to a first winter 'minimum' around the end of December or within the first few days of January, i.e. the 'turn' of the year. There is then a 'false' recovery in mean temperature, before the series minimum is reached in mid-February. In both cases of course, these minima occur after the point of shortest daylength - in the case of the lowest (mean) temperature, a good 7 weeks after this date. The main reason for this marked 'lag' is again a function of the fact that sea temperatures in particular do not reach their lowest values around our islands until well into February or even early March. There are other factors though, such as a greater tendency to northerly or easterly types as the winter progresses and the increasing likelihood of anticyclonic 'blocks' allowing night radiation to become fully effective.

Thickness and its uses

Thickness -- a short explanation!

'Thickness' is a measure of how warm or cold a layer of the atmosphere is, usually a layer in the lowest 5 km of the troposphere; high values mean warm air, and low values mean cold air.

It would be perfectly feasible to define the average temperature of a layer in the atmosphere by calculating its mean value in degrees C (or Kelvin) between two vertical points, but an easier, practical way to measure this same mean temperature between two levels can be gained by subtracting the lower height value of the appropriate isobaric surface from the upper.
Thus one measure of thickness commonly quoted is
= height (500 hPa surface) - height (1000 hPa surface) [ for those of you, like me, too old to catch up with all the changes the world brings, millibars = hPa!, so 500 hPa is exactly the same as 500 mb. ]

In practical meteorology, the most common layers wherein thickness values are analysed and forecast are: 500-1000 hPa [ abbreviated to TT or TTHK] ; 850-1000 hPa; 700-1000 hPa; 700-850 hPa and 500-700 hPa. By subtracting the lower (height) value from the upper value, a positive number is always gained. The 500-1000 hPa value is used to define 'bulk' airmass mean temperature, and can be seen on several products available on the Web. The other 'partial' thicknesses are used for special purposes, for example, the 850-1000 hPa thickness is used for snow probability and maximum day temperature forecasting, as it more accurately defines the mean temperature of the lowest 1500 m (5000 ft) or so of the atmosphere.

Advection is simply the meteorologists word for movement of air in bulk. When we talk about warm advection, we mean that warm air replaces colder air, and vice-versa. These 'bulk' movements of air of differing temperatures can be seen very well on thickness charts, and differential advection, important in studies of stabilisation / de-stabilisation, can also be inferred by considering advection of partial thicknesses.

If you pull up thickness charts from the web, it it useful to highlight the isopleths of thickness, and work out, from either the mslp pattern, or the 500 hPa pattern, whether cold or warm advection is taking place. It should be possible with practice to find warm and cold fronts (tight thickness pattern), and areas where the thermal gradient (spacing of thickness lines) is changing - note particularly areas where developments tend to decrease spacing of thickness lines -->> increased potential for atmospheric development.

Total Thickness (500-1000 hPa) isopleths (when shown in combination with other fields) are conventionally drawn as long-dash lines, with the values either thus [540] or white numerals on a black/solid rectangle. (Where there is no conflict, i.e. the thickness isopleths are the only ones shown, then usually sold/continuous lines are used.) Certain isopleths are considered 'standard', mainly for historical reasons: They are listed hereunder, with the colour code convention used by the UK Met.Office on internal charts.
474 - red 492 - purple 510 - brown 528 - blue 546 - green 564 - red 582 - purple

Operational charts usually show isopleths at 6 dam intervals but some international forecast output will only have the standard isopleths as above: for example the UKMO 2-5 day charts. (If you want to compare forecast values over and near the British Isles with extremes, see here). 

Can the 500-1000 hPa patterns be used to infer the snow risk? Well, yes they can, but because the layer is so deep -- some 5 km, or 18000 ft of the lower atmosphere, its not a good indicator. As a VERY rough guide the following may be used:

Rain and snow are equally likely when the 500-1000 hPa thickness is about 5225 gpm (or 522 dam). Rain is rare when the 500-1000 hPa thickness is less than 5190 gpm. Snow is extremely rare when the 500-1000 hPa thickness is greater than 5395 gpm.

Thickness --- a much longer explanation!


[ Q ] Thickness? ....thickness of what?

[ A ] From time to time in meteorology newsgroups, the word 'thickness' is used, particularly when talking about snow probability, or the prospect of warmer, or colder weather generally. If you simply want to remember that 'Thickness' is a measure of how warm or cold a layer of the atmosphere is, usually a layer in the lowest 5 km of the troposphere, and that high values mean warm air, and low values mean cold air, then you can ignore the rest of this FAQ. If you wish to know a little more, read on!


[ Q ] What is 'Thickness' and how is it measured?

[ A ] Although it would be perfectly feasible to define the average temperature of a layer in the atmosphere by quoting/calculating its mean value in degrees C (or Kelvin) between two vertical points, from the early days of upper air meteorology, it was realised that from the hydrostatic equation, an easy to calculate measure of this same mean temperature between two levels could be gained by subtracting the lower height value of the appropriate isobaric surface from the upper. (** see alternative name below)

The hydrostatic equation, in its simplified form, is -dp/dz = pg/RT ...... Eq(a)

here:
dp being the pressure difference across two defined levels
dz the height difference between those two levels
g the gravitational constant
R specific gas constant for dry air
T average temperature in layer (strictly average virtual temp.)

