Glossary terms M-R


Man-machine mix

The concept whereby a forecast result is achieved as a combination of human interaction with (by intervention), and interpreting (using experience of the atmosphere and conceptual models) NWP output, rather than allowing the models to predict the weather with no interpretation.


Abbreviation used to stand for maritime environments, often seen in aviation forecasts.

mb (or MB)

Often seen as the abbreviation for 'millibar' (see also mbar).


An abbreviation for 'millibar', being one-thousandth part of a bar. The 'bar' is the basic unit of atmospheric pressure as defined in the c.g.s. system of measurement (now regarded as obsolete). 1 bar = 103 millibars = 106 dynes/cm2 (c.g.s. system) = 105 N/m2 (Pa) (SI system).


(abbr) Mesoscale Convective Complex


(abbr) Mesoscale Convective System

Mean wind

The atmosphere in motion near the surface of the earth (the 'wind') exhibits marked variations over very short periods of time, depending on such variables as low level thermal stability, ground-induced friction effects, vertical shear etc. Average or 'mean' winds are defined over periods such as 1, 2 or 10 minutes for reporting in meteorological bulletins. (60 minute periods are often used for climatological purposes). For SYNOP and METAR reports, the period is 10 minutes in most countries, though in tropical storm advisories and measurements associated with US-based organisations (e.g. NHC, JTWC), 1 minute averages are used, and often referred to as 'sustained' winds.


Both predominantly north-to-south and south-to-north airflows (over distances of 100's of km) are termed meridional.

Mesoscale Convective Complex (MCC)

This might be regarded as the 'super' version of an MCS. It is strictly defined in terms of areal extent of coldest tops of the cloud sensed by IR imagery (typically over 300 km), and must last at least 6 hr, with many active systems lingering for upwards of 12hr. Would be regarded as extremely rare for NW Europe - more a feature of central and eastern Europe with average annual frequency around 5.

Mesoscale Convective System (MCS)

(as defined in 'Images in weather forecasting' - other services may define differently)... " a mesoscale grouping of deep convective and stratiform cloud and precipitation, together with associated circulations. The system exists much longer (at least four hours) than the lifetime of an individual constituent cloud. The individual convective clouds contribute to a common upper-tropospheric outflow shield or anvil. The system's convective-scale downdraughts merge at some time to form a continuous zone of cold air in the lower troposphere". What is visible on satellite imagery is the amalgamation of many cirrus outflow heads from the many cells comprising the system. Frequent cloud-to-cloud lightning is a feature of these systems. Although mainly a feature of the central plains of the United States and over continental Europe, they can on occasions affect Britain, mainly affecting the 'English lowlands' with an average frequency 2 to 3 times per year - when they do occur, they are responsible for some extreme/violent convective weather, and behave as a weather system in their own right, often modifying the enviroment in which they form considerably.


METeorological Aviation Report -- a weather observation for a specific airfield at a given time, containing the minimum information necessary for air operators for safe usage: wind/visibility/significant weather type/cloud amount+base/temperatures/pressure settings being the 'core' elements. Usually only issued with all elements when the airfield is operational (i.e. Air Traffic Control is open), but increasingly automated observations are now appearing of varying quality. Major/civil airports issue at HH+20 and HH+50 (i.e. 20 and 50 minutes past each hour), with others hourly only. METAR reports may also have TREND forecasts appended giving a short-range (usually 2hr) forecast of significant changes. SPECI reports are issued when meteorological variables deteriorate/improve through defined levels. For a brief listing of the weather decode for a METAR/TAF/SPECI, click on this link, and there is now a full explanation of the METAR code on this site :- HERE


(abbr) Shallow (below eye level); used in aviation reports in connection with fog (FG).


