Observations and Reports

Some notes on observing and reporting weather events, and the codes associated with this.


Aviation significant weather (SIGWX) charts

This is a brief note relating to what you will see on aviation significant weather (SIGWX) charts that are issued by many centres around the world, but principally looking at those issued by WAFC [World Area Forecast Centre] London (actually a unit within the Operations Centre in Exeter [Devon], having been in Bracknell until 2003, and before that, as RAFC [Regional Area Forecast Centre] London, at London/Heathrow airport until the 1980's).
( Note that the only other WAFC issuing products is called "Washington", but is in fact in Kansas! )

It is NOT intended to replace (and should not be used as) formal advice to pilots/aircrew but rather it is designed to be a quick 'aide-memoire' to interested amateurs who want to make sense of the charts.

Aviation flight-planning charts are usually issued to cover three broad 'operating levels' in the troposphere / lower stratosphere:

In general, the lower the level, the less area a particular chart will cover; they are generally issued every 6 hours, with a fixed validity time 00, 06, 12 or 18 UTC. Bear in mind though that there will be national and military departures from these general ideas.

You will find depicted on these charts all or some of the following:


Two symbols may not be understood - they depict icing and turbulence thus:-

   icing  turbulence
 moderate  moderate icing symbol  moderate turbulence symbol
 severe  severe icing symbol  severe turbulence symbol

(Light icing/turbulence is not found on the vast majority of charts - they may be indicated on some specialist/military charts)


... bold arrow following the wind ... showing location of core of winds >=80kn ... median height of core in FL notation (warm side of jet) ... where jet core strength is >=120kn, the spread vertically above/below the jet core (+/-) in hundreds of feet that the wind equals/exceeds 80kn ... short double-parallel strokes across jet showing steps of 30kn ... conventional speed notation (i.e. triangles 50kn, long feathers 10kn, short feathers 5kn).

Clear Air Turbulence (CAT)

... dashed lines enclosing areas of similar turbulence structure (intensity/height bands) ... base/top of layer (standard FL notation) ... XXX means below base level of chart ... number in square box refers to side-panel where is found the height/intensity information. (NB: not all jets generate significant turbulence).

Tropopause levels

... spot numbers in rectangular boxes (standard FL notation) ... boxes with upward pointing arrow denote tropopause highs ('domes') ... boxes with downward pointing arrow denote tropopause lows ('funnels') ... (tropopause level is where coldest air [aloft] is found in any particular airmass ... thus the level of greatest engine efficiency).

Cloud-weather (icing/turbulence)

... scalloped lines around areas of similar 'cloud' weather .. amounts / types of cloud in standard notation ... moderate / severe icing / turbulence ONLY indicated ... top / base of layer in FL notation ... note carefully: the top / base of the significant icing / turbulence layer is forecast, NOT the top / base of the clouds ... XXX means base of layer is below the lower boundary of the chart (or above the upper boundary of a low-level chart).
Specific notes regarding CB:
> Cumulonimbus (CB) imply severe icing and turbulence ... the symbols are not shown.
> ISOL CB only indicated on civil medium/high charts where embedded (EMBD) in layer cloud. Isolated CB in unstable air-masses that are not likely to be embedded are not shown on WAFC charts. However, they may be indicated on medium/high-level military charts, and also on all low-level charts, e.g. F215/415.
> OCNL/FRQ CB always shown, unless outside the vertical envelope of the chart.


... shown using standard notation AT THE SURFACE ... upper fronts and troughs are NOT indicated (on civil charts) ... occasionally expected movement is shown using an arrow and speed in knots.

Tropical Storms

... location and name (or sequential numerical identifier in some basins) of Tropical Storms (with of course the associated cloud-weather structure).

Volcanic activity

... the location of active volcanoes (sufficient to pose a hazard to in-flight aircraft from ingestion of ash/debris etc.) is shown, with its name and location offset ... users should refer to latest SIGMET's for further information.

Beaufort Letters

When recording weather in a diary, log or similar medium, it might be useful to know that there exists a system of 'shorthand' which can be used for the purpose. This page lists the letters, the method of using them, combinations etc., and some hints/tips.

The system of abbreviations was first used by Francis Beaufort (later Admiral Sir Francis) early in the 1800's, and intended for use at sea. The scheme has been considerably revised since those days.

It is accepted practice to record phenomena at a particular time in the following order:

  1. State of Sky.
  2. Thunderstorm (if present).
  3. Precipitation (if present).
  4. Atmospheric obscurity (if present).
  5. Any other phenomena.

The following is the list of abbreviations used, ordered in sections as above: NB... you can click on the subject lines above to go directly to that section of the document.



b Total cloud amount: 0 to 2 oktas (eighths of sky covered)
bc Total cloud amount: 3 to 5 oktas
c Total cloud amount: 6 to 8 oktas*
o Uniform thick layer of cloud, completely covering the sky (i.e. 8 oktas - no gaps: mainly used with St, Ns and thick As)


* There are often occasions when large amounts of cirrus cloud cover the sky, but the day is bright or even quite strongly sunny. The system does not distinguish between c = cloudy/grey and c = cloudy/bright + fine, so it is often useful to note in plain language what the 'character' of the sky is, e.g. c(bright, fine with diffuse sunshine).

As an optional extra, plus / minus signs can be used to indicate whether the cloud cover has been generally increasing / developing over the past hour (+), or decreasing / dissolving (-), thus: bc+, b-etc.

State of sky letters are always recorded, unless there is deep fog or thick falling snow, such that the sky state is not possible to determine. In this case, the state of sky letters are left off.

Observing cloud amounts: Estimating the amount of sky covered with cloud is not an easy task - even overcast / 8 oktas and sky clear are not straightforward!



l Distant lightning (storm too far away for sound to reach observer)
t Thunder heard; no lightning seen
t l Thunderstorm


l, lightning seen (thunder not heard), is obviously more likely to be observed at night. During the day, if lightning is so bright as to be visible, then the thunder will also be heard, and 'tl' should be recorded.

A Thunderstorm is regarded as being as being 'at' a particular point from the first hearing of thunder, to 10 minutes after the last hearing of thunder, in cases where there is doubt about whether a storm has passed by or not. However, when it is obvious that a storm has passed, then a note can be made to that affect, even if distant thunder can still be heard.

Lightning and / or precipitation may or may not be observed - the important thing for reporting thunder is the sound of same. See note below regarding the coding of the intensity of thunderstorm.

The intensity of a thunderstorm is a matter of great subjectivity. It is based upon the lightning / thunder frequency only, the precipitation is assessed separately. However, in some situations, the 'vividness' of the lightning display and 'loudness' of the accompanying thunder [ notwithstanding the actual frequency of same ], may be relevant to assessment, though some plain language comment should be made. In particular, it is useful, if possible (and safe), to note things like whether the lightning is cloud-to-cloud (CC), cloud-to-ground (CG) etc., and also the actual number of discharges detected per minute.