Eq (a) states that the change of pressure with height (above a point where the total pressure of the column is p), is governed by the mean temperature over the vertical distance involved.
Thus in cold air/low T values, -dp/dz is larger than in warm air/high T. ... or putting it another way, lets start out at 1000 hPa and ascend vertically; in cold air, you would reach the level of 500 hPa sooner (greater rate of change of p) than in warm air. Thus the vertical distance from the level of 1000 hPa to the level of 500 hPa is less in cold air than in warm air
....cold air => low thickness values;
....warm air => high thickness values.
(for a more rigorous treatment/discussion of the hydrostatic equation, see any good textbook on meteorology - there is a list below).

Thickness can be calculated from the heights reported on a radio-sonde ascent, or a thermodynamic diagram can be used to add up the partial thicknesses over successive layers to achieve the net (total) thickness.

An example of the former would be
500 hPa height = 5407 m
1000 hPa height = 23 m
Thickness = 5407-23 = 5384 m (or 538 dam)

Careful note must be made when the height of the 1000 hPa surface is below msl thus: 500 hPa height = 5524 m
1000 hPa height = - 13 m
Thickness = 5524 -(-13) = 5537 m (or 554 dam)

[** NOTE: you will also see 'thickness' charts referred to as 'Relative Topography' charts for this reason - this is especially so on web output from centres in mainland Europe.]


[ Q ] What are the most common layers through which the thickness is analysed / forecast and what are they used for?

[ A ] In practical meteorology, the most common layers wherein thickness values are analysed and forecast are: 500-1000 hPa [ abbreviated to TT or TTHK] ; 850-1000 hPa; 700-1000 hPa; 700-850 hPa and 500-700 hPa.
By subtracting the lower (height) value from the upper value, a positive number is always gained. Some values are quoted in metres, and others dekametres (tens of metres), dependent upon the use to which the value is put. In general, when dealing with the lower atmosphere, metres are used to better refine the output to the inferred surface parameter (e.g. maximum temperature), whilst for the deeper layers, dekametres (10's of metres) are sufficiently accurate.)

500-1000 hPa: (also known by some meteorologists as the 'total' thickness, for historical reasons). This is used to define the broad average temperature for the lower half of the troposphere. From the hydrostatic equation (see Eq(a) and reference (1) below),

Z2 - Z1 = RT/g * ln(p1/p2) ..... Eq(b)

replace p1 and p2 by 1000 hPa and 500 hPa respectively
therefore ln(2) = 0.69
R = universal gas constant for dry air = 2.87 * 10^2 kg^1 K^1
g = 9.81 ms^2
T = mean temperature through layer (K)
Z1, Z2 = heights of isobaric surfaces p1,p2 (in metres)
by substitution, and allowing for the fact that T in the original equation is Kelvin, we have, for the 500/1000 hPa layer....
mean temperature (degC) = (Thickness/20.3) - 273 ..... Eq(c)

thus for 5640 m thickness --- this represents a mean T = +5 degC
5460 m -4
5280 m -13 and so on.
and further, it can be seen that for 10 m (1 dam) change of thickness in this layer, this represents a change in mean temperature through the layer of 0.5 degC. It is useful to remember this when, for example, looking at the Royal Met.Soc 'Weather Log' which shows the deviation from normal of thickness values over a large part of the northern hemisphere ... an anomaly of + 4 dam doesn't mean quite as much as one of + 2 degC!

[ Using this relationship, it is also possible to come up with a crude approximation to the expected surface maximum temperature - for more on this, see here.]

850-1000 hPa: This is useful for defining the temperature structure in the lowest 1500 m or so of the atmosphere, and can therefore be used in such things as rain/snow prediction, maximum temperature forecasting etc.

700-1000 hPa: Similar to 500-1000 hPa but focussed rather more on the lowest 3 km of the atmosphere and therefore an attempt to combine the broader measure of the 500-1000 hPa and the finer details obtained by layers nearer the earth's surface.

500-700 hPa/700-850 hPa: Used in studies of differential thermal advection, particularly when considering possible convection, degrees of instability etc.


[ Q ] What is the historical relevance of 'gridding'?

[ A ] Before the advent of super-fast main-frame computers, and the better understanding of the character and physics of the upper air, upper wind forecasts up to 24 hours ahead depended upon a process known as 'gridding' - the arithmetical manipulation of layer thicknesses. A surface/msl pressure chart would be analysed, then the isobaric pattern would be converted to equivalent 1000 hPa height contours, taking into account the temperature if this deviated significantly from the 'standard'; the thickness pattern would be drawn, using thermal wind relationships and known patterns associated with frontal systems, then the 1000 hPa and thickness patterns overlaid, and at the intersection of the 'grid' of such contours, the resultant 500 hPa (or other height, depending upon the thickness used) could be achieved, using the relationship that h(500) = h(1000)+h'(thickness).
This gave better results than just using the poor network of actual 500 hPa heights/winds, and by using thicknesses (i.e. differences), the systematic errors of differing radio-sondes could be effectively ignored .

More importantly, to produce a forecast upper air chart, first the forecast surface pattern was produced using known empirical relationships/rules of thumb etc., then the thickness pattern adjusted to fit around this forecast pattern, keeping the correct relationship found both in analysis and conceptual models in mind, then again by gridding, or graphical addition of the 1000 hPa contours and the thickness pattern, the upper contour pattern could be produced. It was essentially this process that was used to forecast all upper winds until the work could be put on a more rigorous/mathematical basis by the solving of complex equations using the big number- crunching machines from the late 1960's. (Incidentally, you can tell the vintage of someone working in meteorology by whether they refer to the parameter 500-1000 hPa thickness as the Total thickness ... a hangover from these days of gridding charts.)