To aid the forecasting of condensation trails emitted (or not) from high-flying aircraft, a line marking the critical temperatures (altitude dependent), above which trails are not possible, is marked on a tephigram . The values are approximately -24degC at 1000 hPa (i.e. roughly sea-level), -39degC at 250 hPa (34000ft / 10.4 km) and about -45degC at 130 hPa (50000feet/15km). Using the MINTRA line (as it has come to be called - based on experiments by JK Bannon during World War II with the piston-engined Spitfire), a forecaster will mark two further lines on a tephigram: MINTRA minus 11degC (A) and MINTRA minus 14degC (B). If the ambient temperature (from the tephigram air temperature plot) lies between (A) and (B), then short, non-persistent trails are possible. If colder than (B), then long, persistent trails should be expected. However, some note should be paid to the relative humidity - high values will tip the balance to trailing (or longer/persistent trails.), even with air temperatures warmer than (A); ultra-low rh% will reduce the risk of condensation trails - the design of engines will have an effect as well. In broad terms, warm Tropical Maritime airmasses with a high but cold tropopause will result in a good deal of trailing, whilst cold, polar air-masses with a low, relatively warm tropopause will seldom give rise to significant aircraft trails. (See also "Why do some high flying aircraft leave white trails in their wake?")


Visibility reduced due to the suspension of minute water droplets in the atmosphere: the visibility is >=1km (less than this, and fog is reported), and the relative humidity is ~>95%. However, slavish adherence to this latter figure is not advised!

Mountain Waves

(also known as standing, lee or orographic waves. Often abbreviated to MTW in aviation circles) -- Under certain conditions, which must include a stable layer around/just above a mountain/hill range, air flowing across the range will be found to oscillate vertically in a standing wave configuration with well defined crests/troughs in the wavetrain. The horizontal flow must be reasonably brisk and within roughly 30 degrees of the crest of the hill/mountain range, with speed increasing with height, and directional shear must be small. Airflow is often smooth, but the vertical velocities in the upward/downward legs of the standing waves can be strong, and may break down to give local severe turbulence. Surface winds downstream of the ridge may be notably gusty as well, but paradoxically, they may also be extremely light, or even reverse direction. The presence of MTW activity may be betrayed in visible satellite imagery, if the flow is moist enough, as a series of near-parallel bars of cloud downwind of the ridge causing the airflow displacement. (See entry under: "Lenticular clouds") [ Although standing-wave motion is often referred to as 'lee' wave action, upward motion will be found on the windward / upslope side and of course across the hill / mountain crest too; there are also well documented cases of wave motion being found some considerable distance upwind of a topographical obstacle - perhaps due to some element of 'backwards' propagation of the wave-train. Wave motion may also be seen in cloud patterns where the triggering is due to marked shear in the windflow, well away from a range of hills. ]


(abbr) [historical only - no longer used] Medium Range Forecast (or Global Spectral) model (run by the NCEP). Had two basic formulations: for short-range work, the "Aviation" (AVN) run to 72 hours, and for extended range work (also known as the MRF run) to beyond 144 hours - in one iteration to 240 hours (i.e. 10 days). Now (2002), subsumed within the main NCEP NWP suite as the Global Forecast System (GFS) (q.v.).


(abbr) Mean sea level pressure (also seen as PMSL).

Multi-cell storm

See the FAQ here.



(abbr) National Centers for Environmental Prediction (part of the US National Oceanic and Atmospheric Administration [NOAA], National Weather Service [NWS]). NCEP comprises 9 centres; one of those centres is responsible for running the atmospheric prediction models (see MRF). To find out more, visit: http://www.emc.ncep.noaa.gov/

Net radiation

The difference between absorbed and emitted radiation.




(abbr) see Noctilucent cloud.

Noctilucent cloud (NLC)

These are clouds in the high atmosphere (around 83km) which are thought to be composed of small ice-coated particles (possibly meteoritic debris), which can be observed in mid-summer (June/July in the Northern Hemisphere) against a twilit sky: the sun must be between 6 and 16 degrees below the local horizon. They appear as very tenuous, often interwoven filaments of cloud, mostly white or slightly blue in tone. (Also known as polar mesospheric clouds or PMC's.) [ For a link to more on this, and an aid to identification, see this entry in the main FAQ or this article.