PRECIPITATION (i.e. rain, snow, hail etc.)



d Drizzle, freezing drizzle(*) [ " fairly uniform precipitation composed exclusively of fine drops of water, less than 0.5 mm diameter, very close to one another; the effect of their individual impact on water surfaces is imperceptible " ]
dr Drizzle and rain mixed
h Hail, small hail, snow pellets, diamond dust, ice pellets($)
hr Hail($) and rain mixed
hs Hail and snow($) mixed
r Rain, freezing rain (*) [ " precipitation of liquid water particles, either in the form of drops of more than 0.5 mm diameter or of smaller, widely scattered drops. " ]
rs Rain and snow mixed, or partially melted snow ('sleet')
s Snowflakes($)
sh Snow grains($) ( known in some texts as 'granular snow' )


(*) Freezing rain/drizzle, is liquid precipitation that freezes on contact with surfaces chilled below zero deg.C.

Precipitation is recorded by type, intensity and continuity.


The type of precipitation is indicated by the appropriate letter, or combination of letters if there is a mixture of precipitation. Example: d = drizzle, r = rain ; dr = drizzle and rain : If the precipitation is in the form of SHOWERS, i.e. falling from cumulus, cumulonimbus or altocumulus, then the prefix 'p' is used [originally from 'passing'], thus a shower of snow is ps; a shower of rain and snow mixed is prs etc. Showery precipitation is usually marked by abrupt beginning and end, and by rapid fluctuations in intensity. The particles are often larger than those falling in non-showery situations.

$ Notes relating to the observing/recording of 'solid' precipitation, and cloud types associated:
( notes are appended to the bottom of this page ---- click on the link above to go straight there )


There are four categories of intensity: slight, moderate, heavy and violent. To indicate slight precipitation, the letter denoting the type of precipitation is followed by a subscript 'o', thus; slight rain = ro, slight shower of rain and snow = proso etc. Note that in mixed precipitation, the subscript is applied to both elements (see also the note at the end of this paragraph). For moderate precipitation, the letter alone is used, thus moderate shower of rain = pr. For heavy precipitation, use capital letters. Thus for heavy snow, use S. For violent phenomena, usually applied to showery/convective precipitation, use a subscript 2, thus a violent shower of rain = pR2. On a general note relating to mixed precipitation, when two types of precipitation co-exist, the intensity of the 'heaviest' type governs the coding: thus for a shower which contain a moderate fall of rain, with a slight accompanying fall of snow, this would be noted as: prs. However, some plain language remark should be made in such a situation to amplify.

When some method of measuring the rate of rain / snow falling is available, then the following should be used to decide upon the intensity:

RAIN: (i.e. from layer clouds)

  1. Slight Rain: rate of accumulation is slow, not more than about 0.5 mm per hour.
  2. Moderate Rain: enough rain to form puddles in a short time, and giving a rainfall rate of between 0.5 and 4 mm per hour.
  3. Heavy Rain: Makes a noise of roofs, a splashing off hard surfaces (roads etc.) and gives an accumulation of more than 4 mm per hour.

SHOWERS: (i.e. from cumuliform clouds)

  1. Slight shower: less than about 2 mm per hour.
  2. Moderate shower: from about 2 mm/hr to 10 mm/hr.
  3. Heavy shower: about 10 to 50 mm/hr.
  4. Violent shower: above about 50 mm per hour.

SNOW: (applicable to both layer and cumuliform type clouds - assumes no drifting.)

  1. Slight snow: flakes are sparse. Rate of accumulation not exceeding 0.5 cm per hour.
  2. Moderate snow: visibility impaired. Rate of accumulation 0.5 to 4 cm/hr.
  3. Heavy snow: visibility much reduced - often to or below fog limits - with accumulations more than 4 cm per hour.

As regards noting the the intensity of thunderstorms with rain / snow, the intensity of the thunderstorm is considered separately from that of the accompanying precipitation. The intensity of a thunderstorm is judged by the frequency of the thunder and lightning, so if there is a thunderstorm in progress with little thunder / lightning activity, but heavy rain, this would be noted as: tloR. Note that any intensity qualification for a thunderstorm is applied to the combined letters, tl, not to each letter. So a 'slight' thunderstorm is noted as: tlo.

Continuity (i.e. how long does it go on for!):

When precipitation falls from layer cloud, then the continuity of such precipitation is noted in the following manner:

  1. When considering the past hour, if the precipitation (i.e. rain, snow etc.) has been of a 'start-stop' variety, then the letter for type ( e.g. r ) is prefixed by the lower case ' i '. Thus for intermittent slight rain, this would be noted as..... iro, and for intermittent moderate rain and snow mixed as..... irs etc.
  2. When, over the past hour, the precipitation has continued at that intensity, without a break, then the letter denoting the precipitation type is repeated, thus for slight rain that has continued for an hour without a break this is noted as..... roro, and continuous heavy snow by..... SS etc.
  3. If noting precipitation not specified as continuous or intermittent, the Beaufort letter is used alone. For example R = heavy rain; doro=slight rain and drizzle mixed.

When there is a change of type and / or intensity, then this is indicated by successive use of letters as appropriate. This is best demonstrated by an example:

Consider light snow that has continued for over an hour at the same intensity without a break, this then turns to slight rain / snow mixed, but for a brief (less than one hour) period turns to heavy snow (sky obscured), before becoming light snow for less than one hour, then dies out. The sequence of Beaufort letters would be:

csoso, croso , S , cso , c

Note that each group of letters is separated by a comma, though will see in some texts that the comma should only be used for 'clarification'. In my view, it is best to always use it to avoid any possible confusion.

ATMOSPHERIC OBSCURITY (i.e. factors reducing visibility, other than precipitation.)


f Fog, ice fog ^(visibility < 1000 m)
fe Wet fog(visibility < 1000 m)
fg/fs Shallow land/sea fog(visibility above the fog >=1 km)
F Fog, ice fog ^(visibility < 200 m)
ks Drifting or blowing snow(%)
kz Dust or sandstorm (visibility < 1000 m)
m Mist (visibility >=1 km) [ Relative Humidity >~95% ]**
z Haze


(%) The distinction between the two is that blowing snow is raised well above the normal (adult) 'eyeline', and thus reduces the overall visibility markedly, and drifting snow remains below the 'eyeline', and does not materially affect the prevailing visibility.

^Ice Fog: fog consisting of minute ice crystals (as opposed to water droplets for water fog).
When the visibility varies with direction around an observer, such that in one area the visibility is below 1000 m and elsewhere it is at or above, then fog patches exist, and the letter for the type of fog is prefixed by 'i', thus ' if '

As with precipitation, changes in occurrence, thickness etc., of fog is noted by successive use of appropriate letters: thus bcif, cf, ff, F ..... indicates that fog patches extend to the whole area, which last for at least an hour without appreciable change, then thickens further to reduce the visibility below 200 m.

