The sections of the FAQ are listed below; click on one to go to that section.
It is important to note that this FAQ is not a 'do-it-yourself' course in meteorology. It simply aims to answer some common puzzling questions that might be posed by the non-professional whilst browsing met-related web pages, or lurking in one of the weather newsgroups.
The following is an extract from the Charter for the newsgroup: "This group is essentially for the discussion of daily weather events, chiefly affecting the UK and adjacent parts of Europe, both past and predicted. The discussion is open to all, but contributions on a practical scientific level are encouraged. It may also contain postings of observations during interesting weather episodes. The group is expected to be patronised by both amateurs and professionals (including academics), but it is primarily for weather enthusiasts rather than research scientists. Any discussion of climate issues should be from a scientific standpoint and not a political (or environmental-activist) one." (See here for full copy source)
PLEASE NOTE: Binary files, (such as interesting satellite images, or weather charts) should not be posted into the newsgroup. To do so is a great annoyance to users, and contravenes the Acceptable Use Policy of some ISP's and associated peering carriers. Post such files into newsgroups specifically set up to carry binary-encoded information, and then post to the uk.sci.weather newsgroup the location. Alternatively put on your own web site.
There is a little more about the newsgroup history in the section on USW history.
The original author of most of the material here, Martin Rowley, was an employee of the UK Meteorological Office (now known simply as 'the Met Office'), but has now retired. He has now also retired from maintaining this FAQ, and this web site was started in 2007 to hopefully carry on his good work.
A number of other knowledgeable folk have also contributed to the information here, for this I am very grateful.
Note that the current maintainer, while having a strong interest in the weather, is by no means a meteorological expert! He relies completely on input from the many experts on the uk.sci.weather newsgroup.
Steve Loft 24th January 2008
There is always a problem with units in meteorology, because the 'operational' community use, and are used to, different units to those of an academic/theoretical persuasion, and so our trade is littered with anomalies! In this FAQ, I have used degrees Celsius for temperature as this will be familiar to most, but for height/altitude, both feet (used by the aviation world), and metres/km equivalents are given - mostly approximations. Wind speeds, where given, will be in knots (used in practical observing/aviation forecasting) and metres/second. The relationship between the two units is assumed to be knots=2*m/s, as only approximations are quoted. Note also that other approximations are often used, for example the Dry Adiabatic Lapse Rate is quoted as 10 degC per 1 km, whereas it is calculated to be 9.8 degC/km. (See here)
As its name implies, this jet stream is associated with the marked discontinuity found at the boundary of well defined air masses - polar to the north/sub tropical to the south (in the northern hemisphere), conventionally found at the polar front. It meanders markedly in response to global/regional scale atmospheric changes but has a latitudinal 'home' roughly from 45 to 65 deg N/S. Its altitude is somewhere between 28000 ft to 34000 ft (8.5 - 10.5 km), with its own distinctive tropopause level. Speeds are of the order 80-130 knots (40-65 m/s), but may be as high as 180 knots (90 m/s), and downstream of main continental land masses in late winter/early spring, in excess of 200 knots (100 m/s). Although it is regarded as a 'single' ribbon of strength encircling the hemispheres, in reality the jet is broken and in developmental situations, can become very distorted with new jets re-forming at different levels from the 'main' baroclinic jet. (NB: during the winter half-year, jet-streams can be found at even higher latitudes in both hemispheres, roughly around 18000 ft (circa 5.5 km), which are named Arctic (or Antarctic) jets. These are again tied to a discontinuity between air masses (a frontal zone, or Arctic / Antarctic front), but this time between a very cold (and relatively shallow) 'arctic' airmass and somewhat less cold polar-maritime or polar-continental air. They are most evident (in the Northern Hemisphere), across Canada, the far north of the USA, northern Scandinavia, North Russia & other high-latitude regions. However, at times, they come further south, interacting with the PFJ, with sometimes dramatic developmental consequences.)
The average level of the core of this westerly jet lies at an altitude of about 40 000 ft/12 km, just below the tropical tropopause. It occurs in the latitude range 25-40 deg N/S, and is most marked during the winter and early spring of each hemisphere, but is not associated with any surface frontal structure. Its existence owes more to the fact that air in the high-level (poleward travelling) leg of the Hadley circulation conserves its angular momentum, being effectively 'turned' towards the east and finally concentrated in this ribbon of strong westerly wind. Because the Hadley circulation, and mid-latitude north/south linking flows are governed by seasonal heat differences north-to-south, then the STJ does vary, as noted above. Wind speeds are generally 80-150 knots (40-75 m/s), but can be much greater over eastern seaboards of large land masses, e.g. speeds of 400 kt (200 m/s) have been reported over east Asia/NW Pacific. From time-to-time, the STJ and the Polar Front Jet can interact with marked developmental consequences.
This occurs at times during the winter and early spring when the stratosphere near the poles is much colder than it is further south due to the absence of insolation at these times of the year. Its direction is westerly overall, with high variability, and has speeds in the range 100-200 kt (50-100 m/s) at altitudes around 70 000 ft/21 km and occurs on the poleward side of latitude 70 degrees.
This jet occurs in the northern summer between 10 and 20 deg N, chiefly over or just to the south of high land masses such as in Asia and Africa. Its occurrence is due to a temperature gradient with colder air to the south which produces sufficient temperature differential above 50 000 ft/15 km to give wind speeds of over 100 kt (50 m/s). Because colder temperatures at height are to the south, it is an easterly jet. (This jet is now more usually known as the Tropical Easterly Jet(TEJ) ... perhaps more correctly as it lies some distance from the Equator.)
In NW Europe, when meteorologists refer to a jetstream, it's the Polar Front Jet (PFJ), that is usually meant. As its name implies, it is associated with the classical 'Norwegian model' polar front - the surface discontinuity between cold/ex-polar latitude air, and the warm, relatively moist air originating in the sub tropical anticyclone belt.
When air masses (see "When was the concept of an air mass proposed?" and "So, how is an air mass defined?") lie adjacent to one another, the temperature difference isn't just found at mean sea level, but throughout the troposphere. Because atmospheric pressure decreases more quickly with height in cold/polar air than warm/sub-tropical air, there arises a pressure differential, which gives rise to intense pressure gradients at altitude, and hence the very strong winds observed. Because of the high wind speeds involved with jetstreams, any slight changes, in either velocity or direction, or both, leads to vertical motion in the air below the jet, and is a major player in the processes of atmospheric development.
(For more on upper air meteorology, jetstreams etc., see "Upper Air Meteorology").
First, to visualise what 'stable' and 'unstable' states mean in a physical sense, stand a round pencil on end on a level surface. From Newton's First Law of Motion, it will remain upright until a force is applied. Once displaced, the pencil falls over, failing to pass through its original (upright) position. This is the UNSTABLE state. Now lay the pencil on its side, at the bottom of an incline. Displace the pencil slightly up the incline, then remove the force of displacement. The pencil will return to its original position. This is the STABLE state.
In the atmosphere, whether air that is displaced does so in an unstable or stable environment depends upon the vertical temperature profile of the air -- its lapse rate -- and upon the moisture content of the parcel. These differences are fundamental to understanding why clouds take up the form they do.
In the atmosphere, when a 'parcel' of air moves vertically upwards (or downwards), it cools (upward motion), or warms (downward motion), in accordance with thermodynamic rules ... if the air is unsaturated (air temperature > dew point temperature), the cooling/warming will be at a rate of 3 degC per 1000 ft (or 10 degC per 1 km): This is known as the Dry Adiabatic Lapse Rate/DALR; If the air is/becomes saturated (air temperature=dew point temperature), this rate is roughly halved in the lower troposphere, due to the release of latent heat upon condensation. This rate is known as the Saturated Adiabatic Lapse Rate/SALR.
Such ascent/descent is said to be adiabatic, which means that the energy/heat changes are confined to that particular parcel.Provided the parcel is warmer (less dense) than the environmental air through which it is passing, it is buoyant, and rises. If the parcel is colder (denser) than ambient air, then it will descend, or try to descend. Because the rates of cooling (ascent), and warming (descent) of individual parcels are fixed, the important variable is the overall lapse rate (i.e. the rate of change of temperature with height) of the atmosphere. On average, this is 1.98 (call it 2 degC) per 1000 ft, or 6.5 degC per 1 km in the troposphere, but this average conceals a wide variety of cases which are important in meteorology.
Where the temperature falls off slowly with height, or indeed rises, e.g. in a slow moving anticyclone, or a tropical maritime airmass, then an air parcel subject to lifting/adiabatic cooling will readily find itself colder than its surroundings ... denser ... and try to return to its original position: The air is ABSOLUTELY STABLE. Where the temperature falls off quickly with height, e.g. in a cold/polar air mass over NW Europe in late winter/spring, then an air parcel subject to ascent, although cooling, may still find itself warmer/less dense than its surrounding air ... it will be buoyant, and tend to rise further: the air is ABSOLUTELY UNSTABLE.
Problems arise when, on ascent, the dew point of the air is reached, and the rate of cooling is therefore less - it follows the SALR figure. If, however, the parcel is still warmer/less dense, then it will continue to rise, and the condition of the air is said to be CONDITIONALLY UNSTABLE .. i.e. conditional upon whether the parcel is saturated or not. This is by far the most common situation in the 'real' atmosphere, accounting for some 65-70% of situations taking the troposphere as a whole.
Stable airmasses generally imply the absence of 'free' vertical motion, and any ascent that does occur must be forced, i.e. frontal (dynamic or mass) or orographic (mechanical) ascent, and the cloud structure is essentially layered. (NB: Forced ascent comes about in several ways: frontal ascent due to large-scale air motion within frontal systems, with of course adjacent descent; convergence into an area of low pressure - the converging air can't go down near the surface - it has to go up; and topographical forcing, that is, air is forced to rise over major upland ranges. )
Unstable airmasses imply free vertical motion (given an initial trigger action), and the cloud structure is 'heaped' or cumuliform. If the vertical motionis vigorous and deep enough, and there is sufficient moisture, then heavy showers/thunderstorms are likely. (NB: Trigger action: method of causing air to rise initially, which in the lower troposphere include not only the 'wide-area' triggers noted above under stable conditions, but also smaller/mesoscale mechanisms such as differential heat response between land and sea, coastal convergence, etc.)
For more information on these subjects, see a good textbook on meteorology, for example, Essentials of Meteorology:(Taylor and Francis/D.H.McIntosh and A.S.Thom).
'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)
The 500-1000 hPa value is used to define 'bulk' airmass mean temperature, and can be seen on several products available on the Web.
For more information see here.
(see also here for typical figures, extremes etc.)
The atmosphere is divided up into layers with names which describe the dynamic or thermal structure of that particular layer. The two layers which are of most interest to us are the troposphere and the stratosphere.
Troposphere: (overturning or changing sphere) - The lowest layer of the atmosphere. Positive lapse of temperature (positive lapse rate: temperature overall decrease with height). It is the most important for operational meteorology, as this layer contains almost all the water vapour, and by far the greatest part of the mass of the atmosphere. Because of its mean thermal structure, it is the region of greatest vertical motion (up and down) in the atmosphere, even without the help of vigorous thunderstorm complexes, which in themselves may occupy the entire depth of the troposphere. At some level, there is usually an abrupt change in the lapse rate from positive (decrease with height), to isothermal (no change), or a slight rise. This level is the tropopause. Typical heights of the tropopause, and therefore thickness of the troposphere, are:
In mid-latitudes, the temperate zone, which is of most interest to us in NW Europe, the tropopause is highly variable, from cold to warm season, and from cold to warm air mass. For example, it is lower in winter, and in cold/polar air masses (typically 8 to 10 km/25000 to 30000 ft), than in high summer, and in warm/sub tropical air masses (typically 12 to 14 km/35000 to 45000 ft)
Stratosphere: (the 'layered' sphere) - the next layer ascending through the atmosphere. Isothermal or negative lapse rate of temperature (i.e the temperatures rises with increasing height). Because of this temperature structure, little natural, or un-forced overturning of air takes place, either within the stratosphere, or in exchange with the troposphere. Once gases, particulates etc. penetrate to this layer, they remain there for very long periods, hence the concern regarding such substances due to both the actions of mankind (e.g. CFCs) and those of natural processes (e.g. volcanic ash). However, near the boundary with the troposphere (q.v.), marked vertical motion can occur under certain circumstances (forced by jet-stream actions), which are important in driving developments in the troposphere.
As with the troposphere, the stratosphere varies in thickness, but as an average figure the top of this layer, the stratopause, occurs around 45-48 km (148000-158000 ft).
The importance of the stratosphere (and the primary reason for its temperature structure) is that much of the atmospheric ozone is found within its lower layers - circa 18 to 30 km amsl. The selective absorption by ozone (and oxygen) of solar ultra-violet radiation leads to warming in the stratosphere - this (and other) factors give rise to its markedly stable nature. Very little water vapour is found here, nor dust (except for dust injected by major volcanic eruptions), but when the stratosphere is anomalously cold, then Polar Stratospheric Clouds (PSC) are sometimes visible.
These equivalents are based on the International Standard Atmosphere and promulgated by ICAO:
|mbar (hPa)||Nominal Altitude |
(ft to nearest 1000 ft; metres to nearest 100 m)
|100||53,000 ft / 16,200 m|
|200||39,000 ft / 11,800 m|
|250||34,000 ft / 10,400 m|
|300||30,000 ft / 9,200 m|
|400||24,000 ft / 7,200 m|
|500||18,000 ft / 5,600 m|
|600||14,000 ft / 4,200 m|
|700||10,000 ft / 3,000 m|
|850||5,000 ft / 1,500 m|
For full details, see this article. The article also has the definitions of QFE, QNH, QFF and QNE.
There are four principal types of satellite imagery used in operational and research meteorology. Each has its advantages and disadvantages. Many examples of each type can be found at meteorology related web-sites.
1. Visible Imagery (VIS)
Images obtained using reflected sunlight at visible wavelengths, in the range 0.4 to 1.1 micrometres. Visible imagery is displayed in such a way that high reflectance objects, e.g. dense cirrus from CB clusters, fresh snow, nimbostratus etc., are displayed as white, and low reflectance objects, e.g. much of the earth's surface, is dark grey or black. There are grey shades to indicate different levels of albedo (or reflectivity). Very dependent upon angle of incident sunshine, and of course, not available at night, though some military/research satellite sensors can utilise reflected moonlight to detect cloud.
2. InfraRed (IR)
These images are obtained by sensing the intensity of the 'heat' emissions of the earth, and the atmosphere/atmospheric constituents, at IR wavelengths in the range 10-12 micrometres. The earth, and its components, radiate across a wide spectrum of wavelengths, but for many of these, the atmospheric gases, of which water vapour is an important constituent, absorb a significant proportion of such radiation. Thus so-called 'windows' need to be chosen to allow the satellite sensors to detect such radiation unhindered, and the 10-12 micrometre band is one such. IR imagery is so presented that warm/high intensity emissions are dark grey or even black, and low intensity/cold emissions are white. This convention was chosen so that the output would correspond with that from the VIS channels, but there is no need to follow this scheme - indeed in operational meteorology, colour slicing is frequently used whereby different colours are assigned to various temperature ranges, thus rendering the cooling/warming of cloud tops (and thus the development/decay) easy to appreciate: warming/darkening of the imagery with time indicates descent and decay; cooling/whitening images imply ascent and development.