(abbr) Navy Operational Global Atmospheric Prediction System (also abbreviated to NGP). This model is provided by the US Navy, Fleet Numerical Meteorology and Oceanography Center (FNMOC). To find out more, visit: https://www.fnmoc.navy.mil/PUBLIC/

Normal (environmental) lapse rate

The average change (decrease or increase) of temperature with height. In the troposphere it is 6.5°C / km, or ~2°C / 1000ft.

North Atlantic Oscillation

See "What is the North Atlantic Oscillation?".

Norwegian model

The classical idea of a travelling wave depression on the polar front running forward and deepening, with the cold front moving faster than the warm front, thus 'occluding' the warm sector, with the parent low slowing/turning to the left (in northern hemisphere), and filling up.


(abbr) Nimbostratus (NS in METAR/SIGWX charts etc., Ns otherwise); a deep cloud extending from low through to higher mid-levels, formed by vigorous and widespread rising motion, giving rise to heavy precipitation & a high risk of severe icing.


(abbr) No significant cloud - (No CB, no cloud < 1500m/5000ft or below the highest minimum sector altitude, whichever is greater, and CAVOK is not appropriate); used in aviation forecasts (and latterly in actuals).


(abbr) No significant weather; used with aviation weather forecasts, e.g. TAFs.


(abbr) Numerical weather prediction. The processes that make up the atmospheric system can be represented by fundamental equations, which can be solved in discrete time steps to achieve a numerical forecast. The concept was well understood from early in the 20th century, but needed the arrival of electronic computers in the early 1950's to be of practical use.



(abbr) Outside air temperature: often used in connection with aviation weather reports.


Abbreviation for occluded front (or occlusion).


In general use, often used as an abbreviation for observations; in the specific case of the newsgroup uk.sci.weather, used as part of the 'Subject' line where the body text contains observations in some structured (perhaps semi-coded) way and in a time sequence. The (suggested) format for the Subject is:

[OBS] < location >< date >

(this entry may be amended with experience and validation of the current trial - see also WR)

Occluded front

The merging of two fronts, as when a cold front overtakes a warm front.


(abbr) Occasional (well separated, as in CB in aviation forecasts.)


Abbreviation for occluded front (or occlusion).


(abbr.) 'operational' model


(abbr.) 'operational' model

Operational model

A term used to differentiate the primary NWP output from a particular centre from any ensemble products from the same source. The operational model will almost always be run at a higher resolution than that used for ensemble output. It must not, however be assumed that the 'Op/OPER' is necessarily the best outcome, particularly beyond 3 days or so. (see Ensemble)

Orographic forcing

An airstream encountering a barrier to its passage is forced to go around or over the obstacle. The upward deflection of the airflow is sufficient to give rise to adiabatic cooling, and if the air is moist enough, the formation of clouds, precipitation etc. In addition, convergence of the flow on the windward side (due to a rapid decrease in velocity) when the air encounters a sharply graded barrier not only enhances the vertical motion, but also leads to a deformation of the flow which in turn alters the vorticity of the air particles. Thus, hill and mountain ranges are most important in a study of meteorology.

Orographic rainfall/snowfall

For precipitation to occur, other conditions being satisfied (i.e. enough humidity, required temperature structure, sufficient depth of cloud etc.), there must be a supply of upward motion through the cloud producing the rain, snow or whatever. In orographic precipitation, the forcing agent is provided by large ranges of hills/mountains blocking the flow of humid air in such a way that vertical (upward) currents of air are produced, leading to adiabatic cooling >> condensation >> cloud formation/enhancement >> precipitation element growth. Orographic forcing OF ITSELF usually only produces small amounts of precipitation, but can be the means of enhancing or triggering other mechanisms (e.g. convective activity), and is one of the important elements in the 'seeder-feeder' model (q.v.). Computer models in operational use do now have sufficiently realistic orography and vertical resolution to model such, but the output (usually) does not explicitly define orographic precipitation.