** on the subject of the relative humidity with mist, this can become a bone of contention where several observers are noting the same/similar events. The figure quoted is meant to be a guide, not a mandatory limit.

and although not strictly belonging to this group:

v Abnormally good visibility


What the definition of 'abnormal' is when talking about visibility can be the subject of many an argument between observers. In a polar airflow over many parts of Scotland, 60 km or more would not be abnormal, but in the hazy Thames Valley basin, it might well be.



e Wet air, without rain, snow etc. falling
g Gale (mean speed 34-47 knots over a 10 minute period)
G Storm (mean speed 48 knots or more over a 10 minute period)
i Intermittent (used with precipitation and fog)
j Phenomenon within site but not at the location of the observer (**)
kq Line Squall(***)
p Shower -- see notes against precipitation for usage.
u Ugly, threatening sky (in addition to bc/c/o etc.)
w Dew deposit (make sure that 'dew' is really being deposited,
and not just 'guttation' water extruded from plant leaf surfaces on some cold nights.
x Hoar frost deposit*
y Dry air -- less than 60 % relative humidity

* The letter ' x ' (hoar frost) is used when a white, crystalline deposit of ice is observed on solid objects, after a cold, clear night. Hoar frost occurs when water vapour condenses (sublimates) directly to the ice phase without an intermediate liquid phase (though the process is usually initiated by a vapour-to-liquid phase). Hoar frost should not be used for the freezing of water already present, or for the glaze produced by freezing rain / drizzle, or for the rime produced during freezing fog episodes - all these phenomena should be noted in plain language separately.
** The letter ' j ' is used in combination with various other letters to record phenomena occurring within sight of, but not at the station; thus jp indicates a shower within site but not at the observing point;

for example:

jp .... precipitation within sight (often used with showers); however note that if the 'distant' shower was at the point of observation earlier, and has moved away, jp is not used in this context.

jf .... fog within sight (visibility at the station 1000 m or more )

jks .... drifting snow within sight.

*** A squall is differentiated from a gust by its greater duration: generally lasting for several minutes before decaying again. Squalls are often associated with the passage of fronts, particularly cold fronts, or well defined troughs, or with the 'gust front' from a well defined/mature supercell Cb. To qualify as a line squall, other marked changes are often observed, e.g. change of wind direction, fall of temperature etc. The following definition is used when estimating wind speeds using the Beaufort scale of wind speed: " .... a sudden increase of wind speed by at least three levels of the Beaufort scale, the speed rising to F6 or more and lasting for at least one minute."

$ Notes relating to the observing/recording of 'solid' precipitation, and cloud types associated:

[ for more on each type, including how the precipitation is formed, click on the name. ]
  1. SNOW / SNOW-FLAKES: Precipitation composed of white, ice crystals - often aggregated into clusters/clumps depending upon the temperature in the lowest layers of the atmosphere. Roughly, at temperatures above about - 5 degC, then clustering is likely, but at lower temperatures, individual crystals may be expected. CLOUD OF ORIGIN: As, Ns and occasionally Sc (usually with a high convective element - e.g. downwind of the North Sea in winter); Cu and Cb in showers.
  2. SNOW GRAINS: (also called granular snow) Very small (dia: < 1 mm ) white and opaque grains of ice. The grains are flattish or elongated. When the precipitation reaches the ground, they do not bounce or break up and usually fall in small amounts - not sufficient to cover the ground and not from shower cloud. CLOUD OF ORIGIN: Usually St, Sc (shallow, but sub-zero top) or perhaps Fog.
  3. SNOW PELLETS: (also called soft hail) White and opaque grains of ice. These grains are spherical or conical. The diameter is larger than that of Snow Grains (q.v.), being in the range 2 to 5 mm. The grains are easily crushed and rather brittle and tend to bounce when falling on hard surfaces, sometimes breaking up in the process. Snow pellets generally occur mixed with other shower-type precipitation: e.g. rain or snowflakes, when surface temperatures are around zero deg.C. CLOUD OF ORIGIN: Cu, Cb and sometimes Sc.
  4. ICE GRAINS: (also called Ice Pellets [ a ] or just Ice Pellets) These transparent or semi-transparent pellets of ice usually bounce when hitting a hard surface and make a sound on impact. They are usually spherical and have a diameter less than about 5 mm. Frozen raindrops, or largely melted and refrozen snowflakes. The freezing process is thought to take place close to the ground. CLOUD OF ORIGIN: As, Ns.
  5. SMALL HAIL: (also called Ice Pellets [ b ]) The appearance and behaviour is similar to Ice Grains above. Pellets of snow, encased in a thin layer of ice, which may be the result of interception of super-cooled water droplets, or the pellet itself partially melting, then re-freezing. CLOUD OF ORIGIN: Cb or vigorous Cu
  6. HAIL: Small balls/pieces of ice falling either separately or massed together in irregular lumps. The diameter is very variable, from 5 mm up to 50 mm, or (in really extreme cases more.) Hailstones are composed almost exclusively of transparent ice, or a series of transparent layers, alternating with semi-transparent/cloudy ice, indicative of the formation process whereby the hailstone has spent some time inside the parent Cb, at varying altitudes gathering coats of ice from different regions of the cloud, and having markedly different temperatures from region to region. Where possible, the diameter of the hailstones should be noted, and photographs taken against a scale/ruler etc.
  7. ICE NEEDLES: (or ice prisms, or diamond dust) unbranched ice crystals, in the form of needles, columns or plates: so tiny that they often appear to be suspended in the air, and can give rise to spectacular visual effects when the sun shines through them - indeed it is not necessary for cloud to be present as they are often observed (in polar latitudes) under a clear sky. Need very low temperatures, and conditions are often not right in maritime NW Europe - better inland.


Precipitation Formation Processes

A note regarding how various types of precipitation are formed, with an eye to use by observers trying to distinguish one type of hydrometeor from another.

0. Outline:
This note is intended to amplify the rather 'bald' descriptions attached to the various precipitation types elsewhere; what I have attempted is to describe, using generally accepted current ideas, how we arrive at a drizzle droplet, or why a snow pellet is what it is!

However, cloud physics is a highly specialised field of research and readers are urged to bear this in mind - the text below is an attempt to capture the important elements and thus may not fully describe the processes involved with sufficient scientific rigour.

1. Classification of precipitating clouds:
In very broad terms, precipitating clouds can be classed in one of two ways:


Both types of classification will be mentioned below. The primary division for the purpose of this note though will be that due to temperature structure throughout the precipitating cloud.

2. Warm clouds.
In 'warm' clouds, precipitation formation depends upon the individual tiny cloud droplets (which of themselves are too small to fall through cloud-environment updraughts) growing by collision / coalescence, such that they eventually become heavy enough to overcome the upward currents and fall from the base of the cloud. [ It is of some interest here to note that just because droplets collide, they do not necessarily 'join together' - see specialised texts for more on this. ]

In stratus (St) and stratocumulus (Sc) clouds, which are fairly shallow, precipitation is not common unless turbulence within the cloud causes enhanced collision and capture / coalescence between droplets of varying size - even then the air below the cloud must be humid enough to allow rain or drizzle to reach the ground without evaporation. The cloud-water density within the cloud must also be high, or evaporation within the cloud will offset the growth of large, precipitating elements.
Sometimes, particularly within Sc over the sea, instability causes enhanced vertical motions (and therefore increased chance of growth by collision / coalescence), and rain is more readily achieved.