3. Water Vapour (WV)
This imagery is derived from emissions in the atmosphere clustered around a wavelength of 6.7 micrometre. In contrast to the IR channel, this wavelength undergoes strong absorption by WV in the atmosphere (i.e. this is not a 'window'), and so can be used to infer vertical distribution and concentration of WV - an important atmospheric constituent. WV imagery uses the radiation absorbed and re-emitted by water vapour in the troposphere. If the upper troposphere is moist, WV emissions will be dominated by radiance from these higher levels, swamping emissions from warmer/lower layers; this radiation is conventionally shown white. If the upper troposphere is dry, then the sum of the radiation is biased towards lower altitude WV bands: it is warmer/less intense radiation, and this is displayed as a shade of grey, or even black. WV imagery is very important in the study of cyclogenesis, often being displayed as a time-sequence.
4. 'Channel 3' (CH3)
Imagery from a specific wavelength of 3.7 micrometre, lies in the overlap region of the electro-magnetic spectrum between solar and earth-based/terrestrial radiation. It is sometimes referred to as 'near infrared' (NIR). CH3 images use a mixture of back-scattered solar radiation plus radiation emitted by the earth and atmosphere. It is used in fog/very low cloud studies. Interpretation is sometimes complex, especially in the presence of other tropospheric clouds.
In the troposphere (the 'weather' zone ... see here, the layers are divided up into three broad levels: (approx heights only)
|Polar latitudes||Temperate regions||Tropics|
|High||10 000 - 25 000 ft |
3 - 8 km
|16 500 - 45 000 ft |
5 - 14 km
|20 000 - 60 000 ft |
6 - 18 km
|Medium||6 500 - 13 000 ft |
2 - 4 km
|6 500 - 23 000 ft |
2 - 7 km
|6 500 - 25 000 ft |
2 - 8 km
|Low||Surface -- 6 500 ft |
up to 2 km
|Surface -- 6 500 ft |
up to 2 km
|Surface -- 6 500 ft |
up to 2 km
The heights assigned to the 'divisions' between levels should not be followed slavishly, and assignment of clouds to the various 'groups' should be made with the appearance and composition in mind.
High clouds are primarily composed of ice crystals; Medium clouds are a mixture of water droplets (usually super-cooled) and ice crystals, in varying proportion, and low clouds primarily water droplets, but in individual cases these descriptions are probably simplistic.
(NB: Super-cooled: means that although the temperature of the droplet is below 0 deg.C, it remains liquid - this is a common state in the middle part of the troposphere.)
In the 'Low' cloud classification come: Stratus (St); Stratocumulus (Sc); Cumulus (Cu) and Cumulonimbus (Cb). However, note that both Cumulus and Cumulonimbus clouds often extend well into 'medium' levels, and towering Cu, and Cb extend to 'high' levels.
In the 'Medium' cloud class come: Altostratus (As); Altocumulus (Ac) and Nimbostratus (Ns). Nimbostratus often has a base within the 'low' cloud category.
In the 'High' cloud group are: Cirrus (Ci); Cirrocumulus (Cc) and Cirrostratus (Cs).
The white trails are ribbons of ice crystals. As a by-product of the exhaust of aircraft engines, water vapour is trailed from the engine exhaust which adds to the local humidity of the air the aircraft is flying through, and which tends to super-saturation of the air. However, the exhaust gases are of course hot, and so these hot gases help to raise the temperature of the air and thus is can hold more vapour before saturation is reached. There are therefore two opposing mechanisms at work: the water vapour in the exhaust trying to saturate the air; the hot gases of the exhaust trying to decrease relative humidity. When the balance between outside air temperature (OAT) and local humidity is just right, then condensation trails will occur: usually abbreviated to CONTRAILS, and sometimes referred to, from old coding conventions, as COTRA.
Persistent condensation trails can last for many hours, gradually spreading out to form large, sometimes dense areas of cirriform cloud; they can have dimensions typically several kilometres wide and several hundred metres in depth (thus they can be seen in visible satellite imagery). They spread because of turbulence at the 'trailing' level ( enhanced by the aircraft passage), differences in wind speed along the flight-path and there is also thought to be a contribution from solar heating. Because trails can last so long and come to dominate the upper troposphere in any particular synoptic situation, the production of such form part of the debate on the overall global radiation balance.
Wake trails: As an aircraft passes through a lower troposphere having a high relative humidity, (usually during landing or take-off phases - and for military aircraft, during 'high-G' manoeuvres), very short, non-persistent 'trails' can sometimes be seen coming from the wing tips, or white 'lift-generated sheets' streaming off from the trailing edges of the main wing, control surfaces etc. Both features are due to short-term local reduction of pressure, leading to condensation, though the precise mechanism in each case is different.
(a): "lift-generated sheets": as an aircraft moves forward, air accumulates (pressure builds), at leading edges, with a compensating depletion of air (fall of pressure) across the top of the wing (generating lift) and along trailing surfaces. The reduction of temperature in the near-saturated environment, consequent upon the slight lowering of pressure, can be enough to cool the air to it's dew point, and thin sheets of water droplets are observed.
(b): "wing-tip trails": the flow of air around the wing-tips undergoes marked distortion which manifests itself as a tight-vortical (or 'twisting') motion of the airflow; the vortices are formed by, and will lead to, a local increase and decrease of pressure - in the latter case, if the atmosphere is humid enough, then white trails can be observed. In both cases, the sheets/trails (of minute water droplets) will evaporate quickly again due to mixing with the non-saturated environment in the wake of the aircraft.
Dissipation trails (DISTRAILS): In contrast to the formation of CONTRAILS (see FAQ here ), aircraft on passage at high levels can cause the dissipation of pre-existing cirriform cloud, due to the local increase in temperature consequent upon the ejection of hot exhaust gases from the aircraft engine. The passage of the aircraft will be marked by a clear lane in the cloud. However, it will be obvious from the description (above) relating to condensation trails, that the heat outflow must markedly outweigh the injection of water vapour from the spent fuel, and the phenomenon is rare. The effect may also be caused by turbulent mixing with dry air just above the cloud layer, caused by the aircraft motion, and this mechanism can lead to temporary clear lanes in other cloud forms, e.g. thin stratocumulus or altocumulus. However, beware of a similar phenomenon, whereby the shadow of a 'normal' condensation trail is cast on thin cirriform cloud below - leading to a visibly dark band in the cloud. This is not a dissipation trail.
Incidentally, whilst on the subject of 'trails', if you are looking at visible satellite imagery over the region of a slow-moving anticyclone, and notice lots of thin, white lines criss-crossing the region, which don't appear on the corresponding InfraRed image, these are ships' trails, caused by exhaust particles from the vessel acting as condensation nuclei, and 'seeding' the humid, near sea surface environment, and betraying the presence of the ship by a thin band of water droplets which are not dispersed due to the very light winds and minimal mixing in the anticyclone.
The average condition of temperature change in the Troposphere is for there to be an overall decrease of temperature with increasing height: a positive lapse rate (see here). However, in the 'real' Troposphere, frequent reversals of this 'normal' lapse are observed, particularly in the lower layers - these zones of increasing temperature with height are inversions (i.e. the inverse of the average state), and are very important for both synoptic/mesoscale meteorology (e.g. fog/stratus formation/dispersal), and pollution dispersion studies, as they cap layers of markedly stable and potentially stagnant air masses.
Examples of inversions include those due to anticyclonic subsidence; cooling land by night (nocturnal inversions); and sea-breeze inversions, where cooler sea air under-cuts warmer land air. Where the inversion is associated with an abrupt lowering of the moisture content (sharp fall of dew point), at the altitude of the temperature rise, then interesting radio-refraction conditions occur, familiar to viewers of terrestrial television in stagnant anticyclonic episodes.
In fact, if you are caught out in one, there is no difference. You can still get wet! Meteorologists however distinguish between precipitation (rain, snow, hail etc.) falling from cumuliform cloud in an unstable environment - a shower, from that falling from layer clouds in a generally stable environment which are just called rain, snow, sleet etc.
However, rain from layer cloud in a frontal situation for example can be rather hit-and-miss, especially in a weakening situation, and so forecasters will try and get around such problems by talking about 'patchy rain', 'outbreaks of rain', 'splashes of rain' etc. The opposite problem comes when a well defined trough sweeps across an area, in which the cloud structure is most likely of an unstable type: cumulus, cumulonimbus and altocumulus.
Given the definition above, the short, very sharp falls of rain might be called 'showers' (and probably coded as such by observers), but this would be misleading to members of the public caught out in such precipitation: hence the 'showery outbreaks of rain', 'showery bursts of rain', 'localised downpours' etc.
In day-to-day meteorology, the temperature of the lowest layer of the atmosphere is measured at a height of 1.25 m (about 4 feet) above local ground level. Usually, though not always, this is achieved by placing thermometers in a double-louvered screen with the bulbs of the thermometers, or the sensor heads (for distant reading thermometers), placed so that they cluster around the 1.25 m standard. The temperature so read is usually called 'the air temperature' and it is these values that appear, for example, in the World Cities reports in newspapers/teletext, or plotted on standard synoptic charts, and also it is at this level that the forecast temperatures seen on tv weather maps are based.
When the temperature as measured in this way falls below 0.0 deg C, then an AIR FROST is recorded. For other purposes though, e.g. horticulture, road gritting operations etc., we need to know what the temperature is at the surface of the ground, and most weather stations set at least two thermometers to record these values: a grass minimum thermometer, set just above/in contact with short grass, and a concrete minimum thermometer, set so that its sensor/bulb is in contact with a concrete slab of standard dimensions/composition.
When the temperature as measured by the thermometer set over grass falls below 0.0 deg C, then a GROUND FROST is recorded. (In spring and early summer, when the temperature is expected to produce a frost using the grass minimum thermometer, but not over other surfaces (due to thermal inertia of surfaces such as concrete, tarmac etc.), then the unofficial term 'grass-frost' may be heard in weather forecasts - this is to try and avoid panic by road, railway and airport operators as soon as they hear the word 'frost' but alert gardeners, growers etc., to the risk of damage).
The difference between the two levels can be considerable: On still, clear nights, with air of a low humidity content, 5 degC or more is not uncommon.
A common misconception, is that it must be coldest in the middle of the night, and warmest around midday. On some occasions, mainly due to air mass changes, this may be correct, but not usually. The lowest (minimum) temperature usually occurs a little while after sunrise, and the highest (maximum) temperature usually occurs after midday --- sometimes as late as 3 or 4 hours after midday.
To understand why, it is necessary to consider that thermal energy during the 24 hours is radiating continually from the surface of the earth (at long wavelengths), and incoming solar (relatively short wave) radiation obviously only when the sun is above the horizon. With the sun below the horizon (night), outgoing radiation allows the surface to cool, and the temperature drops. After sunrise, incoming solar radiation counteracts this loss of heat, but only after a lag - which can be up to an hour or so in winter with a low solar elevation.
The minimum temperature occurs when there is a balance between outgoing and incoming radiation. As the sun rides higher in the sky, increasing amounts of short-wave radiation are available to heat the ground, and therefore available to heat the overlying air. Although outgoing land-based radiation is also increasing, solar heating is dominant. The temperature rises, until, past noon, incoming solar radiation starts to decline again.
The highest(maximum) temperature occurs when heat gain due to incoming solar radiation, and heat loss due to outgoing terrestrial radiation balance: this occurs some time after midday.
For any particular sample of air, which is cooled at constant pressure, there will be a temperature below which water vapour condenses to form liquid water drops, assuming sufficient hygroscopic nuclei present. That temperature is known as the Dew Point and is a measure of the Absolute Humidity (see "What is the difference between Humidity and Relative Humidity?").
Absolute Humidity, often just referred to as 'the humidity', is a measure of the actual amount of water vapour in a particular sample of air: measured as a partial pressure (vapour pressure/hPa or millibars); a mixing ratio (gm water vapour/kg of dry air), dew point etc.
Relative Humidity - expressed commonly as a percentage value, is the ratio of the actual amount of water vapour present in a sample (the Absolute Humidity) to that amount that would be needed to saturate that particular sample.
The two terms are not interchangeable and can lead to confusion; e.g. on a cold, raw winter's day close to the east coast of England, the dew point might be 1 degC and an air temperature of just 2 degC. This would give a RH of 93%; a 'high' Relative Humidity, yet few would refer to such conditions as 'humid'. Conversely, on a hot summer's day, with a dew point of 18 degC, and an afternoon temperature of 30 degC, that's a RH of 49%; a 'low' Relative Humidity, but high Absolute Humidity.
Only for the special case of thunderstorms coming up from the south in summer. I have seen many thunderstorms (real crackers as well) in April with air temperatures of 8 degC and a dew point of 4 degC. What is really important is that the air must be unstable (see "stable and unstable air masses"), usually achieved by warming at the bottom or by cooling high up or both. Then you need a trigger to release the instability, usually heating and input of moist air (high dew point), but if the air is unstable enough just the heating will do. Other triggers are forced lifting of air over hills or forced lifting by convergence (e.g. sea breezes).
(thanks to Will Hand for this answer)
At mid to high latitudes in the upper part of the troposphere (above roughly 5 km ), the mean wind flow exhibits a broadly west-to-east motion - this applies in both hemispheres. On many occasions, particularly in mid-latitude/temperate zone regions, the flow is directed more or less directly from west to east, crossing few latitude zones within the same longitude range: this is a 'highly zonal' type - any short-wave disturbances embedded in the flow will be carried quickly along and the weather is ever-changing as a succession of frontal systems, interspersed with transient ridge conditions cross any one point.
However, on both average (e.g. monthly) pressure maps and on individual days, long-wave trough/ridge patterns can be found - some having large amplitude, i.e. the airflow meanders a long way north and south around the loops of the pattern, crossing many parallels of latitude in a relatively limited longitudinal range: a 'meridional' type; Usually, some west-to-east progression of the looped pattern can be seen over a 24 hr period, and the associated surface weather type changes, albeit more slowly than the zonal type described earlier.
However, if the 'loops' in the pattern become locked in one geographical area, then depending where you are in relation to the upper flow, the associated surface patterns are often little changed from one day to another, and in extreme cases, from one week to another - the pattern is said to be 'blocked'.
In, and just to the east of a slow-moving trough in the upper flow, the surface weather will tend to be of a low pressure/convective/showery type, and perhaps cool for the time of year (but not necessarily).
In, and just to the east of a static ridge in the flow, the surface pressure will tend to be high, with settled conditions lasting until the block is destroyed. This latter case is responsible for prolonged dry/hot weather in summer, but cold/sometimes grey conditions in winter, and considerable pollution build-up can occur at all seasons due to the stagnation of the lower level air and high air-mass stability encountered.
For a personal view of some aspects of upper air meteorology, and some further explanation of the terminology used, see "Upper air meteorology".
A trough on a mean sea level pressure chart, (or an upper air contour chart) can be picked out by an arrangement of isobars (contours) which are concave towards an area of low pressure (low contour height) along a particular axis, and that axis is defined so as to lie along the points of maximum curvature on the individual isobars (contours).
If this sounds complicated, it isn't really: the feature is analogous to the 'valley' on an OS map and defined in the same way - pressure, or contour heights 'fall' into the trough line. A front may have troughing along its length, but not all troughs are frontal! Indeed, not all troughs have 'weather' associated with them in the cloud/rainfall sense. Lee troughs found downwind of a major range of hills/mountains are often cloudless, and thermal troughs forming over land during the day due to mesoscale heating may only be found by careful drawing of isobars: if the air is dry and/or stable, little significant cloud will be associated with this feature.