Orographic waves

See Mountain waves.


(in an NWP ensemble suite) When considering a collection of solutions at a particular lead time (from a single-centre, or as part of a 'Poor Man's ensemble), some 'clustering' is usually observed - i.e. a large number of members pointing to a similar outcome. However, as lead times get longer (especially beyond 72 hours), one or two members may depart significantly from the ensemble mean, (and/or mode of larger clusters) - these are termed 'outliers'; such indications carry small weight, but cannot be totally ignored. In particular, at extended range (beyond about 7 days), there may be no clear 'clustering' signal, and an 'outlier' is just as likely to be right as a solution nearer the mean/median of the output. (See also ensemble, clusters, ensemble mean etc.)


(abbr) Overcast 8 oktas (cloud amount, as used in aviation reports, forecasts etc.)


The promise of a fine, sunny day is sometimes spoiled because cumulus cloud builds and spread out into an almost unbroken sheet of stratocumulus by late morning - which then refuses to break up for the rest of the day*. For this to occur, there must be a marked inversion (see "What is an inversion?") within 100 to 300 hPa of the surface, which must be intense enough to stop convective currents 'breaking - through' the inversion even at maximum temperature; the convective condensation level (CCL) must be at least 60 hPa below the inversion level, and the layer between CCL and inversion must have a reasonably high relative humidity. For some rather obscure reason, this phenomenon has come to be called 'overconvection' (at least in the UK, probably originating within the gliding community) - possibly because convective cumulus 'spills-over' to cover the sky? (* though, other conditions being right, the cloud may disperse around or just after dusk.)

Over-running trough

An active, mid-latitude frontal system is associated with a marked short-wave trough. The 'active' weather associated with the front lies forward of the trough, driven by the dynamics associated with it. At some stage in its life though, the trough (or a portion of it) will 'relax' (and effectively weaken), allowing the trough to run well ahead of the lower-tropospheric portion of the frontal system - it 'over-runs' the (surface) location of the front, and the activity at that position will decay. Note however that the upper trough will still have 'weather' associated with it - and may be the means of driving an upper/split frontal structure well away from the classical surface front as drawn on conventional analyses. See also "Why fronts die".



Pascal - allocated in honour of Blaise Pascal, to a unit of one N/m2, the basic unit of pressure in the SI system.


Some atmospheric processes are below the grid-scale/wavelength of operational meteorological computer models and cannot be handled explicitly by such schemes - for example individual showers, which are not only important for local weather, but have a feedback effect within the atmosphere that needs to be included in the NWP routines to maintain a realistic model of the real atmosphere. Larger scale model parameters (e.g. wind vector, temperature, humidity) are used to diagnose and represent the effects of such sub-gridscale processes: this is know as parametrisation. [ See HERE ]

Partial drought

A period of at least 29 consecutive days, whose mean daily precipitation does not exceed 0.01 inches / 0.2 mm. (See also drought; absolute drought; dry spell).


(abbr) Pressure gradient force


Is the study of times of naturally occurring events, such as the first blossom appearance in a long established species, or the departure of migratory birds. From 1875 to 1948, a register of such events was maintained by the Royal Met. Society, but after a period when the science was in the doldrums, the Woodland Trust and Centre for Ecology & Hydrology combined in the late 1990's to 'kick-start' the observing network, recognising that such data can complement studies into long-term climate change. For more detail, see:- http://www.phenology.org.uk/


(abbr) Ice Pellets (was PE); used in aviation weather reports.


(abbr) Polar mesospheric clouds (or Noctilucent clouds).


(abbr) Pressure at mean sea level: often seen in connection with NWP model products.


(abbr) Well developed sand/dust swirls; as used in aviation weather reports.

Polar front

A boundary that separates polar air masses from tropical air masses.

Polar low

(or Polar depression or Polar meso-cyclone) See "What is a polar low?".

Polar mesocyclones

A term now used to encompass the whole 'family' of disturbances resulting from arctic air flowing equatorward over progressively warmer seas; the term 'Polar Low' (q.v/above) is now often used only for 'extreme' systems where gale or near gale-force winds are observed.