Further reading: For a useful summary of this method of upper wind forecasting, see Ref:(2) and for a historical perspective, see Ref:(4) "Bomber Command upper air unit" - RAS Ratcliffe.


[ Q ] Does gridding have any relevance today?

[ A ] The reverse of gridding (graphical addition of thickness values to a lower surface height), is de-gridding, and can be a useful technique to master, both to achieve a surface pattern from a 500 hPa/Total Thickness chart, and to attain a simplified conceptual idea of development due to upper air processes.

From the definition of Total Thickness (h') where: h' = h(500) - h(1000) ..... Eq(d)

where h' = total thickness
h(500) = height of the 500 hPa barometric surface
h(1000) = height of the 1000 hPa barometric surface

re-arranging Eq(d), we have h(1000) = h(500) - h' ... Eq(e)

At levels near msl, the approximate relationship [ see cautionary note below # ] holds that a difference of 6 dam = a difference of 8 hPa. ... Eq(f)

Thus, from Eq(e), if at a certain point on a chart, a 500 hPa contour of 540 dam, is crossed by a thickness isopleth of 528 dam, the value of h(1000) = 540-528 = +12 dam. From Eq(f), this relates to an isobaric value of (12/6)*8 = +16 hPa (or 1000+16 = 1016 hPa) All other intersections of the 540 hPa/500 contour, and the 528 thickness isopleth yield the same value. Consideration of other intersections will build up a pattern of 1000 mb (and hence msl) values, and the surface pattern can then be inferred from these two fields.
We can go further, and see that it is clear from Eq(e) that by either decreasing the 500 hPa contour value (trough approaching), or increasing the thickness values (warm advection), the height of the 1000 hPa surface will lower, and because 1000 hPa and mslp are linked, mslp will lower===> development. Where both terms are strong (omega development, whereby an upper short- wave trough engages an area of strong warm advection), then explosive cyclogenesis is possible, all other factors being suitable. This is of course a very simplistic explanation of development, but it is a useful conceptual idea to keep in mind when trying to interpret upper air charts.

[ # The relationship 6 dam=8 mbar is a very approximate one, and was accepted at the time because upper air charts were drawn to 6 dam intervals, surface charts to 8 mbar intervals (or multiples thereof), and accepting the inferred 0.75 dam per millibar (i.e. 6/8) relationship kept things simple. In reality, using the hydrostatic equation (Eq(b) above), for average mean sea level pressure of 1013mbar, average surface temperature of 15degC and taking a narrow 'slice' of the air around mslp, then the true value is 0.83 dam/mbar: other values are 0.80 dam/mbar for mean temperature 0degC and 1000mbar, and at 20degC and 1000mbar, it would be 0.86]


[ Q ] How are thickness isopleths shown on synoptic charts?

[ A ] Total Thickness (500-1000 hPa) isopleths are conventionally drawn as long-dash lines, with the values either thus [540] or white numerals on a black/solid rectangle. Certain isopleths are considered 'standard', mainly for historical reasons, and are coloured (in UK Met.O use) according to the following convention):
474 - red
492 - purple
510 - brown
528 - blue
546 - green
564 - red
582 - purple
Operational charts usually show isopleths at 6 dam intervals but some international forecast output will only have the standard isopleths as above: for example the Bracknell 2-5 day charts.


[ Q ] What about advection? How can I use it?

[ A ] Advection is simply the meteorologists word for movement of air in bulk. When we talk about warm advection, we mean that warm air replaces colder air, and vice-versa. These bulk movements of air of differing temperatures can be seen very well on thickness charts, and differential advection, important in studies of stabilisation/de-stabilisation, can also be inferred by considering advection of partial thicknesses.

Example:


If you pull up thickness charts from the web, it it useful to highlight the isopleths of thickness, and work out, from either the mslp pattern, or the 500 hPa pattern, (or indeed any wind in the layer) whether cold or warm advection is taking place. It should be possible with practice to find warm and cold fronts (tight thickness pattern), and areas where the thermal gradient (spacing of thickness lines) is changing - note particularly areas where developments tend to decrease spacing of thickness lines -->> increased potential for atmospheric development. See also the section relating to thermal winds

Below, is an example of a mslp and thickness chart drawn from the NWS site. Red (warm) and blue (cold) arrows show some areas of significant advection.


{a copy of an NWS thickness and MSLP map showing warm & cold advection}


[ Q ] What about the various patterns of thickness isopleths?

[ A ] The most obvious patterns that the thickness isopleths can take up look very like those you would see on a surface/isobaric chart: highs, lows, troughs and ridges. A closed high-value contour, usually labelled 'W' is referred to as a warm dome; a closed low-value contour (or series of same), labelled 'K' is a cold pool. The 'W' and 'K' denote WARM and KALT respectively - possibly from the Norwegian/German roots of much of the research into upper air patterns. Cold pools are especially important, being often the first indications that the potential for small/mesoscale deep and vigorous convective activity exists, given a suitable trigger action and sufficient moisture. ( They are sometimes not resolved by NWP suites adequately either, particular those with long grid lengths. ) The horizontal spacing of thickness isopleths is also a useful indicator of the potential for development. Close spacing shows that cold air lies adjacent to warm air -- no doubt with an attendant frontal surface. More importantly, the fact that such a baroclinic zone (surfaces of temperature and pressure intersecting at an angle) exists, means that the potential for development is strong -- a slight displacement, i.e. forcing by a high level jet streak, will lead to substantial falls of pressure and consequent 'weather'.
(Examples of all these features, with further notes, can be found by clicking here.)