However, the classic 'warm cloud' precipitation belongs to the tropical convection group; here vigorous upward currents allow huge quantities of water droplets to grow to a large size quite quickly (by collision & coalescence / wake-capture of drops with varying fall-speeds), before finally overcoming the net upward motion and rushing to earth as a torrential (or 'tropical') rain-storm. This plume of earthward-headed larger drops will also gather (or 'sweep up') other drops on the way in an accelerating (or 'chain-reaction' ) fashion, adding to the high surface rainfall totals.

3. Cold clouds.
In 'cold' clouds, the fact that ice crystals (albeit in fairly low concentration) and water droplets (with temperature below 0degC) can co-exist is the key factor in the precipitation process. Water droplets are usually dominant (at temperatures warmer than about -20degC) but the ice crystals are more efficient centres of growth from the vapour stage; this follows from the difference between the value of saturation vapour pressure over an ice surface, when compared with a water surface. In these 'mixed' clouds, the air is close to being saturated with respect to liquid water, but is super-saturated (an unstable phase) with respect to ice. Consequently, in mixed clouds, ice crystals grow from the vapour phase much more rapidly than do the nearby droplets. This is usually known as the Bergeron - Findeisen process.
[ In fact, the idea could be ascribed as easily to Alfred Wegener who suggested this process as early as 1911; the theory was refined by Bergeron (~1933) and Findeisen (~1938).]

The ice crystals that grow in this way may themselves fall from the cloud as precipitation but in very small quantities & in isolated elements. Usually though, some further growth is required from one (or both) of the following:
(i) the enhanced crystal might grow through aggregation ('joining-together') with other such crystals (snow), or . . .

(ii) coalesce with water droplets in the riming process (graupel formation), which will lead to hail of varying sizes, densities and composition.

The final precipitation element is largely dependent upon the temperature, size & concentration of the water droplets involved - and in many cases, melting below the cloud base will lead to rain, rather than snow, soft-hail etc. All of these are dealt with below.

4. Types of 'solid' precipitation.

5. Detailed notes on observed surface precipitation.
Listing (based on the ordering in the "Observer's Handbook"): the listing for Code 4680 are subject to change.