A modern complication on charts used on the GTS is that plumes of high humidity...e.g. in the case of very humid/warm air coming northward out of France/Iberia, are also shown as 'trough' lines for want of any other identifier. Although with development pressure may become lower along this 'plume' than surrounding areas, and therefore qualify as a trough by the above definition, often the difference is small or initially non-existent.
Find the distance between adjacent isobars in the area that you are interested in - making sure that the isobaric interval is the same as that for which the scale was constructed - often 4 mbar. (Dividers can be used, but a strip of paper suitably marked is just as good.) Using the geostrophic scale for the correct latitude, put one end of your marked distance on the left-hand end of the scale, and read off at the right-hand end the geostrophic wind speed for that isobaric spacing at that latitude.
Remember though that many corrections are needed to find an approximation to the 'real' wind. See the Glossary and any good book relating to meteorology.
(see also "Thickness: what is it?": thanks to Jon O'Rourke for looking up the extreme values.)
As already noted elsewhere, the values of the (total) thickness between levels at 500hPa and 1000hPa give a useful measure of the mean temperature of that layer. In summer, values might range from 546dam (cool, showery northwesterly) to 560dam (warm, settled anticyclonic spell); in winter from 530dam( brisk, chilly, showery flow, with inland night frosts) to 550dam (mild, open-warm sector type). (The values are given for comparitive analysis only, and the weather types of course don't necessarily follow from the values); Values below 528dam in winter would herald the arrival of potentially wintry conditions, and in summer, thickness values above 564dam might be a precursor to some notably high temperatures.
I looked up some 1961-1990 average values for a couple of points across Britain (based on the RMetS 'Weather Log' charts & NOAA-CIRES/CDC Re-analysis project).
For the period 1991-2005 (15 years) based on my own records, thickness values have risen (relative to the above & for a point roughly within the CET 'domain') by about 1.2 dam (representing roughly +0.6degC through the layer; 1997 and 2003 showed an increase on the 61-90 climatology of about +3dam/+1.5C. This accords well with expected changes due to anthropogenic global warming.
The increase noted above across 'Central England' is broadly confirmed by data released by the Met Office / Hadley Centre. They show that, relative to the 1961-1990 reference period, 'lower tropospheric' temperatures have increased by about 0.4degC averaged over the years 1991-2005.
As to extremes, for the UK mainland only, the highest Jon & I could find (from rather small-scale maps) came out around 575dam in July over southeast Kent (SE England), and the lowest around 495dam on the extreme tip of NE Scotland in January. However, for the British Isles, we have a low value of 491dam over Shetland, and values below 500dam can be recorded in exceptionally cold easterly types as far south as East Anglia & SE England; for example, in January 1987, total thickness values in these latter areas were certainly below 498dam, and probably briefly around 495dam. (Record low UK day-maxima on the 12th January, 1987). For a graphical representation of maximum and minimum thickness values for 6 points around the NW of Europe, see "Thickness Extremes".
All are formed within an unstable environment (see "Stable and unstable air masses", and all require the following to be in place: (i) Instability through a reasonable depth of the troposphere; preferably (but NOT necessarily) extending above the freezing level; (ii) sufficient moisture to sustain the cloud-building process - medium level dryness will often kill shower formation unless low-level inflow of moisture is substantial; (iii) a trigger action - i.e. something to kick the whole process into life by lifting the parcel that goes on to grow into a moderate depth cumulus cloud, or a well-developed 'supercell' complex.
Once these conditions are met, then consideration of things like shear, CAPE, helicity, etc., are needed as follows:- (for definitions, see the Glossary, and in particular for helicity, see "What is helicity?")
Single-cell showers: the 'classic' growth/decay model of a Cumulus cloud , whereby a single moist convective cell develops in an airmass that is moderately unstable (CAPE values ~ 100 J/kg), provided of course that there is sufficient depth of moisture and there is an initial trigger action. When the updraught and the precipitation downdraught occupy virtually the same atmospheric column (there is little or no vertical relative wind shear to tilt the cloud), the downdraught quickly swamps the updraught - the shower soon decays (perhaps lasting only a matter of minutes - the cloud would last longer though), yielding small amounts of rain/snow. However, when there is a change of wind speed with height (but little directional change), the updraught column is tilted forward, and the resultant precipitation downdraught is held clear of the downdraught, allowing greater development and moderate intensity showers occur. The cold downdraught though soon swamps the inflow of surface air, cutting off the updraught and the shower decays after about 20 to 30 minutes. These events would be typical of Polar Maritime airmasses.
Multi-cell thunderstorms: Whenever wind shear is present in an unstable atmosphere, the developing convective clouds will be tilted to a greater or lesser extent. As seen above (single-cell showers), when only the wind speed changes, then short-lived, non-propagating showers are produced. However, given *both* change of wind speed and direction with height (relative to the storm motion), and sufficiently high CAPE (> ~ 250 J/kg), then the precipitation downdraught is skewed well to the side of the storm updraught, and does not interfere with it - allowing that storm cell to develop its full potential - other necessary factors (e.g. sufficient moisture) being in place. In addition, the downdraught will hit the surface and spread horizontally as a cold density current (gust front). At some point, this will meet the low-level inflow, and a new 'daughter' cell (see the Glossary) may be initiated which may grow into a full-scale storm cell in its own right. This usually (but not always) occurs to the right of the cloud motion, and the whole storm complex appears then to move to the right .. in fact the daughter cells take over from each successive parent to produce this effect. Large Cumulonimbus (Cb) clouds are produced with these processes; each cell lasting at least half-an-hour, and depending upon external forcing agents (e.g. coastal convergence, synoptic troughs, orographic lifting), the storm complexes may last for several hours.
Supercell thunderstorms: Although in some respects, 'supercell' storms can be regarded as a special (and intense) case of the multi-cell storm, there are important differences as well. The environment is still sheared in the vertical, indeed markedly so in the lower layers, and daughter cells are produced. However, a key distinguishing element between supercell and non-supercell events is the presence of a rotating updraught. CAPE values for supercell events will typically be ~1000 J/kg or more, and helicity will also be high - hence the tendency to rotation of the storm complex, and its individual elements. It is thought unlikely, for example, that giant hail would be possible unless the updraught were enhanced by the presence of rotation within the system.
The overall storm motion may be quite small (e.g. Wokingham storm), with the spawned cells forming close to the base of the parent cloud - often several daughter cells coinciding - these form an almost self-perpetuating system lasting several hours. These mechanisms produce the most severe late spring / summertime thunderstorms with local intense rainfall leading to flooding, plus occurrence of hail, possible tornadoes etc. (Note however that slow displacement of such storms should not be assumed - results from North America show displacements in excess of 40 knots / 74 km/hr ). Potential instability at medium levels (circa 500 hPa / 5 to 6km) is also required, as is an initial inhibiting factor (warm / dry air capping surface based instability) to allow the 'loaded gun' effect to build up.
(thanks to Will Hand for much assistance with this answer)
This is a derived parameter which quantifies the tendency for airflow in the lower levels of the troposphere to 'corkscrew' and thus encourage the formation of storms with strong mesoscale circulations, possibly leading to tornadic activity.
Helicity is related to:
(a): speed shear from surface to 3 km (about 700 hPa) - how much the wind speed changes over this altitude band.
(b): directional change of the wind over the same altitude band.
(c): the strength of the low-level wind contributing to the speed / directional shear (as above).
The numerically-greater each of these elements is, the higher is the helicity available for ingestion into a developing storm complex. (It should be noted that the storm will modify the local wind-field, often quite markedly: this means that caution should be exercised when using standard radio-sonde sounding data, or broad-scale NWP output to assess the likelihood or otherwise of severe local storms.)
Helicity has units of energy and can therefore be interpreted as a measure of wind shear energy that includes the directional shear. If there is no directional shear then the helicity is zero: if the wind backs with height then the helicity is negative; if it veers with height (more normal in storms in maritime NW Europe) then the helicity is positive.
Helicity is usually derived in a storm frame of reference, the 'storm relative' helicity, [ Hr ] between the surface and a height, [ h ] and is calculated as an integral between those limits thus: (Vh - C) x Wh x dh [units=m**2/s**2 ] Where [ Vh ] is the environmental horizontal wind velocity , [ C ] is the storm velocity and [ Wh ] is the local relative vorticity. Often [ Hr ] is calculated between expected cloud base and cloud top.
Studies in North America looked at the use of helicity (ignoring sign) for forecasting the risk of tornadoes. They found the following:
Helicity 150-299 ... weak tornadoes (possible 'supercell')
Helicity 300-499 ... strong tornadoes (favourable for 'supercell' development)
Helicity > 450 ... violent tornadoes
( These figures should be used with caution in the UK where helicity will normally lie between -200 and +200 m**2/s**2 )
(See also this FAQ entry)
(with thanks to Rodney Blackall for advice & suggestions with this and the following entry.)
And why do forecasters find it so difficult to get it right?
Whether snow penetrates to the surface as snow, or melts to rain or sleet on the way down depends upon the height of the 0 degC level (ZDL) above local terrain. It should be easy over relatively flat ground: forecast yes/no for precipitation and use a good forecast model (or dense network of boundary-layer radio-sonde ascents) to find the ZDL. If you are above this level, then expect snow, if below expect rain or sleet.
The 'air-mass' zero degree level is relatively straightforward to forecast. The problem is that snow situations in our part of the world often occur with surface temperatures 'around zero', and minor deviations from the air-mass (or synoptic-scale) ZDL are important, but difficult to predict. There are several factors that must be taken into account when assessing these potential variations in the ZDL. Among these are modification of the temperature profile in the lowest layers of the troposphere due to passage over warm or cold surfaces; cooling due to evaporation of the precipitation elements as they fall through the air and cooling due to latent heat exchanges when snow begins to melt in situations that are 'marginal'. These, and other modifying effects, are discussed this FAQ entry, but they will alter, sometimes dramatically, what type of precipitation actually reaches the surface.
In many of the countries of 'maritime' NW Europe, the major conurbations and the principal highways lie below the 200 m (circa 650 ft) contour. Variations in the low-level temperature structure, often involving changes in intensity from 'light' to 'moderate' or heavier precipitation can cause chaos, yet be difficult to predict and protect against except with very vague generalisations within forecasts. They are also difficult for road, rail & airport authorities, as it can be raining quite happily for several hours (when no precautionary measures can be taken), then all of a sudden, several cm of snow will accumulate as the precipitation intensity changes - or perhaps freezing rain is the result with obvious consequences. A ground height change of more than 30 m (around 100 ft) is quite normal within a town, so it is not uncommon for sleet to fall in one part of the town causing few problems, but snow in another spot nearby.
1. SYNOPTIC-SCALE MODIFICATION of the temperature structure of the lower troposphere. If the air passes over the sea (or similarly warm surface), then the sensible flux of heat to the air above will raise the ZDL, perhaps tipping the balance towards rain or sleet, rather than snow - windward coastal plains may miss out on the worst of the snow. ( However, these same areas may be the only places to experience moist convection in winter and provided the air is cold enough, and the sea is close and upwind, then snow showers can be frequent. ) Heat from major urban areas (provided areally extensive) can also tip the balance in highly marginal situations. If the air passes over an ice or snow-covered surface, then a flux of heat from the air to the surface occurs, modifying the ZDL structure, usually resulting in a sharp, shallow inversion. The air-mass (highest altitude) ZDL is unlikely to be affected but a secondary pair of ZDLs may form as the thermal structure of the lowest 300m is distorted and either freezing rain or ice pellets, rather than 'proper' snow is the result. This is often a difficult situation to get right after a long cold period is trying to break down.
2. EVAPORATIVE COOLING of the air through which the snowflakes are falling. Even with the most intense precipitation, there is always lots of air around the falling raindrops or snowflakes and evaporation of the precipitation elements will occur. This will lead to a microscale cooling (due to latent heat exchanges as the liquid/ice evaporates), which multiplied by the huge number of precipitation elements leads to a net cooling of the environment through which the droplets/crystals are falling. This in turn leads to a lowering of the ZDL. The effect is proportional to the precipitation intensity ( and inversely proportional to the mean wind speed through the melting layer ) and is greater when the ambient relative humidity is well under 95%.
3. PHASE-CHANGE COOLING of the air through which the snow is falling. Rain/snow situations are often marginal at low altitudes. This means that more often than not, snow is melting in the lowest 200m or so and thus the environment is cooled due to heat exchanges consequent upon the melting of the ice crystals into liquid water. Again, intensity of precipitation is a major factor - greater intensity means that there are more precipitation elements involved which means greater overall cooling. The effect compounds that at (2) above, the net effect of evaporative and phase-change cooling is quite significant - lowering the ZDL by some hundreds of metres in prolonged precipitation. This is especially pronounced in stable air and catastrophic in near-isothermal conditions in a frontal zone. Some of the worst low-level icing conditions for aircraft occur in these situations, and of course, the ground isn't very far away!
4. BULK (DOWNWARDS) ADVECTION of cold air due to drag by precipitation elements and by downdraughts in a markedly convective environment. Another effect that is related to precipitation intensity is the cold air that is dragged down by the falling elements and the associated downdraughts. Descending air warms adiabatically so this introduction of colder air from upper levels is offset somewhat and is the least effective modulator of those considered above. [ However, in such situations, the relative humidity will fall (greater separation between air and dew point temperature) so evaporative cooling will become more effective - see (2) above. ]
5. OROGRAPHIC UPLIFT COOLING. As air in a thermally stable environment is forced to rise over a range of hills or mountains then the adiabatic cooling will cause the temperature to fall with height more rapidly than in the undisturbed environment. This will lower the ZDL allowing a greater downward penetration of the snow that might otherwise be expected. (This is important in a few of our major towns and cities that rise into the 'foothills' of major hill ranges, e.g. Manchester, Sheffield & Bradford.)
6. FALLING OR SETTLING PROBLEM. Apart from the factors mentioned at the end of (1) above, snow is pretty well guaranteed to fall and settle if the surface temperature is at or below 0 degC. Rain is almost guaranteed if the surface temperature is above 4 degC. In between there is a degree of uncertainty and quite small changes in intensity can switch between sleet and snow, and between snow thawing faster than it falls or vice-versa.
In the 17th century, when the concept of the barometer was first developed and refined by Torricelli & Pascal (amongst others), atmospheric pressure values were noted in terms of the height of the mercury column supported within a tube which had one end closed and the other end immersed in a bath of the liquid exposed to the atmosphere. Barometers continued to be marked in units of length (inches or mm of mercury) long after aneroid barometers became a common instrument - hence even today it is not unusual to see barometers marked in either inches (British / Imperial and US sources) or millimetres (European / Continental sources).
1 millibar (or hectopascal/hPa), is equivalent to 0.02953 inches of mercury (Hg). It is therefore only necessary to multiply a reading in millibars by the latter figure, to achieve the required conversion. E.g. for 1023 mbar, multiply by 0.02953=30.21 inches. To go the other way, the relationship is 1 inch of mercury=33.8639 mbar; again as an example, to convert 29.45 inches, multiply by 33.8639=997 mbar. (For those of you reading this on the continent, you are more likely to be dealing with millimetres, and the appropriate conversions are: 1 mbar=0.750062 mm Hg and 1 mm Hg=1.333224 mbar. )
The dew point depression (often abbreviated to DPD), is the difference between the air temperature and dew-point of a sample. It can refer to surface (i.e. screen) temperature values, or to measurements in the upper air. The larger the value (wider separation between air temperature and dew-point), the lower the relative humidity.