Meteorologists will always play close attention to zones of 'tight' thickness contouring for this reason. Troughs and ridges denote tongues of cold and warm air respectively, and, from work by RC Sutcliffe (& others) during and after the Second World War, they can also be used to infer the magnitude and sign of development on the surface. The details of the mathematics are beyond this FAQ, but it is only necessary to recognize the patterns as below:


[ Q ] Can the 500-1000 hPa patterns be used to infer the snow risk?

[ A ] Until the advent of NWP suites capable of using model variables to routinely predict the 850-1000 hPa partial thickness (the one now commonly used in the UK to assess likelihood of snow - see below), the 500-1000 hPa total thickness was used much more extensively than now. In the 1950's and 1960's, some studies were published which attempted to refine the TTHK association with snow/rain prediction ... the results are presented below BUT REMEMBER, better predictors are available and should be used where possible.

(1) Lamb: Q Jnl R.Met.Soc. 1955 Critical value for equal probability of rain and snow(NW Europe)

Average: 527 dam (extremes: 521 to 546)
Established snowfields: 536 dam
Windward edge of snowfields: 528 dam
Seas with SST around 10degC and over windward coasts: 523 dam

Lamb's analysis was undertaken for the whole of NW Europe, including places like Riga and Stockholm, which may explain the difference in the average figure from that in the following section. However, Lamb did make a more detailed analysis for inland stations in the British Isles, where he found that when there was snow lying, the critical values were 5305-5335 gpm, and with no snow lying 5225 gpm, which accords more with Murray below)

(2) Murray: 1959 Using an analysis for the UK only, found the following:

Rain and snow are equally likely when the 500-1000 hPa thickness is about 5225 gpm (or 522 dam)

Rain is rare when the 500-1000 hPa thickness is less than 5190 gpm

Snow is extremely rare when the 500-1000 hPa thickness is greater than 5395 gpm; it is rather uncommon when the value is greater than 5305 gpm.


A personal view here, but I have always regarded the 522 dam isopleth as a better 'first guess' at snow vs rain than the 528 value for frontal precipitation. Indeed, I joined the Met.Office at a time when TTHK were still being extensively used for this purpose, and 522 was regarded as the 'snow-line'


(3) Murray: 1959 Also carried out a more detailed investigation using a combination of predictors, the 500-1000 hPa TTHK, the surface (screen) temperature and the height of the freezing level. (I have only shown the TTHK/screen temp relationship as this is the most useful one for those with access to WWW met.products.) He presented the results in graphical form, but for ease of use, I have converted them to tabular format, using the 'standard' TTHK values. The use of a graph implies greater precision than is usual anyway.

(a) percentage probability (P) of type of precipitation in relation to surface temperature and 500-1000 hPa thickness.

SCREEN TEMPERATURE (degC) >> -1 0 1 2 3 4
TTHK (dam)            
 516 A A A A A A
 522 A A B C C C
 528 A B C C D D
 534 A B C D D D

A is P >90%
B is 90% >P> 50%
C is 50% >P> 10%
D is P< 10%

In critical rain/snow situations, one of the variables that is difficult to forecast is the screen temperature. Therefore this method is not the answer it may at first sight seem.

(4) Boyden .

In the course of work to compare various predictors, Boyden gave the following figures. 500-1000 hPa TTHK

5180 gpm: 90%
5238 gpm: 70%
5258 gpm: 50%
5292 gpm: 30%
5334 gpm: 10%

In the course of this work, Boyden confirmed what others have already pointed out, that the 500-1000 hPa thickness parameter is the poorest discriminator for rain/snow. The best was height of freezing level and surface temperature, both difficult to forecast in 'critical' situations, and the next best/least worst was the 850-1000 hPa value, though Boyden devised the correction 'factor' that we all now use to take account of mean sea level pressure and local height.


[ Q ] So, how are the 850-1000 hPa values used.

[ A ] This parameter can be used in several ways, the two of most use to the 'bench' forecaster are: (1) calculation of the risk of rain vs. snow, and (2) forecasting the daytime maximum temperature.

(1). Because the layer from 1000 hPa to 850 hPa covers the lowest 1500 metres or so of the atmosphere, it is better suited to deciding on which phase precipitation will reach the ground ( i.e. whether snow will melt to sleet or rain), than the 'total' thickness layer 500-1000 hPa. Statistical relationships have been produced:

The following are un-adjusted critical values and adjusted values for the 850-1000 hPa partial thickness found by statistical analysis: snow probability:
Probability:............................90%.....70%.....50%.....30%.....10%
850-1000 hPa(gpm)..............1279.....1287....1293....1297....1302 (un-adjusted)
850-1000 hPa(gpm)..............1281.....1290....1293....1298....1303 (adjusted-see below)

It is important to remember that when mslp values differ markedly from 1000 hPa, or the height of a station/area differs greatly from msl, a correction has to be made to the partial thickness before assessing the snow risk. Also, the partial thickness only takes into account the mean temperature of the lowest 250 hPa or so of the atmosphere and not the humidity, which is of vital importance for accurate snow prediction. Downward penetration of snow is greatly aided when precipitation falls into dry air.

Boyden found that the following correction should be made to the partial thickness (850-1000 hPa): (Z - h)/30

where Z: height in metres of the 1000 hPa surface above msl
h: height in metres of the ground asl.
... and then the second line of critical values used in the table above.