Polar mesospheric clouds

(PMC) See entry under Noctilucent clouds.

"Poor Man's" Ensemble Technique

A true NWP ensemble (q.v.) is the product of multiple iterations of a single atmospheric model on a single centre's computer: the individual members of the ensemble run are obtained by perturbing the initial conditions very slightly to simulate the uncertainty that is always present at analysis time. However, long before these techniques were perfected, operational forecasters would (and still do) absorb the differing output from various international centres (e.g. EC, NCEP, DWD etc.) and / or different 'runs' from the same centre - treating all the various outputs as 'members' of what has been dubbed a 'Poor Man's Ensemble'. As with true ensembles, the more model runs that agree at a certain lead time, the higher is the confidence in that particular solution.

Potential Instability

(also known as Convective Instability) Said to exist when forced lifting (e.g. ascent over mountains or broad- scale/dynamic ascent) causes a layer, initially (just) stable to such forced ascent to become unstable. Decreasing humidity aloft is required within the layer, and heavy rain/thunder can be the result. Theta-W or Theta-E(q.v.) difference charts are often used to find such areas of potential instability: the usual levels used are at 850hPa and 500hPa. The value at 850hPa is subtracted from that found for 500hPa, and negative values so found indicate potential instability. Only slightly negative differences can lead to some significant convective activity .. all other factors being favourable of course. [ Such layers can also be inferred using a thermodynamic diagram (or tabular listing of Theta E or Theta W), noting where values decrease with increasing altitude within the low-to-middle troposphere (roughly up to 400hPa). ]

Potential Vorticity

The ratio of the absolute vorticity (q.v.) of an atmospheric column to the (defined) pressure difference across the column. This quantity is used to label air in much the same way as we use other conservative properties. As a column of air moves along, it 'shrinks' vertically (due to mass divergence) in just the right amount to decrease its absolute vorticity; as it expands vertically (due to mass convergence), its absolute vorticity increases. Therefore, Potential Vorticity tends to remain constant following the motion of the flow, for adiabatic motion.


Anything 'precipitated' by clouds (rain, snow, hail, drizzle etc.) is covered by this noun. Often abbreviated to 'ppn' or 'pptn'. (for definitions of various types of precipitation, see:- "Beaufort Letters")

Pressure Gradient

The difference in atmospheric pressure over a defined (usually horizontal) distance. (See "Why does the wind blow?")

Pressure Gradient Force

(abbr. = PGF)The force exerted on the air due to a pressure gradient, causing a tendency for movement (i.e. 'wind') from areas of high pressure to areas of low pressure.(See "Why does the wind blow?")

Prevailing visibility

Used in METAR (& TAF) coding - see Visibility [ reports ].

Prevailing wind

The most frequent wind direction for any particular location in a given period, e.g. a day, month, year or climatological period.


Partial fog (i.e. fog "banks"; substantial portion of airfield covered by fog - but not completely; visibility < 1000m.)


Probability (as used in aviation forecasts, e.g. TAFs; in the latter, under current [2002] rules, only PROB30 or PROB40 are allowed, e.g. 'moderate' probability of an event occurring.

Probability forecasting

Given that there is always a measure of uncertainty in forecasting the weather, the likelihood of an event happening can be expressed as a probability: thus a 70% chance of rain, 20% chance of thunderstorms etc. Often useful in finely balanced situations i.e. rain vs. snow; severe storms vs. no storm etc. (see also Deterministic forecasts).


When large scale features in the upper air, such as a 500 or 300 hPa trough/vortex drift west-to-east this is said to be a 'normal' progression of the pattern. (See also retrogression).