Surface values of DPD (screen or psychrometer measurements) are often used in empirically derived algorithms to forecast overnight minimum temperatures, fog-point temperatures, height of convective cloud bases, likely stratus base due to turbulent-mixing etc. For example, a high afternoon DPD value would suggest low relative humidity for that air-mass (afternoon values usually being assumed to be representative due to good mixing of the boundary layer air), and assuming no air-mass change, the night minima will be relatively low, as would the fog-point temperature. Conversely, if the DPD value was small in the afternoon then, other factors being right (light wind, clear skies etc.), then mist/fog would be a high risk for the coming night - or low cloud if the wind were just a touch stronger. Daytime cumulus bases will be lower with small DPD values, than with a greater separation between air temperature and dew point.
Upper air values are often used to assess likely layered cloud amounts, again other things being equal - i.e. sufficient uplift to lead to condensation and a stable environment. On a thermodynamic diagram, if the 700 hPa DPD is less than about 3 degC, then expect thick, layer cloud to be present. Dew point depression values are also used in the study and forecasting of severe convective storms.
The speed of sound in air is considerably less than the speed of light. It is therefore possible to calculate the distance of a storm, provided you can observe the associated lightning, and the thunderstorm is close enough for the sound (of the thunder generated) to reach you (*).
A 'thunder-clap', or the noise our ears hear (ranging from a sharp crack for nearby, short path-length lightning strikes to a long, low rumble for a distant, long path-length discharge), is caused by the rapid expansion (due to heating) of air in the lightning channel that the stroke passes through. ( If you are frightened of thunder, it is useful to remember that by the time that noise has reached you, the really dangerous lightning strike has already occurred. )
Very roughly, the speed of sound in the lower atmosphere is 330 metres / second (or 1 mile / 5 seconds). If you observe the 'flash' of the lightning stroke, then begin counting (or timing) in seconds until the sound of the thunder just reaches you, using the relationships above will give a rough guide to the distance of the storm. Thus a time difference of 3 seconds will give you three-fifths of a statute mile, or about a kilometre; 6 seconds=> 2 km etc.
((*) thunder, under average atmospheric conditions, should be audible up to 8, possibly 10 km away. Strong winds & low-level temperature variations may though alter these values considerably, with values quoted up to 20 km in some cases; lightning at night can be seen up to 100 km away, depending upon your observing location, obstructions etc.)
[ Also see this FAQ entry ]
In the 'free' atmosphere on our rotating earth, the movement of air is forced by differences in atmospheric pressure between one location and another: this difference, over a specified distance, is known as the PRESSURE GRADIENT.
It might be assumed that once there is a pressure gradient, that air would travel directly from high pressure to low: this doesn't happen, because as soon as it begins to move, it undergoes an apparent deflection owing to the fact that we live on a rotating planet. In the Northern Hemisphere, the 'deflection' is towards the right of air motion; in the Southern Hemisphere it is towards the left. A balance is achieved whereby the force due to the Pressure Gradient (PRESSURE GRADIENT FORCE, or PGF) exactly equals the deflection due to planetary motion (the CORIOLIS DEFLECTION or ACCELERATION [CA]).
The wind direction is that summarised in Buys Ballots Law (q.v.)
The wind resulting from these ideal conditions is known as the GEOSTROPHIC WIND - a theoretical wind (as measured on a chart using a GEOSTROPHIC SCALE) that assumes the following:
(b): the wind is not speeding-up (accelerating) or slowing-down (decelerating) along the line of travel (isobars/contours are parallel).
(c): the pressure pattern is not changing (no atmospheric development).
(d): there are no frictional forces at work, either molecular, or due to passage over tangible obstacles (e.g. the surface of the earth).
(e): the wind blows horizontally - i.e. there is no vertical motion involved.
Looking at these factors in order:
(a): Unless the air motion is gentle, this factor must be allowed for by finding the curvature of the isobars/contours and subtracting (cyclonic curvature) or adding (anticyclonic curvature) a correction depending upon the strength of the geostrophic wind & the radius of curvature of the isobar/contour. For a tightly curved isobaric/contour pattern around a small-scale low pressure area in mid-latitudes, the value will be in the range 10 to 30 knots at geostrophic wind speeds of over 80 knots. Equally important is the anticyclonic correction: even for a gently curved isobaric pattern around a slow-moving anticyclone, an extra 5 to 10 knots can be added to the theoretical calculation - enough for example to tip the balance between a 'Force 7' and a 'Gale'! (Unlike the cyclonic correction, there is a theoretical maximum for the anticyclonic correction of twice Geostrophic value).
If the pattern is curved, then the corrections applied above will give rise to the term known as the GRADIENT WIND, which is loosely taken to be the 'free-air' value (very roughly around 900 mbar/900 m), from which the wind at the surface can be obtained - (see below).
However, even this is problematic, as it assumes that the pressure system is not moving, i.e. the air takes a path exactly governed by the pattern on the chart. In reality, especially with developing depressions, the movement of the low itself will mean that the air-path adopts a differently curved trajectory to that of the feature generating the winds. In particular, for fast-moving depressions in mid-latitudes, the geostrophic (theoretical) wind speed is probably a better approximation to the 'free-air' flow than trying to apply a correction due to curvature.
(b): When air speeds up or slows down due to changing pressure gradients along the line of travel, then motions are imparted which cause the air to deviate from the ideal path. This flows naturally from the fact that the ideal wind is trying all the time to achieve a balance between the PGF and the Coriolis Acceleration (CA). As the latter is proportional to the wind speed, any change in same will alter the deflection; the 'flow' will become unbalanced, and a 'correction' needs to be achieved, but not before the air has deviated somewhat offline (so-called 'CROSS-CONTOUR FLOW'). The effects are very important at jet-stream levels, where they lead to cross-contour movements of sufficient magnitude to cause development in the column of air below. (e.g. Jet exits, entrances), and also give rise to the potential for clear air turbulence (CAT).
(c): This is important near the surface where there are tight gradients of pressure tendency, i.e. as found in advance of, and particularly to the rear of some rapidly developing cyclonic disturbances in mid-latitudes. These ISALLOBARIC effects can be to add (rapidly rising pressure), or subtract (rapidly falling pressure) well over 10 knots to the theoretical geostrophic wind, and can be as much as 40 knots in the most 'damaging' case, where a explosively deepening low has passed by, and pressure rises sharply behind due to events in the upper air (confluent trough, q.v.).
Couple this effect with the possible downward penetration of momentum from the upper atmosphere associated with the dry intrusion (q.v.), and some highly damaging wind-gusts can result.
(d): Of all the 'controlling' additional forces, this one is the most evident from day-to-day. It is the reason why the wind that we observe blows across the isobars at an angle (from high to low pressure), rather than directly along the isobars. As friction due to the roughness of the earth's surface takes hold, the PGF gains dominance over the CA, and 'turns' the wind away from the beloved 'tramlines' of the tv weather presenters. The greater the roughness (i.e. towns, cities) the greater the effect; for this reason, wind directions over the open sea are more closely allied to the isobaric direction averaging just 8 degrees departure, as opposed to some 20 degrees or more over land.
Additionally, overland the stability of the air must be considered. Highly stable air will damp vertical exchange (or mixing) between the friction-less/'free' air (roughly above 900 mbar/900 metres) and the air in direct contact with the earth's surface (within the atmospheric boundary layer). In such conditions, the surface wind can be backed from the isobaric flow by as much as 40 degrees (perhaps more), and this effect is particularly enhanced at night in conditions of light gradient. Conversely, in unstable conditions, where vertical mixing is particularly effective, then the 'real' wind direction is sometimes only 10 or 15 degrees away from that taken from the isobars.
Well, it all depends who you talk to! Meteorologists make a clear distinction between hurricanes, a regional name given to tropical cyclones occurring in the tropical Atlantic and east Pacific basins; and intense mid-latitude storms. Reference to the following will show why:-
Hurricanes, or tropical cyclones, form in an environment of little or no vertical wind shear. Vertical shear (which doesn't have to be throughout the troposphere, but can also be over a very shallow layer) destroys the convection around the centre of the tropical cyclone (the 'eye'). For a tropical cyclone to continue to develop, there must be inflow of warm air at lower levels, and upper level outflow: the convection provides the 'pathway' for the necessary rising air. They form over very warm waters - sea surface temperatures (SST) values above about 27 degC, equator-ward of the sub-tropical anticyclone belt, with the capture/inflow of water vapour and sensible heat from the tropical oceans being essential to the physics of the storm. The storm is warm core, especially in the lower troposphere, with little overall (synoptic scale) 500-1000 hPa thickness gradient, and therefore hurricanes have no 'Norwegian model' fronts associated with them. Low pressure is primarily due to the contrast between the warm core of the storm, and the unperturbed tropical environment, with some contribution from compressional warming of descending air within the 'eye' of the storm, and the speed of movement of the storm is generally less than 15 knots.
(NB: the name 'tropical' should not be taken to mean that these features only form between the Tropics of Cancer and Capricorn; they can develop well poleward of the 'tropics' and indeed persist with tropical characteristics [up to 'Hurricane' category], well into the normally accepted 'mid-latitudes': it is the storm structure - as defined above - that is important, not the exact location of the disturbance.)
Mid-latitude severe storms form in a strongly sheared environment, such as is found at high levels around the parent Polar Front jet core - jetstreams play no (direct) part in the formation of a hurricane. The formation of a mid-latitude storm is triggered by a short wave eastward moving disturbance embedded in the upper flow, with consequent distortion of the pre-existing baroclinic (i.e. frontal) zone. There is a strong 500-1000 hPa gradient involved. There is appreciable disturbance of the tropopause in the vicinity of the storm, particularly to the rear in the 'dry slot', and this is thought to be important in that it indicates the intrusion of dry stratospheric air - a key ingredient in the 'explosive cyclogenisis' aspect of these storms. (At the present time, it is not known for certain whether stratospheric air is involved with tropical cyclones: studies are underway to investigate this). The low (or lowering) pressure at the surface is due to an excess of divergence of mass aloft over convergence below, coupled to strong warm advection. The speed of movement of such storms are often in excess of 30 knots.
So far, so good - meteorologists are not going to get the two phenomena mixed up, but when looking at the October 1987 storm that hit the southeast of England, these clear scientific differences must be balanced against the reality of the event. For example, some very warm/moist air was entrained in the storm, possibly the remnants of a former tropical cyclone. Although the 10 minute mean winds in most cases failed to reach the threshold of 64 knots for a hurricane, two reports within the circulation over/adjacent to the English Channel did exceed this threshold, and although not analysed, 1 minute means, which the Miami NHC uses to classify hurricanes, almost certainly would have reached or exceeded this level, particularly when set against observed gusts of 70-90 knots or more, which are easily attained in mature tropical cyclones. There was widespread damage and disruption, with millions of trees damaged or felled, several people dead, ferries stranded on windward shores and given these facts, it easily matched the OED definition of a hurricane.
Prior to dawn on the 16th October, 1987, the image most members of the general public had of the damage wrought by hurricanes came from television pictures from the US or the Far East. The folk of the south east of England then are surely to be forgiven if venturing out and finding the car under a substantial tree, or whole communities cut off from electrical power, they refer to this event as "... the 'hurricane' of 1987".
(help with information relating to tropical cyclones was supplied by: Sim Aberson, a meteorologist with NOAA's Hurricane Research Division in Miami, Florida.) For more detail, visit the Tropical Cyclone FAQ at: http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html.
[ For more on the use/abuse of the phrase "Hurricane Force 12" see:- HERE ]
The Shipping Forecast, which is provided by the Met Office (under a contract with the UK Maritime and Coastguard Agency), and broadcast four times daily on BBC Radio 4, is highly structured to maximise the use of the available time. The basic order of the forecast is:
- GALE WARNINGS IN FORCE
- GENERAL SITUATION
- AREA FORECASTS: WIND DIRECTION/SPEED: (SEA STATES**): WEATHER: VISIBILITY: (SHIP ICING IF APPROPRIATE)
- COASTAL WEATHER REPORTS AROUND BRITISH/IRISH COASTS (*)
(*) From April 6th, 1998, certain bulletins no longer carry coastal weather reports.
(**) from 2006, some versions of the bulletin under header FPUK71 EGRR have included sea states.
Most of the forecast is self-explanatory, but in the synoptic preamble, and in the weather reports which follows, some terms are used which may not be familiar.
Movement of pressure centres: (in forecast preamble/general situation)
|Slowly||up to 15 knots||(approx: up to 8 m/s or 28 km/hr)|
|Steadily||15 - 25 knots||(approx: 8 - 13 m/s or 28 - 46 km/hr)|
|Rather quickly||25 - 35 knots||(approx: 13 - 18 m/s or 46 - 65 km/hr)|
|Rapidly||35 - 45 knots||(approx: 18 - 23 m/s or 65 - 83 km/hr)|
|Very rapidly||over 45 knots||(approx: over 23 m/s or 83 km/hr)|
Pressure changes:(in coastal station reports/3 hours is a 'standard' time period used in synoptic meteorology in mid/high latitudes.)
|Steady||Change less than 0.1 mbar in past 3 hours|
|Rising/Falling slowly||Change 0.1 to 1.5 mbar in past 3 hours|
|Rising/Falling||Change 1.6 to 3.5 mbar in past 3 hours|
|Rising/Falling quickly||Change 3.6 to 6.0 mbar in past 3 hours|
|Rising/Falling very rapidly||Change more than 6.0 mbar in past 3 hours|
Veering/Backing of wind: When a wind direction changes such that it moves with the clock, e.g. from east to south through south-east, that is a veering wind; A wind therefore that changes against the normal clock motion is a backing wind.
...and for the visibility categories the following apply:
|FOG||< 1 km||< 1100 yds|
|POOR||1 to 3.9 km||1100 yds to 2 nautical miles|
|MODERATE||4 to 9 km||2 to 5 nautical miles|
|GOOD||>=10 km||> 5 nautical miles|
Since 2006, sea states (strictly wind-wave forecasts) have been included in some versions of the bulletin: they employ a crude relationship between the forecast wind and expected wind wave heights (mean of a well-formed wave train) IN OPEN WATER. The categories are as under:-
|Description ||Height in metres|
|Calm||0.1 or less|
|Smooth||>0.1 to 0.5|
|Slight||>0.5 to 1.25|
|Moderate||>1.25 to 2.5|
|Rough||>2.5 to 4.0|
|Very rough||>4.0 to 6.0|
|High||>6.0 to 9.0|
|Very high||>9.0 to 14.0|
As part of the Public Met. Service, the Met Office maintains the National Meteorological Library and Archive (NML&A), which are open to all, particularly those with an interest in meteorology, both amateur and professional. The main facilities are located at Exeter, Devon (see below for Scotland and Northern Ireland). [Historical note: prior to autumn 2003 (Library) and autumn 2004 (Archives), these facilities were in Bracknell, Berkshire.)
use this url to obtain more details:-
(NOTE: there is now an on-line search facility available via this url)
The Library houses weather summaries extending back well into the 19th century, and has an excellent collection of literature, covering most of the earth sciences. It also holds most of the specialist scientific journals on the subject - several from volume 1 number 1, e.g. Weather, Meteorological Magazine, Weatherwise, Journal of Meteorology etc. The Archive holds weather charts from 1867 and observation registers for many sites at home and abroad. (For Met.code buffs, the Library also holds copies of the WMO international coding manuals.)