(2). The 850-1000 hPa is a very good layer within which to determine the air mass likely to affect your station and therefore its use to work out the potential daytime maximum temperature has long been recognised. One such, after Callen and Prescott, relates the partial thickness values.... to the cloud classification for the day ahead.

The relationship between 850-1000 hPa thickness (h*) and the unadjusted maximum temperature (Tu) is given by: Tu = -192.65 + 0.156h* ... Eq(g)

An adjustment is then added to this figure, depending upon forecast 'cloud class' and the time of the year.

The four cloud classes are:(simplified)
Class 0: Low and medium cloud generally less than half cover. High cloud not overcast. Fog only around dawn, if at all.
Class 1: Roughly 50% cloudiness. If fog occurs, it clears slowly during the morning.
Class 2: Mainly cloudy. If fog occurs, clears by midday, but slowly.
Class 3: Overcast with rain/snow etc. Persistent Fog.

Then the following matrix can be used to find the adjustment to be added to Tu. (Whole values degrees C only)

 MONTH  CLASS
  0 1 2 3
 JAN -4 -4 -5 -5
 FEB -3 -3 -4 -5
 MAR -1 -2 -3 -4
 APR +1 0 -1 -2
 MAY +2 +1 0 -1
 JUN +4 +3 +1 0
 JUL +4 +3 +1 0
 AUG +3 +2 +1 0
 SEP +1 0 -1 -1
 OCT -1 -1 -2 -3
 NOV -2 -3 -4 -4
 DEC -4 -4 -5 -5


The original work was based on maxima recorded at Gatwick airport, using upper air ascents (12Z) from nearby Crawley radio-sonde .. both sites now no longer providing the appropriate data. They should NOT be used for latitudes well away from the south of England, particularly in the 'winter half-year', when insolation values (due to differing sun angle and daylength parameters) will differ markedly with latitude.
Also, where marine influence is strong then the values will be highly modulated by local sea surface temperatures.
There are a series of graphs, from which it is technically possible to read off the correction to decimals of a degree, and to refine the correction dependent upon the position in the month, but for practical meteorology, these will do!


Some reading/references:
(1): Essentials of Meteorology auth: D.H. McIntosh and A.S. Thom Taylor and Francis Ltd
(2): The practice of weather forecasting auth: P.G. Wickham HMSO
(3): Introduction to Meteorology auth: S. Petterssen McGraw-Hill Book Company, Inc.
(4): Meteorology and World War II ed: B.D. Giles Royal Meteorological Society
(5): Handbook of Aviation Meteorology: The Met.Office/HMSO
(6): Source book to the Forecasters' Reference Book: The Met.Office/College

Sutcliffe Development Theory

Notes relating to atmospheric development diagnosed using total thickness (TTHK) charts.

[ 1. This note was put together to give a 'flavour' of the ideas surrounding diagnosis of development from thickness charts. If you have come to this page as a first-year student of meteorology, then this is NOT for you! ]
[ 2. Inevitably, given the history of the times (1940's), the efforts of meteorologists on the 'winning' side has assumed dominance. However, it is clear that much work was done in Germany both before and during the Second World War, and this contribution should be remembered. If I find more in this, I will add it.]

The science of forecasting has come a long way since the days of ancient weather lore and a belief in the whims of the gods! During the mid-19th century, the birth of a scientific basis to weather forecasting was witnessed, with the use of the developing electric telegraph networks to exchange data, the discovery of 'laws' relating the wind field to the pressure distribution (Buys Ballot, 1857), and later in the century, the analysis of weather types associated with depressions and anticyclones (Abercromby, 1883).

During the 'Great War' of 1914-1918, as is well-known, the 'Norwegian' frontal & air-mass theories were thoroughly researched and once hostilities ceased, they were enthusiastically adopted & developed by the leading meteorological services around the world - indeed, perhaps a bit too enthusiastically, as they didn't necessarily apply to sub-tropical / tropical regions or indeed to all occasions in the mid-latitudes. Not only that, work on the dynamic basis for atmospheric development tended to be somewhat overshadowed.

Nevertheless, the work of Bjerknes (father & son), Bergeron & Solberg formed the basis of 'front-line' forecasting work up to and including the Second World War. To produce a 'PROG' of the weather 18 to 24 hours ahead involved use of empirical techniques which moved the fronts based on gradients across them, moved the lows (or highs) following continuity and rules based on flow patterns around these features, and in large part, experience of situations past: the upper air (even if it was available), didn't get much of a look-in in the process.

The foregoing might imply that somehow the 'upper air' was ignored! Not a bit; much work was undertaken using primitive kite and balloon ascents during the early part of the 20th century, and increasing air-flights produced more information. Meteorologists realised that to understand & predict the 'weather', they would need more information on & understanding of, air flow well above the surface. The problem was - lack of data!

The Second World War provided the impetus (and the data) for research into upper air patterns and their influence on the surface weather. As part of the procedure for analysis and forecasting of upper air patterns, thickness charts (partial and total) became the stock-in-trade of weather services (particularly for RAF Bomber Command, the USAAF & the Luftwaffe), and out of these charts came the ideas of development theory tied to forcing aloft - R.C. Sutcliffe in the United Kingdom had already researched this immediately prior to the outbreak of war (alongside the work of others, particularly Brunt & Petterssen), and by the latter half of the 1940's he had enough data to publish his seminal work in 1947 (see references below).