 Alternatives:  (none)
(4677 ww):
(FM 12/13-VII)
 21, 25, 58, 59, 60, 61, 62, 63, 64, 65, 80, 81, 82, 91, 92, 95, 97
(4680 wawa):
(FM 12-IX)
 21, 23, 40, 41, 42, 43, 44, 57, 58, 60, 61, 62, 63, 80, 81, 82, 83, 84, 92, 95
 METAR (w'w'):  RA, SHRA (& variants / combinations)
 Beaufort letter:  r, pr (and variants for intensity, continuity)
 Symbol:  image of rain symbol
 Standard description & additional notes: precipitation of liquid water particles, either in the form of drops of more than 0.5mm diameter or of smaller widely scattered drops. The droplets should make an impact on a still (or near-still) water surface such as a puddle, pond, lake etc.
 Clouds producing precipitation: Stratocumulus (Sc), Altostratus (As), Nimbostratus (Ns), Altocumulus (Ac), Cumulus (Cu), Cumulonimbus (Cb).
 Physical processes: In mid & high latitudes the Bergeron-Findeisen process (see elsewhere) is important, as ice crystals grow, aggregate into snow-flakes, which upon melting fall as rain at the surface. However, other mechanisms produce rain at the surface, chief amongst them being the formation of 'graupel' or 'snow pellets' which form by riming within a deep cloud with strong vertical motion (convective types), and then outside of deep winter airflow, these elements melt before reaching the surface to give the rain. Another process involves rapid collision / coalescence of unstable water droplets in warm clouds - those with the temperature wholly above freezing-point: this is primarily a tropical phenomenon.
 Alternatives:  (Glaze, Glazed Ice, "Ice storm", Rain-ice)
(4677 ww):
(FM 12/13-VII)
 24, 66, 67
(4680 wawa):
(FM 12-IX)
 21, 25, 40, 41, 42, 43, 44, 47, 48, 64, 65, 66, 80
 METAR (w'w'):  FZRA (and variants for intensity)
 Beaufort letter:  (no distinction from rain - see above)
 Symbol:  image of freezing rain symbol
 Standard description & additional notes: rain the drops of which freeze on impact with the ground or with objects on the earth's surface or with aircraft in flight. (NB: with changes to the METAR coding, this is now reported with the air temperature below 0degC, whether or not ice is actually deposited.)
 Clouds producing precipitation: (as for rain - see above)
 Physical processes: The original precipitation is produced as for rain (see above), but at some stage, the droplets, having been at a temperature just above zero, encounter a near-surface sub-zero layer which brings the temperature of the liquid droplets back down close to or below 0degC. If these then encounter objects with temperatures below zero, then glazed ice is the result. Of particular importance to low-level aircraft operations, for public-service safety forecasts (ice on roads, pavements etc.), and for advice to utilities having overhead transmission networks.
 Alternatives:  (none, apart from dialect e.g. 'mizzle', though in North America it may be called 'mist')
(4677 ww):
(FM 12/13-VII)
 20, 50, 51, 52, 53, 54, 55, 58, 59
(4680 wawa):
(FM 12-IX)
 21, 22, 40, 41, 42, 43, 44, 50, 51, 52, 53, 57, 58, 80
 METAR (w'w'):  DZ (and variants for intensity & combinations)
 Beaufort letter:  d (and variants for intensity & continuity)
 Symbol:  image of drizzle symbol
 Standard description & additional notes: fairly uniform precipitation composed exclusively of fine drops of water, less than 0.5 mm diameter, very close to one another. The effect of their individual impact on (still or near-still) water surfaces is imperceptible - i.e. it does not produce 'ripples' etc. Unless a drizzle-producing event is long-lasting (many hours), then this type is unlikely to produce quantities sufficient to trigger automatic rain-gauge 'trips'. As it is also not picked up by standard weather radar imagery, human observation is essential to detect such precipitation. Visibility is usually moderate or poor in association with a drizzle event.
 Clouds producing precipitation: Stratus (St): Drizzle falls from a fairly continuous and dense layer of stratus, usually with a low base, sometime touching the ground (fog / hill fog). Stratocumulus (Sc), may also produce drizzle under certain circumstances [ high relative humidity below the cloud ], though usually Stratus is present & it may be unclear whether it is truly the Sc or the St that is producing the precipitation.
 Physical processes: Coalescence of the very small cloud droplets to such a size (albeit still very small) that they can just leave the cloud against any weak upcurrents that are involved in the cloud-formation process. These St & Sc clouds are essentially a 'warm' cloud type (i.e. presence or absence of ice crystals are not a factor in production of preciptiation) and the clouds must have a high liquid water content to precipitate, or evaporation will offset the precipitation-forming process. The relative humidity below the cloud base must also be high (well above 90%) or again, evaporation will come into play. And, although vertical (upward) motion must be small (or the elements would not fall from the cloud), there must be a reasonable amount of turbulence through the cloud-layer to lead to efficient collision & coalescence. The cloud must also be reasonably thick - various studies suggest somewhere between 400 & 600 metres (or roughly more than 1300 ft), but this is highly dependent upon such things as condensation nuclei present, humidity content, updraught strength etc.
 Alternatives:  (Rime, Rime-ice)
(4677 ww):
(FM 12/13-VII)
 24, 56, 57
(4680 wawa):
(FM 12-IX)
 21, 25, 40, 41, 42, 43, 44, 47, 48, 50, 54, 55, 56, 80
 METAR (w'w'):  FZDZ (and variants for intensity)
 Beaufort letter:  (no distinction from drizzle - see above)
 Symbol:  image of freezing drizzle symbol
 Standard description & additional notes: drizzle the drops of which freeze on impact with the ground or with objects on the earth's surface or with aircraft in flight. (NB: with changes to the METAR coding, this is now reported with the air temperature below 0degC, whether or not ice is actually deposited.)
 Clouds producing precipitation: (as for drizzle - see above)
 Physical processes: The original precipitation is produced as for drizzle (see above), but at some stage, the droplets, having been at a temperature just above zero, encounter a near-surface sub-zero layer which brings the temperature of the liquid droplets back down near to or below 0degC. If these then encounter objects with temperatures below zero, then glazed ice is the result. Of particular importance to low-level aircraft operations, for public-service safety forecasts (ice on roads, pavements etc.), and for advice to utilities having overhead transmission networks - however, as the droplet size and intensity of fall are usually small, these do not pose the same problems as for freezing rain (above).
 Name:  SLEET
 Alternatives:  (Rain and snow mixed, 'slushy' or wet snow etc.)
(4677 ww):
(FM 12/13-VII)
 23, 26, 68, 69, 83, 84, 93, 94, 95, 97
(4680 wawa):
(FM 12-IX)
 21, 24, 40, 41, 42, 45, 46, 67, 68, 80, 92, 95
 METAR (w'w'):  RASN (and variants for combinations & intensity)
 Beaufort letter:  rs (and variants for intensity & continuity)
 Symbol:  image of rain and snow symbol
 Standard description & additional notes: precipitation of rain and snow mixed, or of partially melted snow.
[ On an historical note: the word 'sleet' has a completely different meaning in the United States: where it relates to Ice Pellets (q.v.), but in the British Isles, sleet is reserved for a mix of rain and snow, or for melting snow. The word 'sleet' can be traced, through variants, back to at least the 13th century. Because of the potential confusion, meteorologists try to avoid the use of the word - not very successfully! ]
 Clouds producing precipitation: Altostratus (As), Altocumulus (Ac), Nimbostratus (Ns), Stratocumulus (Sc), Cumulus (Cu), Cumulonimbus (Cb).
 Physical processes: as for rain and snow, but at some point, either the snow element partially melts, or the combination of melting and evaporation in the layers near the surface allow rain and snow to penetrate to the surface together.
 Alternatives:  (none)
(4677 ww):
(FM 12/13-VII)
 22, 26, 70, 71, 72, 73, 74, 75, 85, 86, 93, 94, 95, 97
(4680 wawa):
(FM 12-IX)
 21, 24, 40, 41, 42, 45, 46, 70, 71, 72, 73, 80, 85, 86, 87, 92, 95
 METAR (w'w'):  SN, SHSN (and variants for combinations & intensity)
 Beaufort letter:  s, ps (and variants for intensity & continuity)
 Symbol:  image of snow symbol
 Standard description & additional notes: precipitation of ice crystals, most of which are branched (sometimes star-shaped), and often having a light, feathery structure. The branched crystals are sometimes mixed with unbranched crystals. At temperatures higher than about -5degC, the ice crystals are generally agglomerated into snowflakes [ water coating aids adhesion of individual flakes ]. Small flakes, up to 4 or 5 mm in diameter, especially those occurring at the beginning of a snowfall in very cold weather, often show a six-rayed star-like structure of great beauty. Larger flakes usually consist of tangled aggregates of such crystals, so that the geometrical structure ceases to be evident.
 Clouds producing precipitation: Altostratus (As), Altocumulus (Ac), Nimbostratus (Ns), Stratocumulus (Sc), Cumulus (Cu), Cumulonimbus (Cb).
 Physical processes: Deposition of vapour to ice (on ice nucleus - sublimation). Individual crystals grow / aggregate with others within the cloud, then fall through updraughts to precipitate to the surface. In the range -4degC to 0degC, most crystals carry a thin film of super-cooled water & aggregate easily: this is why large collections of flakes are lumped together at these sort of (surface) temperatures. Conversely, at much lower temperatures (but with no definite values involved), the crystals are 'dry' (i.e., they do not carry this water coating), and individual crystals fall.
 Alternatives:  (Soft hail, Graupel, 'Tapioca snow')
(4677 ww):
(FM 12/13-VII)
 27, 87, 88, 93, 94, 96, 99
(4680 wawa):
(FM 12-IX)
 21, 40, 41, 42, 45, 46, 80, 89, 93, 96
 METAR (w'w'):  GS (and variants for combinations and intensity)
 Beaufort letter:  ph
 Symbol:  image of small hail symbol
 Standard description & additional notes: precipitation of white and opaque ['cloudy'] grains of ice. These grains are spherical** ['rounded'] or sometimes conical** ['drawn to a point']; their diameter is about 2 to 5 mm. ( they may also be observed lumped into irregular 'graupel' coagulations. The grains are brittle and easily crushed (hence 'soft' hail); when they fall on hard ground, they bounce and often break up (which helps to distinguish them from snow grains) - but of course, on grass surfaces, they are often preserved intact. Precipitation of snow pellets generally occurs in showers, together with precipitation of snowflakes or raindrops, when surface temperatures are not far from 0degC. In other words, to contrast them from other (similar) types, they are regarded as a 'cold-weather' phenomenon.
[ ** whether graupel is rounded or pointed appears to depend upon how the element falls: if it rotates on the way down, then growth is fairly even, and a spherical object is achieved; if it comes straight down, the element is drawn-out to a 'tail' with the 'head' pointing downwards. ]
 Clouds producing precipitation: Cumulus (Cu), Cumulonimbus (Cb) [perhaps Stratocumulus over/downwind sea/coasts (Sc).]
 Physical processes: Either snowflake intercepting supercooled water drops (in which case, on careful examination, the original snow element may be seen) or cloud-ice particle (grown by vapour sublimation) intercepting supercooled water drops which freeze on contact & collect more super-cooled drops - both essentially a heavy 'rime-ice' process within the cloud. Growth is therefore due to collision & coalescence, in this instance with relatively small droplets; the opaque character is due to the trapping of air within the ice as it forms on the initial particle, when the liquid water content of the parent cloud is small. Areas downwind of relatively warm seas in winter / early spring are favoured, e.g. Northern Ireland, NW Wales.
 Alternatives:  (Granular snow)
(4677 ww):
(FM 12/13-VII)
 20, 77
(4680 wawa):
(FM 12-IX)
 21, 22, 40, 41, 42, 45, 46, 77, 80
 METAR (w'w'):  SG (and variants for intensity)
 Beaufort letter:  sh (and variants for intensity & continuity)
 Symbol:  image of snow grain symbol
 Standard description & additional notes: precipitation of very small white and opaque ('cloudy') grains of ice. They resemble snow pellets in external appearance, but are fairly flat or elongated; their diameter is generally less than 1 mm. When the grains hit hard ground, they do not bounce or shatter (c.f. snow pellets - in fact, snow grains in my experience in the British Isles usually just 'float' to the surface in a rather lazy fashion). They usually fall in very small quantities, mostly from stratus or fog, and never in the form of a shower. [ An observer I used to work with called these 'sago', which is not a bad description!]
 Clouds producing precipitation: Stratus (St), Stratocumulus [low/deep](Sc), [perhaps Fog].
 Physical processes: These elements are the cold-weather equivalent of drizzle. They form in / fall from shallow clouds only. Cloud particles (ice & water drops) aggregate / coalesce and manage to fall through any weak updraughts. Variation of the rime-ice process (with very small supercooled water drops involved). Vertical motions are small, therefore the resultant precipitation is small in size and light in accumulation. The physical structure can be quite variable: from the AMS Glossary ... "very fine, simple ice crystals; tiny, complex snow crystals; small, compact bundles of rime; and particles with a rime core and a fine glaze coating". The cloud type, and the fact that they usually fall in very small quantities distinguish them from snow pellets - the latter are primarily a convective type, these fall in highly stable air mass situations.
 Alternatives:  (Ice Grains, Ice Pellets type [a], Grains of Ice (British only), Sleet (US only))
(4677 ww):
(FM 12/13-VII)
 23, 79
(4680 wawa):
(FM 12-IX)
 21, 40, 41, 42, 45, 46, 74, 75, 76, 80
 METAR (w'w'):  PL (and variants for intensity)
 Beaufort letter:  h (and variants for intensity)
 Symbol:  image of a pellet symbol
 Standard description & additional notes: precipitation of transparent [ clearly see through ], or translucent [ see vaguely through ] pellets of ice, which are spherical [ rounded ] or irregular (only rarely conical [ with a distinct point ]), and which have a diameter of 5 mm or less. The pellets of ice usually bounce when hitting hard ground and make a sound of impact. Usually ice pellets are not easily crushable. On close inspection, the structure of the original pellet or flake is not evident. [To distinguish them from small hail (below), this type falls from layer cloud.]
[ On an historical note: the word 'sleet' is in common usage in the United States for this type of precipitation (since at least the latter part of the 19th century), but in Europe (more particularly in the British Isles), sleet is reserved for a mix of rain and snow, or for melting snow. The word 'sleet' can be traced, through variants, back to at least the 13th century. Because of the potential confusion, meteorologists try to avoid the use of the word - not very successfully! ]