It would be advisable, before making a lengthy journey, to contact the Library Information desk to discuss your requirement and confirm opening times etc. See the web site for contact details.
email@example.com for the Library
firstname.lastname@example.org for the Archive)
Archives for Scotland & Northern Ireland are held in Edinburgh and Belfast respectively.
Not long after the electric telegraph made simultaneous (i.e. 'synoptic') observations possible in near 'real time', it was realised that in regions of 'disturbed' weather, (i.e. close to what we now call a depression), two different 'streams' of air could often be found converging into the disturbed zone - each having markedly different properties.
In the British Isles, Robert FitzRoy, the first director of the Meteorological Office is usually credited with highlighting this fact in 1863, though other workers, particularly in France, Germany, Holland and the United States were thinking along the same lines at the same time. Upon the death of FitzRoy, the concept tended to falter, until later workers took up the theme and elaborated upon it: Abercromby in 1887, Napier Shaw and Lempfert in 1911 and of course by the 'Bergen school': V and J Bjerknes and H. Solberg and others during and just after the Great War.
These latter workers proposed the now familiar 'Norwegian model' of the life-cycle of a mid-latitude depression, whereby a minor wave develops along the boundary between two well defined air masses, amplifies (develops) and is carried forward in the general flow. The poleward air mass has an east-to-west component of air motion at low levels, is relatively cold (ex. Polar), and therefore dense, and has a relatively lower humidity value (lower dewpoint) than the 'opposing' air mass. This latter has a generally west-to-east component of motion (at all levels in the troposphere), is warmer (ex. sub-Tropical) and therefore lighter, and has a higher humidity/dew point value. The colder air mass was designated Polar Maritime, and the warmer air mass Tropical Maritime. The boundary between the two air masses came to be known as the Polar Front (see also here and here for other FAQs in this area).
An air mass is classically defined as a large body of air (many hundreds to a few thousands of km in extent),having quasi-uniform horizontal temperature and humidity characteristics. Indeed, once upper-air soundings became available on a regular basis, it could be seen that this uniformity extended vertically, such that each air mass has a distinct vertical profile of temperature and humidity.
To attain these uniform (or nearly so - nothing is that clean-cut in meteorology!) signatures, a large body of air has to remain over one area for a considerable time - measured in weeks rather than days. This requires a pressure pattern which allows stagnation of the air - and this usually means a slow-moving anticyclone such as is found in the great sub-tropical high pressure belts, the polar high pressure regions or the Asiatic (or other great continental) winter anticyclones. These are said to be the 'source' regions of an air mass. Once an air mass leaves its source region, it is modified, depending largely upon the type and temperature of the underlying surface over which it moves.
For example, air that moves polewards from the sub-tropical high pressure belts encircling the earth will be cooled from below as it passes over progressively colder seas, and this will in turn affect the relative humidity (increasing it leading to formation of cloud/precipitation), and although these processes may slightly lower the absolute humidity, it will still have a higher humidity value than air coming from polar latitudes, which will be warmed from below and will become increasingly buoyant as heat is input to the lower layers.
Air Masses can be classified as 'polar' (having originated in cold/high latitude regions), or 'tropical' (having come out of the stagnant regions around the sub-tropical high. They are further sub-classified as either 'maritime': having passed over a sea surface, or 'continental' having moved over a land mass. This then gives rise to the four principal types of interest to us in north-west Europe:- tropical maritime (Tm or mT), polar maritime (Pm or mP), tropical continental (Tc or cT) and polar continental (Pc or cP). There are of course many modifications , and a full treatment of air masses is outside the scope of this FAQ. See the list of recommended reading.
(thanks to Keith Dancey for this answer....which is his reply to a question in the newsgroup.....)
If more heat is pumped into the system (system=earth) then more water vapour will be put into the atmosphere. How that would affect our (local) weather depends upon how it would affect the world's climate, and how the world's climate affects our (local) weather. Precise answers to these questions are not known. There are climate models which can be run, and they are improving, but they are not 100% accurate, and they may never be! Such models require knowledge of the atmosphere and oceans that are beyond us at the moment, and computing power that can represent all the processes that are going on all the time. A rather tall order. You might be interested to learn that the Gulf Stream (a natural phenomenon that defines, to a large extent, the UK's mild climate for it's latitude) might even become disrupted under certain conditions in some ocean models. So whether global warming is happening, and how far it might go, is really very important, even to us.
Global warming, per se, can be tested by measuring the average temperature of the surface of the sea, and keeping records for a long time. We have historical records, of varying accuracy and varying coverage. We now have instruments orbiting the globe that can measure the sea-surface temperature to breathtaking accuracy. The data indicates warming. The period is rather short. But we don't know (for certain) that this is because of us (human economic activity) or some natural phenomenon that we have yet to discover. Most scientists working in the field believe the former. Increased rainfall (and other local climate change) for the UK and Europe can indeed be an outcome of global warming. When a possibly chaotic system such as the world's climate is perturbed, it might be impossible to predict the outcome, other than there is going to be change. Global warming does not necessarily mean "drier". It certainly does not mean "drier everywhere". And it also does not mean "warmer everywhere".
(NB: 'synoptic' in meteorology is used in the sense that the weather is analysed over a wide area at approximately the same time.)
A. In the early 1950's, Hubert Lamb expanded upon a classification system originally proposed in an article in 'Weather', into the now widely used Lamb's circulation types. The late Professor Lamb(*) was responsible in the UK for much work involved with deciphering the climatological changes that have undoubtedly occurred, and will continue to occur. Indeed, in the early days, the work was rather unfashionable, but is now required study given current concerns. Professor Lamb consolidated his distinguished career by taking a professorship, and the post of first director, at the University of East Anglia's Climatic Research Unit. (see the section on climate change). The system is based on the analysis of the direction of the overall isobaric pattern (not the individual wind direction at any one place) over the region 50-60N, 10W-02E. Once one of the 8 compass point directions, from which the wind blows is allocated, the curvature of the flow is considered, and the directional letters are prefixed by either: A, anticyclonic or C, cyclonic or it is left unclassified (neutral or irregular), when a qualifying letter is not used. Three other categories are recognised: A=anticyclonic (i.e. a notable high pressure itself over the region), C=cyclonic (i.e. a notable low pressure over the region), or U=unclassifiable. This gives rise to 27 classes.
Visit the UEA site to find out more, and to view the catalogue maintained by them.
(*) Professor Lamb died on Friday, 27th June, 1997
The British Isles are well served by English language magazines, periodicals etc., that cover a wide range of interest in the subject, from the keen amateur to the 'cutting-edge' academic end of the spectrum. More details are set out in the section on reading material. Also, several sites listed elsewhere have good links/information on such matters: for example:
Indeed YES! For a start, the uk.sci.weather newsgroup itself is always a good place to post if you've seen something that might be of interest, particularly in the 'unusual' or 'severe' category. The best way to approach this is to 'lurk' for a short while to get an idea of what interests us, then dive in when you feel happy. If you are interested in the 'weather', this is the place for you. To try and help, there is a complete section in this FAQ which deals exclusively with weather observing ... not intended to be exhaustive, but might give you some ideas, tips etc.
To aid those who do not want to plough through observational data, we have a system of 'indicators' [WR] and [OBS] used in the Subject line of any thread containing observational remarks; for more on this see here.
With notable 'convective' weather, e.g. a decent thunderstorm, whirlwinds, tornadoes, etc., then TORRO (see here) would be interested in a report. Visit their site at: http://www.torro.org.uk/
And, how about becoming a member of the Climatological Observers Link? This organisation has been in existence since 1970 and is open to anyone with an interest in meteorology. Both amateurs and professionals are registered observers. Visit: http://www.met.rdg.ac.uk/~brugge/col.html for more details.
Also, a UK Weather Diary can be viewed and added to, which can be seen at: http://www.met.rdg.ac.uk/~brugge/diary.html
[ Although the national domain is 'uk', don't be put off reporting/joining up to the above outside the United Kingdom. The weather knows no national boundaries! ]
And now (autumn, 2000), the BBC Weather Centre is keen for you to email weather observations direct to them ... they will take reports on: email@example.com
(This note prepared with the help of: Dr. Rob Wilby, Department of Geography, University of Derby.)
In very crude terms, it is possible to visualise the mean sea level pressure patterns affecting the north-east Atlantic as varying (or 'oscillating') between two extremes: At one extreme is a minimally perturbed westerly type, with disturbances rattling swiftly across the Atlantic, hurried along by very strong mid / upper tropospheric winds. At the other extreme lies a weak, perhaps ill-defined pressure pattern, but with a strong tendency for stagnation of weather types over and downwind of the north-east Atlantic (a persistent 'blocked' type). [ use of the word 'oscillation' in the popular mind implies some sort of regularity, which in reality is not observed, at least not on short time scales. ]
One method is to use a measure of the 500 mbar strength between defined latitudes: By taking the difference between mean 500 mbar contour heights between latitude 35 and 60 North, this simple method yields high numbers ( a high zonal index ), for strong westerly types, and low numbers ( a low zonal index ), for weak westerly, or blocked types. Another method would be to categorise circulation types using, for example, Lamb's Weather Classification.
However, a simple measure, using observed msl pressure differences from long-term 'normals', can be employed, and can of course be extended back to times well before upper air information became routinely available. Upwind of the British Isles / NW Europe, stations in Iceland and the Azores (and / or Gibraltar) are used, by convention, to define the North Atlantic Oscillation Index (NAOI).
The method of calculation means that lower than normal mslp over the Iceland region and / or higher than normal mslp in the Azores / Gibraltar zone gives rise to a +ve NAOI. The converse situation gives rise to -ve NAOI values. The Index is of most use during the winter, when highly positive values are associated with warmer, wetter, windier winter seasons, especially over NW Europe.
In a record since 1823, the following major 'divisions' can be identified in the winter (December, January, February) season:
( @ Note: The strongly positive NAOI since the 1980s has resulted in a higher frequency of unusually wet and mild winter conditions over most of NW Europe and Scandinavia during this period, with concomitant changes in regional runoff.)
to see data relating to the NAOI go to: http://www.tiempocyberclimate.org/portal/datanao.htm and other interesting links therefrom.
In studies of the climate for any region, locality etc., it is important to have a homogeneous record to describe such atmospheric variables as rainfall, sunshine, temperature which eliminate as far as possible changes in site (both location and characteristics), observing practice and so on. Professor Gordon Manley (1902-1980) developed one such series which dealt with the temperature of 'central England', defined as the area stretching from the Lancashire Plain southwards across the Midlands, and constructed using stations in the Lancashire (including the modern-day Merseyside/Gtr.Manchester) area, and the east and west Midlands.
The series has been maintained since his death, although there have been changes in stations used, as some have closed/altered. Corrections also have to be applied, particularly to latter-day observations to take account of urbanisation, however the 'CET' series remains one of the longest and most widely used of its kind in the world. The record now extends, on a monthly basis, back to 1659, and on a daily record back to 1772. However, the values prior to 1721 are regarded as less reliable than later data, a fact acknowledged by Manley amongst others. Nothwithstanding this caveat, useful clues to changes in climate can be gleaned from this work. It has also been shown that the CET series is a statistically useful indicator of changes of mean temperature for a somewhat wider area than just the 'English Midlands'.
To see the monthly series maintained by the University of East Anglia go to:
... and for data, and more information on the data-set maintained by the Hadley Centre/Met Office, go to:
Remember though that values are often revised after initial issue: it is a good idea to check back periodically to capture such changes.
The 'El Nino' phenomenon, or more strictly the warm El Nino -Southern Oscillation (ENSO) event is coupled closely to remarkable shifts in weather patterns in the immediate Pacific basin, and adjacent areas: e.g. parts of North America. For example, it is clear that the altered distribution of warm/cold water across the equatorial Pacific is the primary reason why excessive rain can fall in places like Peru, and a general deficit of rainfall is experienced in Indonesia, parts of Australia and the Philippines. There is also a generally accepted link between a less-than-'normally' active Atlantic hurricane season and the notably warm event that characterises what has come to be called, THE El Nino.
It is becoming clear from recent studies that we can now rule out the 'No Effect' case: this leaves us with two options -
(a) There IS an effect, but it is on a scale that is dwarfed by regional variations closer to home, e.g. long-term thermal inertia in SST distribution in the N. Atlantic, or continental/oceanic temperature differences across the North America - North Atlantic - Eurasian 'super-region'.
(b) There is a direct, and marked effect that leads to verifiable modification of the weather types across the NE Atlantic/European - Mediterranean region.
(a) appears to be the most likely if we take the year overall; indeed, even in studies published which set out to prove the link between warm/cold ENSO regimes, and impacts over Europe, caution is always advised relating to local/regional scale modification.
(b) is climbing higher in the 'probability' stakes, at least if the 'winter' season only is considered. There are an increasing number of studies published that show a direct link between a warm ENSO season, and, for example, altered rainfall/temperature anomalies across west/central Europe. No lesser person than J.Bjerknes postulated in 1966 that altered activity in the equatorial Pacific appeared to significantly alter the strength/orientation of the PFJ over and downwind of the NE Pacific, which in turn must have at least some effect on the long-wave structure downstream. This appears to have been accepted in later studies & developed further using datasets going back over two centuries or more.
However, this topic will be kept under review, as will this Q/A, and our ideas may change ... for the moment though, for more on El Nino/ENSO etc., see the following sites:
[WMO home page]
[El Nino theme page sponsored by NOAA/TOGA-TAO]
... and of course, a search of the WWW will throw up many active sites dealing with El Nino.
In addition, on this site there is set out in summary format some of the arguments/references that subscribers to uk.sci.weather asked for. See it here.
When a lightning discharge occurs, radio waves are emitted over a broad spectrum of frequencies. For the vast majority of people, such 'atmospherics' (or 'sferics') are simply a nuisance, leading to the familiar 'crackle' that can be detected on a home radio set, particularly in the 'AM' medium or long wavebands.(@see note 1 below)
However, in the 1920's and 1930's, Robert Watson-Watt, a British scientist (and sometime employee of the UK Meteorological Office), developed a method of displaying the discharge information on a crude cathode ray tube, and by taking simultaneous observations on the same flash, the source could be located with reasonable accuracy. (@see note 2 below) This triangulation method continued in use in the United Kingdom until 1988.
The system now employed by the UK Met Office is the Arrival Time Difference (ATD) system. The origin of the lightning flash is computed from the time difference of an atmospheric arriving at several widely-spaced 'listening' stations - located across the UK, the Mediterranean & northern Europe. Each lightning stroke has an individual signal, or wave-form (in the Very Low Frequency part of the electro-magnetic spectrum), and by using accurate (atomic) clocks, and synchronisation between the detector stations, and the control station (Exeter, Devon (UK), originally at Beaufort Park, near Bracknell), an accuracy of at least 5 km (often down to 2 km) can be achieved, although in GTS SFUK bulletins (known within the Met Office as SFLOCs - SFeric+LOCation), the accuracy is limited by the code form to 0.5 deg lat/long.
One of the listening locations is automatically designated as a 'reference' (by the control station) for each discharge, and reports the time at which it detects a 'sferic' event to the control. The control station then 'asks' the remainder of the outstations for detailed wave forms of sferic events close to this time and calculates the time differences and so computes a set of possible locations. Provided more than three outstations are active (out of the 7 available) in acquiring that event, a unique location can be determined for that particular return.