Sutcliffe showed that development (expressed in terms of relative divergence between 1000 and 500 hPa levels) can be diagnosed from total thickness charts: in crude terms, the equation can be expressed as . . . .

[div(500-1000)] = [LAT] + [STEER] + [DEV]

where:-
div(500-1000) represents the relative divergence through that column.
[LAT] .. the 'latitude' term
[STEER] .. the 'steering' term
[DEV] .. the 'developmental' term

the thermal wind (500-1000hPa) appears in each term, so the stronger the thermal wind (i.e. the tighter the thickness gradient), then the more effective is the vorticity-driven development that takes place.

The [LAT] term, which diagnoses the variation of the Coriolis parameter with latitude in the direction of the thermal wind - is generally small, but is important for example in the creation of the notorious 'Scandinavian High' in winter, and the significant areas of low pressure following cold outbreaks from the north over the west & central Mediterranean in late autumn / early winter.

The [STEER] term is proportional to the strength of the thermal wind and to the variation of surface vorticity in the direction of the thermal wind. This term is dominant when the pattern of surface vorticity is well marked & the thermal wind almost 'zonal' (or running west-to-east), i.e. immediately before distortion of the Polar Front undergoing wave-development. This confirms the subjective assessment that small-scale mid-latitude lows are 'driven' along coupled to the thickness gradient aloft - tighter gradient, swifter movement.

The remaining term [DEV] is proportional to the strength of the thermal wind and to its variation of vorticity along the flow. Curvature and shear of the thickness pattern contribute to the latter, and give rise to the 'standard' developmental patterns indicated below.

With developing low pressure at the surface, horizontal convergence at low-levels implies upward motion through the troposphere, and divergence aloft (in the region of the tropopause). The converse applies for developing high cells. The circulations are indicated on this classic diagram:-


Developmental diagram


The work by Sutcliffe (and others) can be summed up neatly in a graphical format; there are four basic patterns of thickness isopleths (indicated below). With each pattern, there is associated a major and minor (vorticity-driven) surface development area.

Chart showing developmental regions associated with thickness (500-1000 hPa) patterns.

Diagram of development regions

C: cyclonic development (circled = major / most effective forcing)
A: anticyclonic development (circled = major / most effective forcing)
Green-dashed lines: approximate thermal ridge / trough axes
Magenta arrows: direction / proportionate strength of thermal wind

For more on all these matters, see the references below.


References:
'A Contribution to the Problem of Development', R C Sutcliffe; QJRMetS/RMetS, 73, 1947
'The Theory & Use of Upper Air Thickness Patterns in Forecasting', R C Sutcliffe & D Forsdyke; QJRMetS/RMetS, 76, 1950
'Weather Map', Meteorological Office/HMSO, 1956
'The Meteorological Glossary', Meteorological Office/HMSO, 1972
'Dynamical meteorology: some milestones', B W Atkinson; RMetS (in "Dynamical Meteorology, An introductory selection"), 1981


Selected glossary of terms:
Buys Ballot's Law: as originally formulated: " if you stand with your back to the wind (in the northern hemisphere), then low pressure lies on your left-hand side ". This gives rise to the standard patterns (in the Northern Hemisphere) of anticlockwise winds circulating around a low pressure area, and clockwise motion around an area of high pressure (reverse for the Southern Hemisphere).
Convergence: When air flows in such a way that the area occupied by a particular 'group' of air particles lessens ('drawing together'), the pattern is said to be convergent. Convergence in the atmosphere is associated with vertical motion, and hence development (or weakening) of weather systems. For example, convergent flow near the surface is coupled to, and may be the primary cause of, upward motion, leading to cloud formation/shower initiation etc.
Coriolis parameter: as a consequence of Earth's rotation, air moving across its surface appears to be deflected relative to an observer standing on the surface. The 'deflection' is to the right of movement in the northern hemisphere, to the left in the southern hemisphere. (also known as the Coriolis acceleration, or deflection)
Divergence: When air flows in such a way that the area occupied by a particular 'group' of air particles grows ('spreads apart'), the pattern is said to be divergent. Divergence in the atmosphere is also (along with convergence/q.v.) associated with vertical motion, and hence development (or weakening) of weather systems, depending upon the level where the divergence is dominant in a particular atmospheric column. For example, divergent flow aloft is coupled to, and may be the primary cause of, upward motion, leading to widespread cloud formation/cyclogenesis etc.
Norwegian model: The classical idea of a travelling wave depression on the polar front running forward (usually west-to-east) and deepening, with the cold front moving faster than the warm front, thus 'occluding' the warm sector, with the parent low slowing / turning to the left (in northern hemisphere), and filling up.
Thermal wind: a theoretical (vector difference) wind that relates the magnitude of the horizontal temperature gradient in a defined layer to the real winds that blow at the top and base of that layer. The speed of the thermal wind is proportional to the temperature gradient.
Thickness (or Relative Topography): The difference in height between two layers in the upper air. The most commonly used being the thickness between 500 mbar (or hPa) and 1000 mbar (or hPa), and normally expressed in dekametres. The larger the value of thickness, the warmer the column of air.
Tropopause: the (usually) abrupt change from falling temperatures with height in the tropopause, to near-uniform, or rising temperatures in the stratosphere.
Troposphere: lowest layer of the atmosphere, with an average depth of 16 to 18 km around the equator, 9 to 12 km temperate latitudes and well below 9 km much of the time in arctic regions. There is a general fall of temperature with height (i.e. a positive lapse rate), with an average value of some 6.5 degC / km (or 2 degC / 1000ft).
Vorticity: a measure of the 'spin' of a portion of a fluid - in our case, of atmospheric particles. Vorticity in a cyclonic sense is designated 'positive', and in an anticyclonic sense is designated 'negative'. In synoptic meteorology, we often only consider vorticity in a horizontal plane - i.e. the 'spin' behaviour of air particles as they move along in the atmospheric flow as depicted on classical 'weather maps'.
Zonal: A predominantly west-to-east airflow is termed zonal (and an east-to-west airflow is negative zonal). The strength of the flow in any sector may be expressed in terms of a zonal index given by the difference in average contour height between two latitude circles through the sector.