[ Additional confusion was caused (in my humble view) by the no doubt well-meaning attempt to clarify the above via the WMO declaring that there would be two forms of 'Ice Pellets' ... type [a] and type [b]. As far as I can work out, this was promulgated sometime between 1951 (the founding year of the World Meteorological Organisation) and 1956 (when the WMO International Cloud Atlas was published, and probably around 1954/55). What were Grains of Ice (British) and Sleet or Ice Pellets (US) were assigned to the nomenclature: "Ice Pellets type [a]" & what were widely known as small hail were re-named "Ice Pellets type [b]": yet the formation, originating cloud type and indeed appearance of the two types appear to be quite distinct. The use of 'type a/b' gained much ground in the 1960's, but appears to have fallen out of favour now - probably for the best!]
 Clouds producing precipitation: Altostratus (As), Nimbostratus (Ns).
 Physical processes: These are regarded as frozen raindrops or largely melted and refrozen snowflakes. The freezing process usually takes place near the earth's surface, after the precipitation elements have descended through a layer where the temperature is below 0degC, which in turn must be deep enough to cool the droplet below 0degC. The freezing process proceeds from the outer skin of the droplet inwards. These precipitation falls are indicative of "freezing rain" temperature structure which may accompany this phenomenon, and are therefore important for aviation forecasting & for advice to utilities with overhead transmission systems.
 Alternatives:  (Ice Pellets [b], Graupel)
(4677 ww):
(FM 12/13-VII)
 27, 87, 88, 93, 94, 96, 99
(4680 wawa):
(FM 12-IX)
 21, 40, 41, 42, 45, 46, 80, 89, 93, 96
 METAR (w'w'):  GS (and variants for combination), SHPL (see below)
 Beaufort letter:  ph
 Symbol:  image of small hail symbol
 Standard description & additional notes: precipitation of transparent [ clearly see through ], or translucent [ see vaguely through ] pellets of ice, which are spherical [ rounded ] or irregular, rarely conical [ with a distinct point ], and which have a diameter of 5 mm or less: they have a glazed appearance & on close inspection (with a magnifying glass), the structure is still evident. The pellets of ice usually bounce when hitting hard ground and make a sound on impact. At first glance, these might be confused with snow pellets (see above), but small hail is not easily crushable, and regarded as belonging to classically 'thundery' weather (a spring / summer-time phenomenon, and you should be able to at least see 'vaguely' through these (unlike with snow pellets, which are highly - white particles). Also, to distinguish them from ice pellets (above), this type falls from heap or cumuliform cloud.
[ Most observers will code the METAR present weather as SHGS (or TSGS etc). However, the code form does allow for SHPL (i.e. shower of ice pellets), which could be used when it is clear that convection is involved.]
 Clouds producing precipitation: Cb (cumulonimbus), vigorous / deep Cu (cumulus congestus)
 Physical processes: Snow pellets/Graupel/soft hail (the core) encased in a thin layer of ice which has formed from the freezing either of droplets intercepted by the pellets or of water resulting from the partial melting of the pellets. Perhaps best regarded as hail 'thrown-out' of a Cb 'factory' before it has had time to grow, or the hail-formation process is not vigorous or the Cb is decaying.
[ Confusion was caused (in my humble view) by the no doubt well-meaning attempt to clarify the difference between various forms of 'ice pellets' via the WMO declaring that there would be two forms ... type [a] and type [b]. As far as I can work out, this was promulgated sometime between 1951 (the founding year of the World Meteorological Organisation) and 1956 (when the WMO International Cloud Atlas was published, and probably around 1954/55). What were Grains of Ice (British) and Sleet or Ice Pellets (US) were assigned to the nomenclature: "Ice Pellets type [a]" & what were widely known as small hail were re-named "Ice Pellets type [b]": yet the formation, originating cloud type and indeed appearance of the two types appear to be quite distinct. The use of 'type a/b' gained much ground in the 1960's, but appears to have fallen out of favour now - probably for the best!]
 Name:  HAIL
 Alternatives:  (none)
(4677 ww):
(FM 12/13-VII)
 27, 89, 90, 93, 94, 96, 99
(4680 wawa):
(FM 12-IX)
 21, 40, 41, 42, 45, 46, 80, 89, 93, 96
 METAR (w'w'):  GR (and variants for combination)
 Beaufort letter:  ph (and variants for intensity)
 Symbol:  image of a hail symbol
 Standard description & additional notes: precipitation of small balls or pieces of ice (hailstones) with a diameter ranging from 5 to 50 mm or sometimes more, falling either separately or agglomerated into irregular lumps. Hailstones are composed, almost exclusively, of transparent [see 'vaguely' through] ice, or of a series of transparent layers of ice, at least 1 mm in thickness, alternating with translucent ['cloudy'] layers. Hail falls are generally observed during heavy / violent thunderstorms. [ for practical observing, if solid precipitation falls, and it might otherwise be identified as snow pellets, graupel etc., if the diameter is =>  5 mm, then such elements will be called 'hail' by default.]
 Clouds producing precipitation: Cumulonimbus (Cb).
 Physical processes: A cloud ice crystal acts as a nucleus, which is carried vertically large distances - encounters (collides with) varying sizes of super-cooled water droplets - this is known as 'riming'; alternate clear and opaque ice shells may form, before the hailstone becomes heavy enough to fall out of the Cb. The standard 'hailstone' is regarded as an extreme form of the riming process in a cloud of vigorous convective motion (strong updraughts) and having high liquid water content (as a proportion of the total cloud mass).
If, however, a hailstone collects (and grows from) super-cooled liquid droplets at too great a rate, the latent-heat release (water > ice) may raise the surface temperature of the stone to 0degC, and some of the water will then remain liquid: the hailstone is said to 'grow wet'. Some of this water is shed (to take part in growth of other crystals / ice particles), whilst the remainder precipitates as 'spongy' hail.
Hailstones collected & cut in half can exhibit clues to the formation process; they are often seen to be made up of dark and light layers of ice in alternating fashion. The dark layers are 'cloudy' [or opaque] ice, containing a high proportion of small air bubbles, and the lighter layers are 'clear' [ transparent or translucent] ice, and are effectively bubble-free. Clear ice is more likely when the hailstone is 'growing wet', i.e. the freezing process is effectively 'slowed' to allow air to be expelled. Cloudy ice implies a rapid freezing process, where air bubbles are trapped.
In spring & summer, there are usually large quantities of super-cooled water drops - large frozen centres - and hail as defined here is most likely. In winter (and like situations), there are smaller quantities of small super-cooled water droplets, and large (or 'standard') hail less likely, and small / soft hail perhaps more prominent.
 Alternatives:  (Diamond dust, Ice crystals, Ice needles)
(4677 ww):
(FM 12/13-VII)
(4680 wawa):
(FM 12-IX)
 11, 21, 40, 41, 42, 45, 46, 80
 METAR (w'w'):  IC (no variants)
 Beaufort letter:  h
 Symbol:  image of diamond dust symbol
 Standard description & additional notes: a fall of unbranched ice crystals in the form of needles, columns or plates, often so tiny that they seem to be suspended in the air. The crystals are visible mainly when they glitter in the sunshine (hence the alternative name "diamond dust"); they may then produce a luminous pillar or other halo phenomena.
 Clouds producing precipitation: these may fall from a cloud (usually stratus [St]) or from fog, or indeed a cloud-less sky.
 Physical processes: Ice prisms are frequent in polar (and other bitterly cold ) regions and occur at very low temperatures and in stable air masses. Updraughts are negligible - therefore allowing individual ice crystals ( grown onto ice nuclei by vapour deposition and enhanced by very light riming) to precipitate.
 Alternatives:  (Snow crystals)
(4677 ww):
(FM 12/13-VII)
(4680 wawa):
(FM 12-IX)
 21, 40, 41, 45, 78, 80
 METAR (w'w'):  SN (light variant only)
 Beaufort letter:  s
 Symbol:  image of snow crystal symbol
 Standard description & additional notes: an extreme form of snow; only found in very cold, still conditions.
 Clouds producing precipitation: various, mainly Stratocumulus (Sc), Altostratus (As).
 Physical processes: Essentially the same process as is involved with snow generation (q.v.), but the crystals so formed do not have other elements in place to allow them to either grow large, or to aggregate into larger snowflakes.