The system is highly effective, though on a few occasions, 'spurious' returns can be seen (usually easily eliminated by reference to IR imagery); also when very active clusters are detected, isolated events elsewhere may not be adequately resolved - though these are now rare events. In addition, be wary of occasions when lightning returns 'appear' to all stop at once; this is usually due to a failure of the central computer, which needs re-setting before the system will function properly.
The system is fully automatic, and theoretically can detect lightning over a large portion of the globe. The system is in a state of continuous development - a major upgrade having been put in place in Autumn, 2001.
For more on the system see the Met Office Education page at this link:- http://www.metoffice.gov.uk/education/secondary/students/lightning.html
(@1:This means that an ordinary home radio set can be used as a crude lightning detector, by tuning to a portion of the waveband -- try the LW section -- that is not used by a broadcast station. During lightning activity, irregular crackles will be heard, and with a little experience it will soon be possible to pick out 'close' from 'distant' discharges by this method - a good reason to hang on to your old portable radios after the 'digital revolution'! )
(@2:This use of triangulation of signals was later adapted in his method of aircraft detection used during the early part of the second World War.)
Unless you are situated at some considerable altitude ... say above about 3000 ft (about 1000 m), then it is best for *home* use to have your barometer indicate the pressure at mean sea level. You can then relate your reading to those in newspapers, television charts etc. However, you should not expect a high degree of accuracy when using many barometers bought for 'decorative' use, and if you intend making weather reports for the synoptic network using a precision aneroid barometer (or similar), then the appropriate professional authority that collects and checks weather data should be consulted - the procedure is very different and involves careful periodic checking against a reference barometer and the use of correction tables/algorithms. (See also the Observer's Handbook)
For most people though the following will suffice:.... Choose a day when the atmospheric pressure is not changing greatly...in association with a slow-moving anticyclone is best (but see also below re: checking over a range). Log onto a site that gives out hourly METAR reports (see UK Weather Information for soem good sites), and pick a station/airfield nearest to your location. If there is no such location, then you may have to plot out several reports for the same time...draw a few simple isobars...then interpolate to find a value. With most home barometers, the nearest whole millibar is about the most you can expect in accuracy. Adjust the barometer by (usually) turning a recessed screw to the rear of the unit until the reading is correct. Keep tapping (gently!) the barometer to overcome friction within the mechanical linkage. Replace the barometer in a shaded/indoor location free from the possibility of accidental damage etc.
You should try and maintain, for say a month, a check against an adjacent site over a wide range of pressure values. By logging your values against those of this nearby site, you will be able to see if there is a systematic or random error in your reading. The former can be allowed for by slight re-adjustment or 'on-the-day' correction; the latter means you have a faulty unit, or its sited poorly -- in direct sunshine for example.
Incidentally, please take NO NOTICE of the absurd descriptive terms often placed around the dial of a barometer. When these originated is not known for sure, but it is known that Robert Hooke, the inventor of the 'wheel' barometer used such terms from about 1670: 'Change' was set at 29.5 inches; then 'Rain', 'Much Rain' and 'Stormy' at each half-inch on the lower side, and 'Fair', 'Set Fair' and 'Very Dry' on the high side. The regular spacing gives the clue to the lack of scientific credibility of such a scheme, and in my opinion they have no practical value.
Through the action of widespread and vigorous duststorms over places such as the Sahara, huge quantities of very fine desert sand can be carried to high enough levels (around 12000 ft/4000 m), where it can be dispersed for considerable distances downwind of the source. On many occasions, such dust is so diffused vertically and horizontally that there is little or no effect observed at ground level.
However, sometimes the dust remains in sufficiently high concentrations, and can become involved with a medium level weather system, which results in the dust being transported towards such places as France, Britain and Ireland. If rain falls, the dust falls as well. This is primarily due to washing out of the dust by large raindrops. This leaves a dusty residue on car windscreens, rain gauges etc., with the most common colour being similar to old mortar: i.e. light beige, but deeper brown, orange and red hues have been observed. The effect is usually noted after light, showery rainfall, often involving medium level instability - heavier rainfall tends to wash the evidence away. A warm, southerly (Tropical continental) low-level airstream, together with a strong southerly middle level flow (circa 700 mbar), originating from the North African area are the conditions required for such events in the northwest of Europe.
Such reports are always of interest...some guidelines are contained in this section (Notes on observing.)
(with effect from 1st April, 2000, the scheme whereby letters were assigned to fronts & centres on Bracknell analysis and short-range prognosis charts ceased. The 'tracking' of low pressure centres was also stopped. This was a direct consequence of withdrawal of one of the support rosters within the NMC. "NMC Bracknell" itself 'ceased to be' in 2003, when the Met Office HQ relocated to Exeter.
To avoid re-numbering the FAQ series though, I shall maintain this abbreviated entry for the time being, as there is some historical interest.)
[ the text below, and that for 2B.17, was kindly supplied by Martin Stubbs. ]
As far as is known the practice in the UK Met Office was always to allocate letters to the features on the charts (up to April, 2000) although it is thought that this may have been a numbering system which was started in about 1944 (Lettering of pressure centres, particularly areas of low pressure, may in fact date back to the latter part of the 19th century). In fact the WMO International Analysis Code (still in existence) actually caters for the identification of fronts or systems using a number defined by the code 'NN'. The actual code form is 99NNSS where NN is defined in the WMO Manual on Codes as the identity number of the system or front. Thus analyses prepared at the Central Forecasting Office in Dunstable in the 1940s and early 1950s for example, may well have had identification letters, but when coded a depression with the letter 'A' would be coded as 990100, and possibly referred to on the outstations as Depression '1'. The UK actually coded its analyses and forecasts in three different ways after the Second World War. The coded analysis/prebaratic that went out on the international circuits carried the identifier group 99NNSS (for example, a depression labelled 'A' would have been coded 990100 81297 59346 . . . etc.), the analyses that went out internally within the Met Office converted this to plain language (for example, LA 81297 59346 . . . etc.) and for the marine bulletin to the ships on the North Atlantic the identifier was dropped altogether (81297 59346 . . . etc.).
Prior to May 2003, there was a legend, 'ASXX' or 'FSXX', which identified the product as either an analysis (A), or a forecast (F) relating to the surface (S) level - strictly mean sea level - for an undefined region (XX). The words 'Analysis' or 'Forecast' are now used where required, and/or "xx hr", where xx=the number of hours from analysis time of the forecast chart, so "84hr" is the expected situation 84 hours from the analysis time.
[ The use of ASXX and FSXX dates from the time when the bulletins were coded using the International Analysis Code when ASXX/FSXX were the headers in much the same way as SMUK is the header for a bulletin of synoptic reports made at a main hour from the UK. ]
In addition, the letters EGRR used to appear on UKMO charts, which are the ICAO identifying letters which are assigned to the Exeter Operations Centre [EXO] (before late August 2003 to the Bracknell Telecommunications Centre) - these again have been dropped from 2003.
Other abbreviations include MSLP (mean sea level pressure) and UTC - universal co-ordinated time (an acronym chosen to satisfy both the English and French speaking communities since it does not have a direct equivalent in either language - in French UTC is read as temps universel coordonne). UTC is based on an atomic standard, but for all practical purposes is equivalent to GMT.
For international transfer of pictorial information the bulletins/files containing that information carry headings such as PPVA89, PPVI89 and so on. The letter P (or Q) indicates pictorial information, the second letter P indicates the information refers to pressure, the V defines the area for which the information is provided and the fourth letter indicates whether an analysis (A), or a forecast where E,G,I,J,K,M and O are for 24/36/48/60/72/96/120-hour forecasts respectively). The figure 89 refers to any parameter at sea level.
In the top-lefthand corner, there is a geostrophic scale - used to find the (theoretical) friction-less wind speed from the spacing of the isobars;(use with caution - see "How do I use a geostrophic wind scale?".
There is no longer a distance scale on the charts, but a tip to find distances ... remember that one degree of LATITUDE is equivalent to 60 nautical miles (n miles), (thus 1.5 degrees is equivalent to 90 n miles and so on) Therefore, if you want to measure off how far a depression has travelled over the period of six hours between analyses, step off the distance with a ruler or pair of dividers, then lay this distance along a line of LONGITUDE in the same area of the chart, and count the number of degrees latitude that this represents. For example, if the distance measured off is five degrees of latitude then this is equivalent to 300 n miles (i.e. 5 times 60, which is 300 n miles in that 6 hours; The overall speed of movement of the feature is even simpler to define for that 6 hours: one has only to remember that 1 degree latitude in 6 hours (i.e. 60 n miles/6hr)=10 knots; therefore if a feature 'steps-off' 3 degrees of latitude in that 6 hours, it must be moving at 30 knots.
These simple calculations can also be used to forecast the expected movement of fronts using the simple methods described in text books (for example, active cold fronts can be advected (moved) at a speed of four-fifths the measured geostrophic wind measured just ahead of the front, the vector being in the direction of the warm sector isobars. A factor of two-thirds can be applied to the measured geostrophic wind to give the expected movement of the warm front.
Isobars are drawn every 4 millibars on charts originating in the UK, the USA and Canada but note that a 5-millibar spacing is more common on charts originating in countries in continental Europe. Isobars are labelled with values in whole millibars (or hecto-Pascals/hPa), starting at 1000 hPa.
Fronts are drawn with heavy solid lines, distinguished by solid 'triangles' for cold fronts, solid 'bobbles' for warm fronts, and a mixture of the two for an occlusion. The triangles/bobbles point in the direction that the front is heading/thought to be heading and placed on alternate sides of the line when the front is quasi-stationary. Heavy lines with no such additions indicate troughs (the word 'TROUGH' may or may not be indicated beside the line). There are occasionally variations in the graphical representation of fronts. If the front is significantly weakening (frontolysis) then a cross hatch is placed across the frontal line (itself broken) between the triangles/bobbles, and if a front is considered to be forming (frontogenesis) then the solid line is replaced by dots between the 'bobbles / spikes' [ However, in my experience, its mighty difficult to tell the difference between the two! ].
An example of many of the fronts, pressure features etc., can be seen here.
On the 'medium-range' charts (e.g. T+48, 72 etc.), there are additional long-dash lines. These are the 500-1000 hPa total thickness lines at 18 dekametre intervals.
(The medium-range charts are currently listed as Additional Products within the context of the WMO Resolution 40 (WMO Twelfth Congress 1995) and may not always be available on the Web.)
The output is now produced using on-screen analysis and field modification tools. The days of hand-drawing charts for both actual and forecast purposes (at least in the UK service) has come to an end.
(this quoted directly from the Meteorological Glossary, HMSO): " A warm, calm spell of weather occurring in the autumn, especially in October and November. The earliest record of the use of this term is at the end of the 18th century, in America, and it was introduced into the British Isles at the beginning of the nineteenth century. There is no statistical evidence to show that such a warm spell tends to recur each year. "
C.E.P. Brooks, in his 'Climate in everyday life', notes that it is the counterpart of our 'Old Wives Summer', here in Europe, and tends to follow the first severe frost and to persist for several days.
It is thought that the phrase was coined by European settlers on the Atlantic coast of North America. Paul Marriott, in his 'Red Sky at Night, Shepherd's Delight', says..." strictly an Indian Summer is a lengthy dry sunny spell from late September into November. The name probably derived from the N. American Indians who relied on a similar fine spell in late autumn for harvesting. " Philip Eden, in his 'Weatherwise' (see the 'books' section of this FAQ) also ascribes this reasoning to the term. However, I have been advised of a belief in the USA that the phrase may be a rather pejorative one coined by the early settlers, which implies that a late ( autumnal / 'Fall' ) spell of warm, sunny weather is not to be relied upon: they found the native inhabitants (in their view) were not to be trusted in like fashion.
(There is yet another theory of the origin: Merchant vessels plying the Indian Ocean would have one of the 'load-lines' marked "IS" (for Indian Summer), to show the maximum load level for ships crossing that ocean in the post-monsoon fine weather season in the latter part of the year. It has been suggested that this might be the origin of the term; I have difficulty with this. The phrase can be traced back to at least 1778, yet the common marking of vessels in this way was not standardised until 1875 (Samuel Plimsoll, MP suggested the famous 'Plimsoll Line'). It is also difficult to see why the term should come to be associated with a phenomenon in North America.)
Such spells of fine, warm dry weather may be 'reliable' in the Atlantic states of the USA; this is not so for our own climate, and Marriott (amongst others) found expectation of a period of such weather in the U.K. to be misplaced.
When arctic-origin air in winter flows southward (northward in the southern hemisphere) across (relatively) warmer seas, strong surface heating acts both to enhance the degree of instability, and trigger vigorous moist convective towers. This is sufficient alone to give rise to heavy, wintry showers/cumulonimbus clusters etc., but often marked troughing, or even a closed circulation in the isobaric flow is found; the resultant low-level convergence/positive vorticity enhancement, plus the localised concentration of the latent heat energy released, enhances development within the system, and an intense (but synoptically small) area of rain, hail, sleet or snow & squally winds can result - a polar low (or polar depression or polar meso-cyclone in some texts).
The dynamics of such systems are not fully understood, and it is only with the (recent) arrival of very high-resolution satellite imagery & sensors in a wide variety of spectral bands that the detail within such systems can be studied. Even so, for operational meteorologists, careful monitoring of all available data is required; Geostationary satellite imagery has a rather course resolution at high latitudes, and the visible channels are of little use in the winter season. Polar orbiter passes (which give much higher resolution imagery) may not be frequent enough to maintain a continuous watch on developments.
Numerical models also have difficulty with such events; they are born in data-sparse regions, and most schemes 'paramaterize' convection i.e. models don't explicitly forecast each individual convective event, but rather indicate the degree of instability expected, its areal extent etc., and thus have problems going one step further and turning an area of disorganised (model) convection into an organised self-sustaining polar low/trough, where upper troughs are not the primary forcing mechanism. The one remaining Norwegian weather ship, and a handful of research and fishing vessels may be the only clues to developments taking place in, for example, the Norwegian Sea.
Polar Lows can develop, and move (in the prevailing flow) with surprising speed, and lead to considerable dislocation of normal life in regions directly affected. Preferred locations for genesis are to the west of large, slow-moving occluded depressions - i.e. those with a pre- existing rear-flank arctic flow. It may be that the geography of the regions in question play a significant part in genesis of polar lows - Dave Wheeler, who I am grateful to for checking much of the above, suggests that vortices shed by high-arctic island groups (e.g. Svalbaard) are enhanced by the land mass of Scandinavia (Norway) to the east and Greenland and its ice shelf to the west.
Many are interested in the 'weather' as an absorbing hobby, perhaps introduced to the subject via school / college (often part of the geography syllabus), or because of a sporting / recreational activity e.g. yachting, surfing or gliding. A good number of books have been written over the years and the first port of call I suggest is to go to your local lending library and see what is on the shelves. Don't be put off because a book has been written for 'sailors' or 'pilots'. These are often very well written by professional meteorologists who have a keen interest in the sport / activity involved, and will cover the elementary facts you need to know in a clear way, without the use of complex mathematics.
At some stage you will want to purchase one or more books that you can refer to at leisure. Some ideas are given in the books section of this site. Not all the books are currently in print, although as an addicted browser of second-hand bookshops, I have found it surprisingly easy to pick up good quality books relating to meteorology in this way. It is also worth asking about availability of titles from a good bookseller. In addition, the Internet now boasts several on-line book suppliers of both new and second-hand items.