Thermal Winds

A thermal wind is defined as the vector difference between two actual winds at different levels in the atmosphere, conventionally calculated (as in the case of thickness q.v.), by subtracting the lower-level wind from the upper-level wind. Put differently, and more practically, it is that velocity component (remember: velocity has both speed and direction), that must be added to a lower level wind to produce the upper level wind. Graphically, these statements can be demonstrated thus:....



Although the principal thermal wind used in operational meteorology is that through the layer between 1000 hPa and 500 hPa, in fact thermal wind calculations can be applied between any two levels in the atmosphere.




In the atmosphere, air moves (the wind 'blows'), because of pressure differential between two points. This is usually demonstrated at mean sea level, using as examples the sea breeze, or the Asia monsoon. Why this should be so can be demonstrated when it is remembered that heat gain or loss by a column of the atmosphere produces expansion/contraction of that column:

consider

 

1. Initially, with both columns having the same mean temperature, there is no pressure differential between them at any level.

2. Column B is now warmed, and expands.

3. Whilst the total amount of atmosphere above P(L)A=P(L)B, i.e. there is no pressure difference at the lower level (usually taken to be msl), there is now a pressure difference at P(U), whereby P(U)B>P(U)A, due to the expansion of the column: there is a bit more of the atmosphere above P(U) than there was before.

4. So, a thermal difference exists between the two columns, this giving rise to the notional "thermal wind", which is a convenient way of visualising the way real pressure differences are set up, leading to real winds at upper levels.

thus strong thermal winds imply marked difference in actual wind with heights, and strong thermal winds (and therefore strong 'real' winds), are associated with frontal boundaries and strongly-sheared convective situations. An example is shown below:

  • Upper trough advances eastwards - positive vorticity advection 'spins up' the atmosphere in a region within which warm air is being advected northwards - increased convergence - destabilisation due to colder air overlaying warm/humid air.
  • As cold air (associated with upper/short-wave trough) butts up to warm air (associated with northward-moving plume), the enhanced thermal gradient (i.e. thermal wind component) leads to stronger upper level winds.
  • The associated development areas, (which are a result of un-balanced forces due to accelerations/decelerations aloft), are made more active, and rather than 'pure' convective activity, bulk lifting of the troposphere occurs, releasing potential instability, and leading to organisation of convective storms, rather than isolated cells/clusters.
  • The resultant thermal forcing often manifests itself in a pseudo -frontal boundary at the surface, and aloft, taking over from the pre-existing, and often weak boundary between the cool/maritime air (rPm), and the very warm/humid (modified Tc) air.
  • The enhanced pressure gradient at the surface leads to a quickening of the northwards advection of the warm/humid air, with the upper trough overrunning, releasing further deep/vigorous convection.
  • The main air mass cold front, driven by the main upper trough sweeps through and displaces the humid air/thunderstorms, thus cutting off the activity.

 


Thermal winds 'blow' so as to obey Buys Ballot's Law with cold thicknesses to the left in the NH.

The 'standard' layer through which the thickness (and associated thermal wind component) are calculated in synoptic meteorology is the 500-1000 hPa layer. However, whereas it is possible to calculate the notional 1000 hPa height, even if it is below the msl, things are a little more difficult with thermal wind calculations.

  1. 1000 hPa often in the friction-affected BL (within about 1500 m above local ground level), and the measured wind is backed/reduced as a result, and therefore does not reflect the true gradient at 1000 hPa.
  2. 1000 hPa may be at/below msl, and no meaningful 1000 hPa wind can be measured.

The lower level wind is often replaced by the 925 hPa or 900 hPa wind, or even the 850 hPa wind if these are not available. However, in these cases, care needs to be exercised when using such thermals alongside the accompanying thickness plots.


Because low level winds in a developmental (mid-latitude) situation are much less than those at jet levels, from the definition of the TWC, the polar front jet is really a large-scale hemispheric thermal wind -- blowing in response to large scale changes in distribution of air of differing temperature -- i.e. air mass discontinuities. Change the arrangement of the blocks of warm and cold air, and you change the direction, and strength, of the upper level jet. This is why, for example, study of sea surface temperature anomalies are important for long-term climate change.