6. Alphabetical listing of precipitation types.

Diamond dust
Freezing drizzle
Freezing rain
Glazed ice
Grains of ice
Granular snow
Ice crystals
Ice grains
Ice needles
Ice pellets
Ice pellets [a]
Ice pellets [b]
Ice prisms
Ice storm
Isolated star-like crystals
Rain and snow: mixed
Rain showers
Sleet (US)
Small hail
Snow crystals
Snow grains
Snow pellets
Snow showers
Soft hail
Tapioca snow

7. Diagram showing precipitation formation ideas.
This diagram attempts to show the various processes involved in the production of rain, snow, hail etc. It is based upon (and added to), the diagram published in 'Clouds, rain and rain-making', written by Mason.

8. Bibliography.


Precipitation Diagram

diagram of precipitation processes


Recording/reporting the event

An important thing to get right is the time of your observation, and even more important the time standard. In the UK, during the summer, we add 1 hour to GMT (or UTC, or 'Z' time...they are all interchangeable for our purposes), and call it British Summer Time. However, international meteorology runs effectively to GMT. So, when you report a phenomenon, use GMT. When BST is in force (in the uk), this means taking one hour off your watch time, so a waterspout seen at 7 pm on the south coast in mid-July should be reported as occurring at 1800 GMT. If you suspect your watch/clock might be in error, check against a time-signal (or similar e.g. teletext clock), and adjust accordingly.

Double-check the date as well! This might not seem an obvious point, but particularly when you are reporting something a day or so later than the event, its easy to get the days mixed up. Watch particularly the time around midnight when we are on BST. Something happening at 15 minutes past midnight by your watch on the 15th, should actually be reported as happening at 2315 GMT on the 14th.

Locations are most important. Its easy to report an exciting event as having happened, and forget to tell us where you are! Have a 'sig file' made up that includes your location, height amsl and other important facts, and use it as appropriate. Obviously if you are out and about, then include as much information about the observing point as possible, or if on a car/train journey, the area where the observation was made. This includes not only the lat/long and/or grid ref. (or town/village if that's easier), but a description of the terrain, location of adjacent water surfaces, direction/height of hill/mountain ranges etc., if they would not be universally known. These latter points of information might help diagnose phenomena in difficult situations, and allow others to relate their observations to yours.

Get into the habit of having a scrap of a pencil and some paper with you to note down important details for later transcription to the newsgroup. Even better of course would be a dictating machine, or a palmtop to record the events. Also, keep a copy of what you post, if your newsreader doesn't already allow this. Someone may want to follow up your report weeks, or even months in the future. Don't rely on your memory...it plays tricks! Always note down the important features ASAP after the event, preferably as it is occurring.

When photographing events, as well as the usual location, time, date etc., note down the readings from the camera...f-stop, film speed/type, shutter speed etc. What state the sun (or other illumination) was in..cloudy/part cloudy/clear; behind/in-front of observer etc. The information may not be needed, but then again, it might! Camcorders are becoming more popular for recording weather events. However, even if you think you have the world's finest footage, make a brief note of the event in longhand just in case what you saw doesn't quite live up to expectations when the tape is re-played. Try to get objects of known dimensions and distance in your shot so that some comparative assessment of the tornado/waterspout etc., can be made. In the specific case of photographs of hailstones, include some form of measure, such as a centimetre rule.