If you are involved in sports / activities such as gliding, sailing, ballooning etc., the associations or clubs that you will probably belong to may offer courses, ad-hoc instruction etc. Contact them for details. The Royal Yachting Association (RYA) in particular encourages its members to be aware of weather processes, availability and interpretation of forecasts etc: visit their web site at: http://www.rya.org.uk
Residential study courses (e.g. Field Study Courses, Met Office College courses), are available, which are an excellent way to get to grips with meteorology in a somewhat deeper way, as well as enjoying some congenial company and the benefit of an experienced instructor. Find out about these from 'activity' magazines, good travel agents, newspaper weekend supplements etc. For the Met Office College courses, visit their web site at: http://www.metoffice.gov.uk
The Met Office Education section is well worth a visit, not only for information on teaching / learning resources, but also for some current data which may be of interest .. use the main Met Office url above (and follow the link via 'Education')'), or find them direct at:- http://www.metoffice.gov.uk/education/index.html
The BBC Weather Centre (which is closely allied to the Met Office) also has a host of useful information - far too much to list here, but if you are starting out on your hobby / passion of meteorology, give this site a go:- http://www.bbc.co.uk/weather/
Subscribe to one of the magazines listed in the periodicals section: 'Weather' or 'The Journal of Meteorology' will provide much of interest. Don't be put off if you are 'new' to the subject; there is much to appreciate about the daily changes in the atmosphere which surround us for which you don't need a degree in Maths! These magazines will also publicise new books coming on to the market, and 'Weather' in particular often carries articles that deal with elementary meteorology.
And of course the Internet itself is increasingly a help with self-education. Many of us are trying to work up information pages on basic meteorology ... There are a few articles here on this site, and this FAQ and its Glossary attempt to cover some topics that frequently appear in the newsgroup. Use a search engine to do a bit of hunting: many North American sites carry some elementary instruction.
Now, moving on to study of the subject on a professional level, then you need to decide in which area your interests lie. My job was part of what loosely can be regarded as 'operational meteorology', i.e. forecast (& allied advisory) services for the general public, aviation, maritime and commercial customers. But this is one small area of what we might regard as the atmospheric sciences discipline. The 'flavour of the moment' is of course the study of climatology, both in historical terms, ( trying to reconstruct the climate of centuries past to detect long-term trends ), and for the future - for example predicting how atmospheric gas composition will change due to industrial and other processes, and how these changes will affect the weather (and therefore us) in the decades and centuries to come. Atmospheric chemistry (for example the study of ozone in the high atmosphere) has an important part to play in the protection of human (and other) life on the planet from harmful solar radiation, as well as being important in understanding the heat budget of the atmosphere. Another specialism is the study of 'micro-climates' around mature woodlands, or in urban situations for example. Studies also increasingly cross formerly rigid disciplinary boundaries, such as into the realms of oceanography and vulcanology. And, under-pinning all, the modern 'weather' business would be nowhere without IT specialists - larger employers such as the Met Office, will give you basic training in meteorology, even though your work is more to do with complex coding of the web site.
Which line you intend to pursue will dictate the requirements in terms of preliminary study, basic qualifications etc. If you intend to go for the 'theoretical' meteorology side of things, with a view, for example to modelling the atmosphere in terms of the basic equations governing physics, then a solid grounding in mathematics and science is required, almost certainly to a degree standard. However, when dealing with practical (or applied) meteorology, then this standard need not be so rigorous. There are many avenues open now which approach the subject via the geography / environmental sciences route. A solid understanding of mathematics (and some physics) is desirable, but this can be 'bolted-on' at a later stage, rather than being a pre-requisite.
The field of opportunity is now so vast (and also changing quickly) that it would be best to investigate the requirements at an early stage: the Internet is an excellent source of such information. In particular, The Royal Meteorological Society see here) have some excellent advice on their web site ... to go directly to this page, go to http://www.royal-met-soc.org.uk/education.html
To find the latest information about University courses within the UK, and a host of other information about tertiary level education in the atmospheric sciences field, visit the Universities and Colleges Admissions Service (UCAS) site at http://www.ucas.co.uk/
Finally, a word of warning. If (as I did) you decide to go into 'front-line' forecasting, be prepared for some sleepless nights! Not only is shift-work required (& weekend working), but you must brace yourself to expect the disappointment of being woken at 4am to the sound of rain lashing against your window, when you confidently expected it to hold off until lunchtime at least: a thick skin, and a sense of humour are a requirement!
For books, it is always a good idea to give the reference section of your local lending library a chance. What you seek may be held on the shelves and you can search for titles, subjects etc., at many of the larger libraries and order them if they are not immediately available. Books long out of print, and not held in a public library may be found sometimes in second-hand book shops. The National Meteorological Library may also hold the item and for serious research, a call to them might be beneficial (see here).
Between them, the UK Met Office & Royal Meteorological Society offer a selection of charts, leaflets, educational packs and the like for sale which often will be of great help, for example to students and teachers in the subject. Other publications may also be obtained (if currently in print) from the Stationery Office (formerly HMSO).
During cases of rapid cyclogenesis (see the Glossary), the long accepted 'Norwegian' frontal/cyclone development model is not appropriate. M.A. Shapiro and D. Keyser, in a paper published in 1990, proposed an alternative which has gained widespread acceptance. I am grateful to Dr. David Schultz (NSSL) for permission to quote the following from an article written (with co-author H. Wernli) for the 'Mariners Weather Log', which to my mind is an excellent summary of the differences between the 'classical' frontal depression model and that proposed for rapid cyclogenesis events ...
"The Norwegian cyclone model, so named to honor the Norwegian meteorologists (e.g. Bjerknes, Bergeron and Solberg) who first conceptualised the typical life-cycle of midlatitude cyclones in the 1910's and 1920's, presents the evolution of a cyclone from an incipient frontal wave with cold and warm fronts, to a deepening cyclone with a narrowing warm sector as the cold front rotates around the cyclone faster than the warm front, and finally to a mature cyclone with an occluded front. Typically, a Norwegian cyclone is oblong, orientated roughly north-south with the cold front more intense and longer than the weak and "stubby" warm front.
The Shapiro-Keyser cyclone model is named after the authors of the study that first presented this conceptual model of the frontal structure in some marine cyclones. As with the Norwegian cyclone model, an incipient cyclone develops cold and warm fronts, but in this case, the cold front moves roughly perpendicular to the warm front such that the fronts never meet, the so-called 'T-bone'. Also, a weakness appears along the poleward portion of the cold front near the low center, the so-called 'frontal fracture' and a back-bent front forms behind the low center. (In the final stage), colder air encircles warmer air near the low center, forming a warm seclusion. Typically, the Shapiro-Keyser cyclone is oblong, elongated east-west along the strong warm front".
Schultz & Wernli then go on to state (I paraphrase) ... an important factor in determining which evolution will be preferred ... is the nature of the large-scale (i.e. mid/upper tropospheric) flow. NWP experiments have indicated significant sensitivity to the profile of the wind speed across the jet flow and other studies have indicated that the along-jet variations of wind speed can be important. Cyclones embedded within diffluent flow (e.g. jet-exit regions) tend to evolve like the Norwegian cyclone model, whereas cyclones embedded within confluent flow (e.g. jet-entrance regions) tend to evolve like the Shapiro-Keyser cyclone model.
The noise we hear when a thunderstorm is in progress is caused by the near-instantaneous expansion of air (due to intense heating) along the path of the lightning discharge. The type of noise we hear (rumble, crackle or bang) depends upon the distance of the observer from the lightning path and the path 'structure', e.g. simple stroke, branched pattern etc.
A lengthy drawn-out rumble/crackle of thunder is due to the slow speed of sound (330 m/s) through air - the lightning 'flash' of course travels at the speed of light (several orders of magnitude faster) - effectively our eyes see the discharge as soon as it happens; the sound wave takes much longer to reach our ears, unless the lightning strike is very close by - when we would experience a violent 'bang' . . and hopefully nothing worse!
When lighting undergoes much 'branching', both within and outside the cloud, although the whole flash would take place over a small fraction of a second, the sound waves from each part of the path (and the branches), take different periods of time to reach an observer: the sound will be heard as either a continuous crackling (relatively close-by storm), or low-intensity 'rumble' (distant storm), or a mix of the two, again depending upon the character of the discharge path.
The reason why near thunder 'cracks' and distant thunder 'rumbles' is this: sharp cracks are composed of sound waves with high frequency (or short wavelengths); these are rapidly damped owing to irregularities in wind & temperature. The lower frequency (longer wavelength) portion of the sound wave is not absorbed anything like as much, and therefore it is these we hear from distant storms.
Thunder, under normal atmospheric conditions, has an absolute maximum 'travel' of about 20 km, and more usually 8 to 10km. In mountainous areas and under conditions of 'anomalous' low-level refraction, then greater distances may be achieved. (See also here)
In the NE Atlantic / NW European area (and like locations), we are used to the idea of the four seasons: spring, summer, autumn and winter. For climatological 'accounting' purposes, these are defined using three calendar month blocks thus:-
However, terminology used in forecasts when describing temperature levels (in the UK) relative to average values (e.g. 'mild', 'rather cold', 'hot' etc.) employ a modified form of the above: the spring descriptions apply from mid-March to mid-May; summer-time is mid-May to mid-September; autumn from mid-September to mid-November and winter runs from mid-November to mid-March. Note though that as with the climatological seasons (above), in any one year, the 'bounds' may not be appropriate: they are only a guide and the terminology they allow do not find favour with all!
Moving away from the four 'classic' seasons, other periods have been suggested which fit more closely the various climatological phases of a year: for example, Hubert Lamb (1950), a noted British climatologist, proposed the following:
This classification was based on his (and others, e.g. Brooks) analysis of British Isles weather periodicities.
And of course we must appreciate that our desire to neatly parcel the year into equal-length units (in the case of the 'standard' definition above), does not sit well when transferred to other parts of the world. For example, tropical (& adjacent) areas subject to the migration of the ITCZ will have a 'dry' season and a 'wet' season (perhaps two) - not necessarily of equal length. In south Asia, it is more sensible to talk about the periods affected by the various monsoon wind regimes, with short transition periods. In the interior of large continents, and at high (arctic / sub-arctic) latitudes, the "seasons" are observed only by convention: the year often consists of extended deep winter & high summer spells, again with short transitional periods of about a month.
One classification that does NOT find favour with meteorologists though is that used by astronomers. They use the dates of equinoxes and solstices to define the start/end of seasons.
The newsgroup: "uk.sci.weather - its history, myths & legends"
On the 1st March, 1996, Philip Eden, a highly respected meteorologist based in the UK, proposed that a new newsgroup be formed, with the title: "uk.environment.weather". A 'request-for-discussion' (RFD) was called for, and after overwhelming agreement, and a suggestion that it might more appropriately be part of the "uk.sci.-" hierarchy, on the 4th April, 1996 a 'fast-track' procedure was invoked to enable the group we now know as "uk.sci.weather" (usw for short) to be enabled 7 days later.
On the 20th April, 1996, the charter for the newsgroup (also proposed by Philip) was formally published, and this was probably the first post into the newsgroup (an extract from this is contained in this section of the FAQ). The newsgroup was the 131st ng created in the "uk" hierarchy. Newsgroup traffic has grown dramatically since about 1999, and the group is particularly busy when major snow events are expected (real or otherwise) or when significant outbreaks of severe thunderstorms are possible. The group is populated by a wide variety of people from many countries (NOT just the UK) with all sorts of backgrounds - as long as you are 'weather-aware', you are very welcome!
Knowing where you are might seem rather obvious but when logging weather events and setting up web sites etc., it is useful to be able to state with some precision a location in terms of latitude and longitude and/or by grid reference.
For the UK, then it is recommended that you use http://www.streetmap.co.uk/ or http://www.multimap.com/ then if you use the search facilities (I find the Post Code method good), you will find a read-out of the location.
The best way to determine your altitude above mean sea level (again UK only), is to use an Ordnance Survey map with contours marked at 5m intervals. Use this link: http://www.ordnancesurvey.co.uk/oswebsite/getamap/ accept the terms/conditions, then use the search facility to find your location and zoom in to the maximum extent. It should be possible to determine with acceptable accuracy the height using the contours and spot heights given.
Observations are the lifeblood of meteorology. The newsgroup is a valuable resource in this respect and many of us contribute reports of what is happening 'out of the window'. To enable those who don't always want to look at every report of fog, snow, rain etc., a system of prefixes in the Subject line of a post has developed as below:-
This follows from a suggestion by David Buttery in early 2001, and is used to highlight reports of weather in plain language. This has proved very useful. Example (in the Subject line) ...
[WR] 1050 GMT snow falling now in Reading
with the body text then amplifying this brief 'heads up'.
this follows from a suggestion (in mid-December 2004) by David Mitchell, as refined by Dave Ludlow, that those posting a sequence of observations (perhaps semi-coded), could usefully indicate this in the Subject line thus:
[OBS] Bracknell Wed 15 DEC 2004
Use of this convention would then enable a search engine to pick out and list data from defined stations on specific dates etc. It is further suggested (BUT NOT MANDATORY) that some form of approximation to the METAR code could be used to list these sequential observations. For more on the METAR code follow the appropriate see here.
However, please note that it is not intended to follow this slavishly: most will heavily amend the format and looking at the newsgroup will make this clearer.
(subject to review)
As the nights draw in, and the yellowing leaves are blown hither & yon in the autumn gales & rain, thoughts of many on this newsgroup turn to ... the "Scandinavian High"! Reason? Well, for those of us living in the 'maritime' region of NW Europe, to get any sort of prolonged cold, wintry weather, we really need a large (in horizontal extent), slow-moving, intense anticyclone - primary centre northern/arctic Russia (probably in excess of 1045 mbar central pressure) - with a strong and persistent ridge extending westwards over Scandinavia - spawning occasional discrete but reasonably 'solid' individual high cells around the periphery; these cells from time-to-time taking over as the primary focus of high pressure.
The air at low levels should be bitterly cold, with low thickness values (indicative of cold air in depth: see "Thickness: what is it?"). In addition, to produce the required snow, Atlantic depressions / fronts will approach this 'block' (see "Why does the weather sometimes get 'stuck in a rut'?") along latitudes south of 50 degN, attempting to displace the beast and in the process we end up with snow .. or sleet .. or freezing rain .. or blizzards, or any combination of same. Some good examples occurred in the winters of: 1946/47, 1962/63 & 1978/79 as well as January 1940 and December 1981 (not meant to be exhaustive).
However, in recent years (this written in autumn, 2005), these situations have been notable by their absence. What 'high' blocks there have been stay teasingly just too far east and more often than not, a broad band of high pressure extends from the Azores area, east-north-eastwards towards the Biscay / English Channel region - perhaps now & then displaced towards the Alps, as storm upon storm sweeps in from the North Atlantic, hurried along by an often powerful upper jet (see "What are jetstreams?"). Rain, gales and above average temperatures prevail, with any 'wintry' weather confined to brief incursions of Polar Maritime west or WNW'lies, or perhaps a temporary Arctic Maritime blast from the north - which is shunted away as the next surge of mild air hurries in from the west. The apparently semi-permanent belt of high pressure in the 'wrong' place has been christened ... "The Bartlett High", in honour of Paul Bartlett, a luminary of this ng, who used to put his experience of forecasting to the test by publishing a reasoned winter forecast for all to see. As Les Crossan has noted (also a stalwart of this ng), this has come to be regarded as a 'slug' - nothing moves it, not even extracting a pair of dividers and skewering the said beast as it sits dominating any particular synoptic chart!