Thickness and maximum surface temperatures

Approximate relationship between Thickness (or Relative Topography): 500 - 1000 hPa & the maximum screen temperature (oC) [ NW maritime Europe; also see cautionary notes below]

 Thickness 500-1000 hPa (in metres) Nil / Poor insolation (a) Moderate / High insolation (b) High summer:
strong sun
 5100  -6  -1  
 5150  -3  2  
 5200  -1  4  
 5250  2  7  
 5280  3  8  
 5300  4  9  
 5350  7  12  
 5400  9  14  15
 5450  11  16  17
 5460  12  17  18
 5500  14  19  20
 5550  16  21  22
 5600  19  24  25
 5640  21  26  27
 5650  21  26  27
 5700  24  29  30
 5750  26  31  32
 5800  29  34  35
 5820  30  35  36

Notes:
(a): Generally overcast, thick cloud cover - minimum effective insolation; also generally applicable to the mid-winter period of low solar angle. Precipitation would imply the figure might be a degree or two lower still.
(b): Fine, sunny much of the time up to day-maximum occurrence. If the sunshine is particularly strong in spring & summer, then this figure may be a couple of degrees (at least) higher. These figures though would NOT apply to mid-winter, low solar-angle events.
(c): In mid-winter, with low thickness values in particular, temperatures will be lower still - perhaps by as much 5degC due to limited insolation.

These figures are based on the theoretical relationship between the average lapse rate within the lower troposphere (2 degC/1000ft) and the implied mean temperature (within the layer 500 - 1000 hPa) given by the total thickness values. They take no account of the type of surface (sea, dry land, snow-cover etc.) and also there is no allowance for seasonal variation of solar energy available.
Sunny figures further imply that a dry adiabatic lapse rate (3 degC/1000ft) exists at time of maximum temperature from the surface to 850 hPa.

However, this is a very crude relationship, and the figures given above should be regarded as the rough 'limits' of expected maxima. In practice, they will vary by several degrees. This is why forecasting day maxima based on total thickness has fallen out of fashion.

Thickness Extremes

The 6 figures presented below show maximum and minimum values of total thickness (h[500] - h[1000]) in dekametres. PLEASE READ THE CAUTIONARY NOTES BELOW:

6 panel showing max/min thickness values

HIGHEST BRITISH ISLES:.... 576 dam in July & August around the Channel Islands.
LOWEST BRITISH ISLES:..... 491 dam in January around the Shetland Islands.

  • Locations/values are deliberately displayed in general terms as these data are extracted from small-scale charts. They are here given to roughly define the 'reasonable' limits of total thickness values.
  • The series covers the period 1945 - 1993, so extremes may not be up to date. Also the early years analysis would not be as accurate as middle/latter parts of the series.
  • Data are extracted from records held at the National Meteorological Library. They are displayed here purely for amateur/educational use. If you wish to use these values in any way - i.e. to support an article, you must contact the NML (see the uk.sci.weather FAQ for details), and obtain detail and permission.

Thickness Pattern Definitions

The chart shown here is intended to show some common features to be found on the 500-1000 hPa thickness chart over the North Atlantic & Europe. It does not represent any one particular situation, and is rather artificial as a result. Refer to the text below for an explanation of the features.

(last updated 29 JAN 2000)

specimen thickness chart


A: COLD POOL (QUASI-STATIONARY, LONG-LASTING TYPE).
B: WARM DOME.
C: COLD POOL (TRANSITORY/MOBILE TYPE).
C': COLD POOL (TRANSITORY/SLOW-MOVING TYPE).
D: COLD POOL (LOW-LATITUDE TYPE).
E: BAROCLINIC ZONES.

COLD POOLS:
There are at least three distinct types of cold pool:

1. Large (in areal extent), slow-moving vortices at latitudes poleward of roughly 50deg N & S (Example A). They are often over, or immediately downwind of the source regions of polar or arctic air masses, in such areas as the Canadian Arctic or Siberian Russia. They take up the characteristics of quasi-permanent features, often appearing on monthly, or even seasonal average maps. The column of very cold (dense) air associated with this version of the cold pool, is reinforced by net outgoing radiation which produces a highly stable, bitterly cold near-surface environment, with a surface ridge or anticyclone on MSLP charts.

2. Once cold air from the source region as detailed in (1.) above is caught up in the circulation of mid-latitude depressions, the characteristics of the air mass are altered. Warming from below (over relatively warmer sea surfaces for example), leads to instability developing, and with an accompanying injection of moisture (after a reasonable length of passage over the sea), cloudy convection (showers, thunderstorms) is triggered. Cold pools (Examples C, C') in these situations are often maxima of such activity, with surface charts showing west or northwesterly airflow, often with cyclonically curved isobars. The cold pool will move in the general synoptic flow, and will eventually warm out (disappear) due to sensible & latent heat exchanges within the environment of the cold pool. However, as at example C', sometimes the cold pool will transform into a slow-moving, longer-lasting entity, particularly if the feed of ex-polar air on its western flank is maintained - this occurs in highly meridional situations.

3. A pool of cold air can also become 'detached' at lower latitudes (Example D), i.e. away from the mid-latitude westerly zone, and drift slowly over relatively warmer seas, (e.g. the Mediterranean), and lead to intense convective development, often taking on marked cyclonic characteristics through the troposphere, and giving rise to locally severe condtions due to heavy rainfall, severe thunderstorms and squally winds. Remnants of these types of cold pool will sometimes drift polewards in summer and bring outbreaks of severe convective activity to mid-latitude regions, as these features will destabilise hot/humid airmasses.

WARM DOMES:
This is a term that is not often heard nowadays, but is applied to the opposite case of the cold pool (Examples B) , where relatively warm (high thickness value) air is enclosed within a closed contour value. The associated low-level weather will be quiet, settled with little vertical development of cloud, if any at all.

BAROCLINIC ZONES:
Areas where there are marked contrasts between cold and warm air masses (Examples E). These can be determined on a thickness chart by a packing together of contours. Usually associated on a msl chart with classical fronts, and therefore an area for potential cyclonic development.