(abbr: Polar Stratospheric Clouds) During the polar 'night' (i.e. the period in the middle of the winter when insolation does not penetrate to ultra-high latitudes), the stratosphere cools significantly leading to 'closed-loop' circulations (both vertical and horizontal) which virtually isolate these polar stratospheric regions - the "Polar night vortex" is found, within which temperatures can be found well below (minus)75degC. In these extremely cold conditions, clouds are observed to form in the stratosphere, which appear to be composed of a combination of nitric acid and water. Stratospheric clouds can also form from ordinary water ice (i.e. as in the troposphere) but these are much less common at these high altitudes as the stratosphere is normally very dry and water-ice clouds only form at the lowest temperatures. The presence of PSC's and the part they play in the chemical interactions at these levels have been a subject of much debate in recent years. (See main FAQ here for the Stratospheric Night Jet and here for Stratosphere & various web-sites dealing with upper atmosphere ozone depletion.)

Pulse storms

(a term often used in North America) Random air-mass thunderstorms forming in an environment of little or no vertical wind shear, which appear as individual returns (without any obvious organisation) on radar/high-resolution satellite imagery systems. They usually last 20 to 30 minutes, perhaps up to 60 minutes, and give rise to small hail, sometimes heavy rain and perhaps weak tornadoes. They can be regarded as a more intense version of the single-cell convective type discussed in the main FAQ here, i.e. higher CAPE values are involved than for an 'ordinary' shower.

PVA region

An area where marked advection (movement) of positive, or cyclonic vorticity (q.v.) is occurring - hence Positive Vorticity Advection; often associated with a small upper trough running through the broadscale upper pattern. Cyclonic development will occur - other factors being favourable.



Pressure at airfield level; set on an aircraft (pressure) altimeter when height above local aerodrome level (strictly the official threshold elevation) is required.


Pressure at mean sea level (reduced according to actual/mean temperature).


Pressure at mean sea level (reduced according to ISA profile); set on an aircraft (pressure) altimeter when height above local mean sea level is required.



(abbr) Rain; as used in aviation (e.g. METAR/TAF) reports.


This is the transmission of energy by electromagnetic waves, which may be propagated through a substance or through a vacuum at the speed of light. Electromagnetic radiation is divided into various classes on the basis of wavelengths; these are, in order of increasing wavelength: gamma radiation, X-rays, ultra-violet (UV) radiation, visible (VIS) light, infra-red (IR) radiation and radio waves.


(sometimes abbreviated to 'R/S') An instrument that measures temperature, pressure and humidity of the atmosphere as it is carried aloft on a balloon. The "sonde" transmits its measurements to a ground-based radio receiver via radio signals, and by accurate tracking (radar or satellite) of the sonde unit, upper winds can be deduced.

Rain day

A period of 24hr, conventionally beginning at 09UTC, during which precipitation of 0.2mm or more has been recorded. (See also Wet day).

Relative humidity (RH)

See the main FAQ here.

Relative Vorticity

The vorticity (or tendency for air particles to 'spin') relative to the earth. It can be considered for practical purposes (and crudely assessed on meteorological charts) as the combination of two factors: (i): the 'spin' imparted due to the curved path that air takes in its passage through the atmosphere (cyclonically curved contours=positive, anticyclonically curved contours=negative). (ii) the other factor is due to the shear developed along the flow due to the differing velocities of the moving particles. Swiftly moving air will generate a 'twist' element relative to the lower-velocity flow on either side [ shear vorticity ]: where the 'twisting' generated is in a cyclonic sense, that is counted as positive; where in the anticyclonic sense then it is negative. ( See also Vorticity; Absolute Vorticity.)


When the amplitude of a trough decreases with time, the trough is said to have undergone relaxation. The change is usually measured in terms of a latitude change of a chosen contour or thickness line.


When an upper trough (or ridge) moves against the normal west-to-east flow in mid-latitudes, the feature is retrogressing, or undergoing retrogression.


(abbr)Relative Humidity (expressed as a % value).


See Upper ridge

Ridge amplification

When contour heights along the axis of an upper ridge increase, the ridge is amplifying.

Right entrance

On the warm side of the jet core, in the region of maximum acceleration of flow. Often associated with marked cyclogenesis.