(prepared with help & advice from Les Crossan - a stalwart of this newsgroup, who sums it up perfectly by going " ... ' freak ' (argh): ' mini - tornado '- AAARGHHHH - CALL TORRO FAST! " If you want to help lower his blood pressure, read on ..... )
Within the memory of many of us 'oldies', there was a view amongst quite respectable meteorologists in this country that tornadoes just didn't happen in the British Isles. Any damage was caused by a "freak gust" - and that was that. Over the last 40 or so years, thanks largely to work by British stormchasers, officials and members of TORRO (see the 'useful sites' section), the fact that British & Irish tornadoes do indeed occur has entered the public consciousness, and more importantly, the Press / Media are now also aware. The British Isles are recognized as being the most tornado - prone landmass on Planet Earth (Fujita) and have a disproportionate number of weaker T1-2 events (Brooks) - the TORRO site has more on this.
However, it is unfortunate that the prefix 'mini' has come into use somewhere along the line, presumably in an (unnecessary) attempt to try and differentiate between our events, and the sometimes more powerful tornadoes of the North American continent. 'Mini', as used for the small car, or a small skirt, or a short-lived spell of heat might be appropriate, but not for 'our' tornadoes, which do indeed belong to the same family of Whirlwind events observed on the other side of the Atlantic (& elsewhere around the world).
A much better qualification would be 'weak' (or 'moderate' if applicable)[or the U.S. term 'landspout' could be used - coined by Les Lemon and others for describing non - supercellular tornadic events]. Even weak tornadoes cause some damage - but 'weak' is presumably 'wimpish' in media-speak (for some): for us in this newsgroup though, we avoid the rather derogatory term " mini-tornado ". And as for 'freak' .... well that's another story and covered in the TORRO FAQ (via the 'useful sites' section), as are many other facts related to tornadoes, waterspouts, funnel clouds etc.
Following a discussion on the use of Total Thickness (500-1000 hPa) in the winter of 1996/97, it was suggested that I (Martin Rowley) turn a posting on the subject into a FAQ, and this was done, the result being formally posted / uploaded on the 1st March, 1997.
There then followed suggestions that a more general FAQ be provided - I undertook to manage / collate same, and the first FAQ in this series was published mid-May, 1997. The FAQ was updated routinely, and a companion Glossary was added in 1998. In addition, several other documents have been provided, which are all linked from the FAQ / Glossary series. The aim is to help those who are not steeped in all the jargon to get the best out the discussions, and simply enjoy our varying weather / climate.
As of Christmas / New Year 2006/07, Martin ceased to be responsible for updating this FAQ. It has now (2007/8) moved to this new site (http://weatherfaqs.org.uk) and is now maintained by myself (Steve Loft) using the Drupal content management system.
It is worth pointing out that the area embraced by the phrase 'adjacent parts of Europe' has grown to include Australasia and North America! As long as it's interesting weather, and within the context of the rest of the Charter, then that has come to be accepted as OK!
From time-to-time, threads in the group can become 'heated', usually when the weather has not gone according to (someones) plan. Usually things calm down and we are all friends again, but as the forum is not moderated, newcomers should be aware that it can get a bit hectic: a good night's sleep, avoiding personal abuse, and reading what you have just typed before pressing the 'send' button can usually work wonders.
Some of those lurking in the uk.sci.weather newsgroup must find this fascination with severe winter weather (as well as other 'severe' events) rather odd, or even perverse. However, it must be appreciated that our interest, from both amateurs and professionals, is primarily in the meteorology of such events ... the rapidity with which the scenario can 'kick-in' and the mechanisms which maintain such blocks as are described here. ( For example, the 'severe/snowy' period, lingering until early March in 1947 did not really start until well after mid January of that year, & was immediately preceded by an unusually mild spell around the middle of that month.)
We do appreciate that bitterly cold, snowy weather is the cause of great inconvenience, not to say distress to many people, especially the elderly, disabled and those on low incomes. Make no mistake, although the incidence of severe winters has decreased, this must not be taken to imply that they will always give us a miss. Subscribers to this newsgroup will perform a valuable service in monitoring conditions as they develop. (See various/other sections on making weather reports in this FAQ series e.g. here, and the section on observing and reporting and links therefrom.)
A more detailed explanation can be found here.
[Used in the order: Qualifier + Descriptor + Phenomenon ... thus for heavy thunderstorm with rain=+TSRA; light freezing drizzle=-FZDZ]
(some* use PE)
(used by auto-METAR)
ice crystals (or diamond dust)
small hail/snow pellets
* although PE was the original two-letter abbreviation recommended for the METAR/TAF weather code, since the change whereby two (or more) weather groups can be used came into force, some users objected to the possible combination of rain (RA) and pellets (PE).
well developed dust/sand whirls
NB: all phenomena are considered to be 'at the station' unless prefixed by VC (= vicinity); thus VCFG is fog in the area, but not affecting the airfield. Vicinity is defined as within 8 km of the airfield.
(see note below)
(i.e. supercooled and depositing rime/clear ice - but see note below)
NB: When used with FG, the qualifier 'PR' is used for fog banks, i.e. an extensive area of fog impinges upon an airfield, reducing visibility over part of same to less than 1 km; 'BC' [ patches ] would be used when a discrete, small-scale area of fog drifts/forms over the airfield, again reducing visibility below 1 km but not in all directions. In practice, it is not easy to tell the two apart!.
NB: When use with FG, the qualifier 'FZ' is now used to mean BOTH fog depositing rime AND fog occurring with an air-temperature below zero deg.C; this latter may or may not be depositing rime ice.
Questions relating to sources of information (Where can I find... ?)
There is a map of the areas at:
Les Cowley maintains a very interesting site which explains all you need to know about halo phenomena, including arcs of contact, parhelia (mock suns) etc. Plenty of interesting diagrams at:-
Several members of the newsgroup have helped put together some pages explaining some aspects of numerical weather products that can be seen on the WWW via various sites. Not only are the lines, shading etc., explained, but there are also some simple notes pertaining to the various outputs. The site also links directly to NWP output sites. Start here.
Colin Martin has given me permission to host this information relating to ducting of radio emissions due to atmospheric conditions; find the page here.
Keith Harris has worked hard to develop this site which pulls down the groups appended to routine, internationally exchanged SYNOP's (see here on this site), and provides a decoded summary daily for everyone to enjoy. Keith's site is at http://www.southendweather.net.
There are many sites that deal with these phenomena, and it can be bewildering to "home in" on those that offer the best background data and current advice. The Met Office have a fine page on their site with links to the main centres around the world, plus the MetO own guidance bulletins on current storm activity, and you could do no better than to start there; go to the Met Office home page, and follow the various links to 'World Weather' & 'Tropical Cyclones' (or via 'Weather & Climate'):- http://www.metoffice.gov.uk/
This site has all the answers, including photographs, a FAQ on the subject, methods of recording & reporting these phenomena etc. - http://www.nlcnet.co.uk/
The Satellite imagery FAQ at http://www.faqs.org/faqs/sci/Satellite-Imagery-FAQ/ may answer some of your queries, and the UK Met Office has a useful summary of satellite operations at http://www.metoffice.gov.uk/education/data/index.html (find it by following the 'Satellite' link at the bottom)
There are many sources of information on the current debate relating to our changing climate, both its natural variation, and anthropogenically forced change. These are now contained within a separate section of this site.
For the ECMWF suite of models go to http://www.ecmwf.int/
For the UK Met Office go to http://www.metoffice.gov.uk/research/nwp/index.html
For the NCEP models go to: http://www.emc.ncep.noaa.gov/
For the US Navy, Fleet Numerical Meteorology and Oceanography system (NOGAPS) go to https://www.fnmoc.navy.mil/PUBLIC/
and follow the links dealing with model specifications & characteristics.
For the DWD suite of models go to http://www.dwd.de/en/FundE/FundE.htm
The UK Met Office have an information sheet on the use of radar in meteorology: http://www.metoffice.gov.uk/corporate/library/factsheets/factsheet15.pdf
(You will need a PDF reader for this).
For the images themselves, start by going to:
There are also useful notes attached on the use and configuration of the network.
for the BBC Weather Centre site:
http://www.bbc.co.uk/weather/ and follow the appropriate links labelled 'UK Weather' & 'rain'. However, it is unclear whether this imagery is now (from 2005) 'pure' radar output, or some amalgam of model and radar - not as useful as it used to be.
For 15-minute imagery, with good navigation, speed control etc., MeteoGroup provide the following:
Note that you can also select other areas in Europe to display - truly a major advance in availability since this FAQ sheet was started back in 1997!
and for imagery over Belgium, the Netherlands, NW Germany and NE France (includes East Anglia & SE England on some views), go to: http://www.meteo.be/meteo/view/en/123361-Radar.html or http://weerkamer.nl/weer/radar/ or http://www.buienradar.nl/
and for other links to European radar imagery, try the 'Top Karten' site:
and follow the links via "Sat & Radar".
(NB: there are now commercial sites offering higher spatial and temporal resolution data, but for a fee, e.g. the Met Office, MeteoGroup, AvBrief etc.)
For BEAUFORT WIND FORCE ESTIMATIONS go to: http://www.zetnet.co.uk/sigs/weather/Met_Codes/codes.htm
For BEAUFORT LETTERS USED IN WEATHER RECORDING go here.
The "Temp, Humidity & Dew Point ONA" (Often Needed Answers), FAQ might be useful and can be found at http://www.faqs.org/faqs/meteorology/temp-dewpoint/
For information on the chemistry, and current distribution, of low-altitude ozone (and other pollutants) in the UK, visit the DETR site at: http://www.airquality.co.uk/
For the FAQ relating to stratospheric (high-altitude) ozone concerns, go to: http://www.faqs.org/faqs/ozone-depletion/
... and for current and past stratospheric ozone measurements and other information relating to the stratosphere and its structure go to: http://www.metoffice.gov.uk/research/stratosphere/index.html
With the increased availability of web-space for home use provided by many ISP's, many contributors to usw have provided data on past weather events for anyone to view. The list here is begging to be added to, so if you have such a source of data (for European events), then let me know.
TORRO host a site detailing extremes for such as temperature, wind speed etc., at:
Dr. Trevor Harley, Dundee University has worked up the following interesting site of 'notable' events: (via)
Martin Rowley has a series of pages on his site which might be of interest:
The Met Office/National Climate Information Centre publish summaries at:
these are produced & uploaded as soon as practicable after the end of the month. You will also find some summaries of 'notable' events via this link as well - well worth a visit!
The BBC Weather Centre publish some monthly summary information - find it on their site, via 'UK' / 'Year in Review':
Philip Eden is providing a growing catalogue of past data, along with current indices (CET, EWR etc.) at this site:-
There are a few in the article on thermodynamic soundings.
Both these sites require registration, but at the time of writing, this process is free: you may be charged for hard copy of the output.
[ Archive of Dundee University ]
[ Archive of EUMETSAT .. for the Meteosat suite of satellites ]
There are a growing number of sites that fit the bill: the ones listed below are simply a selection that I use routinely. Some will archive data for limited periods (say a month or less), others longer.
[ rolling archive of European SYNOP's ]
[ archive of METAR's, TAF's, SYNOP's etc., both coded & decoded ]
[ archive of coded data, including upper air ]
Many sites have been developed which supply such data & the list below must only be regarded as a 'snapshot', but ones which I certainly find of use:
a WMO sponsored site for basic climatological information:
a comprehensive site provided by Canty and Associates of Virginia, USA:
a fine site for climate, recent history etc., plus lots of other useful information - very well worth 'bookmarking' for general use:
another great site with plenty of information, not just climatology:
and one more, the Buttle and Tuttle site at:
and for a full listing of world record temperatures, see:
For the British Isles, then the Tornado & Storm Research Organisation (TORRO) has, since 1974, co-ordinated research into the full range of severe convective storm events - tornadoes, severe thunderstorms, waterspouts etc. Their site can be found at http://www.torro.org.uk/
Another developing, and very useful site with a focus on current data relating to severe storms within the UK, and educational material about such events can be found at http://www.severewx.co.uk/
In 1997, an organisation was formed to perform similar functions covering Germany, Austria and Switzerland. Follow this link for their site: http://www.tordach.org/
Les Crossan maintains a very interesting site to cover his passion relating to all 'storm' phenomena at http://www.uksevereweather.org.uk/
If you want a site that explains many of the mysteries surrounding weather terminology with an ocean-going slant, or a handy site of forecast marine-weather links, or somewhere to find out how to get the best out of NAVTEX broadcasts, then look no further than Frank Singleton's page at:
also, Martin Stubbs has a set of pages with very useful links of interest to the maritime community, and the section relating to email request of products via the US NWS is particularly worth a visit:
The Met Office of course has a sub-area devoted to marine forecasts: this was overhauled in Autumn 2006 with clickable maps for the forecasts and warnings. Find this area via:
[ and you can also use this page to link-through to the Maritime & Coastguard Agency / MCA, for whom the Office provides the basic 'public-service' forecasts. ]
The RNLI site is very useful, and emphasises the part that weather awareness plays in enjoying your sport safely - find their home page at:
And for the forecasts issued by the Met Office on behalf of UK agencies (principally the Maritime & Coastguard Agency) follow these links:
Shipping Forecast ( offshore activities, beyond 12 miles of the shore-line)
(this is the forecast broadcast 4 times a day on Radio 4 LW)
[ Met Office site ]
[ BBC site ]
Inshore Waters Forecast(within 12 miles of shore / short version)
(this is the forecast broadcast twice a day on Radio 4 LW, and copied to CEEFAX page 409)
http://www.bbc.co.uk/weather/ukweather/inshore.shtml [ BBC site ]
Inshore Waters Forecast (within 12 miles of shore / long version)
(forecast provided through funding from the Maritime & Coastguard Agency)
http://www.metoffice.gov.uk/weather/marine/inshore_forecast.html [ Met Office site ]
The BBCi website has all the forecasts (& other information, e.g. gale warnings, tides) in one useful area:-
and choose the category as listed: Gale (& other marine) warnings are found via the 'Shipping Forecast' tab.
And finally, a site for text bulletins of forecasts issued by National Meteorological Services (hosted by Météo France), is now available at:-
To quote from that site: " this web site provides the marine weather information broadcast via Inmarsat-C SafetyNET by all National Meteorological Services (NMS) appointed as Issuing Services within the framework of the Global Maritime Distress and Safety System. " (It is sponsored by WMO).
There are many (mainly US based) sites which can be used to follow the progress of current tropical storm disturbances (Hurricanes, Typhoons, Tropical Depressions etc).
These two will start you off, and there are links from the Met Office url to other primary centres.
The UK Met Office Global Model is regarded highly in tropical forecasting circles, and it is appropriate that they should maintain this useful page (and associated links), from which you can 'drop-down' to other world-wide centres. Use the links in the blue panel for current activity:-
and a site that is neatly ordered, and used graphical representation of current and past storms is that provided by the Tropical Storm Risk Consortium (TSR). There are also seasonal forecasts & background information on the subject: the primary purpose of the site is to provide a risk assessment of storm activity in general and for specific disturbances:-
and another site with graphical visualisation of global tropical storms at:-