As well as your own report, try to gather other eye-witness accounts, particularly in the case of 'severe' weather events. Newspaper cuttings are invaluable, even if they turn out to be rather sensationalist. If you do actually 'cut' out the report, make sure you annotate with the date of publication, publisher's address etc., in case anyone wants to follow up the news item. Make a note of any local radio and tv reports, and in the latter case try to record the news report when any action shots are broadcast.

See also this question in the FAQ.


Visibility and atmospheric aerosols

The visibility (or how far we can see), is determined by the mass and size distribution of aerosols in the air; these are either naturally produced or man-made and can be in the form of liquid water, crystals of ice or solid 'particulates' from various natural & human-activity sources. Most of this note deals with horizontal visibility, but vertical visibility is of great importance, especially to aviators & astronomers.

The distance at which any object can be seen (& recognised for what it is), depends upon the position and 'personal attributes' of the viewer (i.e. keen-ness of sight) and of course, the 'blocking factors' (or degree of obscurity) between the two. For an object to be clearly visible, the eye must receive enough light energy to be able to resolve the object sufficiently for the brain to recognise what it is: packets of light flowing from an object will be subject to absorption & scattering on its way from object to eye, scattering being by far the most important factor.

Water-based atmospheric aerosols

Fog, mist, cloud and precipitation are obvious reasons why you might not be able to see as far as possible, these being composed of either water droplets or ice particles of varying sizes: these scatter the light being emitted from an object, dependent upon the number, character and size of the particles: the greater the scattering (diffusion) the poorer the visibility(**).

In a marine environment, spray due to very strong winds will produce a large amount of minute water droplets which will also reduce the visibility, similar to the action of a thick mist or fog.

(**): Although precipitation will reduce the visibility, even if only slightly, under certain circumstances it can actually lead to an improvement on overall visibility as the rain etc., 'washes-out' impurities in the air.

Solid (or 'particulate-based') aerosols

Suspended solids (or "particulates") can be highly effective restrictors of atmospheric visibility, under the right circumstances & in the appropriate quantity.

Air contains a variable amount of impurities, such as dust, soot and salts. The category of "dust" includes the whole range of pollutants due to industrial processes & the exhaust from combustion-based transport activity and also the lifting & suspension of fine soil-based particles, where very dry, exposed fields are subject to a persistently strong wind.

The main source of natural dust is the arid regions, such as deserts and steppes. Course material, lifted by strong and turbulent surface winds, is never carried far from its source, but minute dust / soil particles are readily distributed throughout the lower troposphere and carried far from the source.

Industrial regions, forest (or heath / scrubland) fires & volcanoes provide the main source of soot (or particulates). Smoke particles of course can produce a dramatic, though relatively temporary drop in visibility.
Combustion at very high temperatures is usually an efficient process, producing only small amounts of unburned residue. However, many fuels (including most used in transport) are burnt at relatively low temperatures, and carbon residues are injected into the lower atmosphere and under the right conditions can be carried large distances.

Air also contains a considerable amount of salt. Through the action of the world's winds, spray is whirled up from the oceans, and when it evaporates the salt remains in the air in the form of minute particles. Indeed, of all the particles floating around in the atmosphere, by far the greatest quantity are in the form of sea-salt of varying sizes. This is hardly surprising given the greater proportion (relative to land areas) of exposed sea-water covering the surface of the earth.

All these particles (above) which constitute the impurities of the air are so small that they cannot be seen individually with the naked eye, but their effect on visibility & on the colouring of distant objects is easily observed as they interfere with (scatter) the passage of light from the distant object.

What conditions give the 'clearest' visibility?

The "cleanest" air, at least in maritime mid-latitudes, will potentially occur with ex-polar air masses, where the absolute and relative humidities are low, and are brought along on a reasonably brisk breeze to mix the lowest layers and reduce or eliminate any surface-based potential for visibility reduction. Although showers may lead to temporary (& often marked) reduction in visibility, the showers usually pass swiftly, allowing a return to the prevailing excellent conditions. Also the flow is often tending to greater instability (cold air flowing over relatively warmer surfaces), and this enhances the vertical mixing.

... And what conditions produce the poorest visibility?

Air masses moving poleward (i.e. from the south in the northern hemisphere) are tending to stability and damped vertical mixing: in the extreme case (anticyclonic inversions in winter), no vertical mixing occurs over a large area. Moderate to poor visibility due to man-made processes are often a feature of 'blocked' anticyclonic episodes, where the flow aids transfer of the pollutants from nearby or distant sources, and the associated inversion of temperature (highly stable environment) works against vertical mixing of low-level air, trapping the dust, particulates etc., within the lowest few 10's of metres of the troposphere. In extreme cases, the concentration of pollutants can reduce the visibility to fog limits [< 1000 m], though the relative humidity is <90%.

Air masses arriving from a south or south-easterly direction (in NW Europe) often bring a high 'dust loading', given the likely source region in the arid areas of the Mediterranean & North Africa. This is particularly so when the source regions are subject to intense convective storms - e.g. the Sahara in the 'summer-half' of the year. These storms lift huge quantities of very fine sand particles into the air which are carried along for many hundreds of km, and can markedly reduce the visibility some considerable distance from the source region.

Astronomy: visibility & 'seeing'

For astronomers, what constitutes 'good' visibility (or 'seeing') is of course directed away from the meteorologists (or aviators) horizontal bias (except of course for celestial objects close to the horizon): As a basic requirement, absence of cloud (or deep fog) is required, but even with 'clear skies', viewing of deep space objects (which are effectively point-source light-emitting objects) require a steady atmosphere - this means that light must not be subject to the distortion caused by high winds (jet stream turbulence) in the upper atmosphere. Surprisingly, this means that immediately to the rear of a swiftly-clearing cold front, whilst cloud cover may be minimal & the stars are shining strongly, images of distant star clusters will appear indistinct due to the presence of the polar jet to the rear of the front. For objects within the solar system, the effect of atmospheric turbulence is not so important.

Visibility measurement: methods of

Assessing the visibility in the past relied upon the observer knowing the distance of known objects from the observing point, and then determining the furthest of these objects that could be seen clearly. Provided that the visibility did not vary markedly from the value found (i.e. due to adjacent mist / fog), then the distance of this object would be the declared visibility. In recent times, most reports for aviation work have used the concept of 'prevailing' visibility where the most prevalent (rather than the minimum) value governs the issued & recorded figure.

With greater use of automatic weather stations [AWS], visibility is more likely now to be determined by a 'transmissometer' (or similar), where the attenuation of a known-strength fine beam of laser-light over a short base-line can be accurately measured and converted into a visibility figure. An added refinement to assess the effect of precipitation is to 'fire' pulses of light energy within the immediate vicinity of the observing point & then detect the amount of scattered light returned. By combining the two methods, a highly accurate readout of local visibility is achieved, but it should be remembered that these systems cannot assess reduction in visibility due to 'adjacent' factors such as passing showers or a fog bank. However, provided they are calibrated & maintained properly, they give a much more consistent record of visibility than that based on a variety of human observers.

What to look for

There are four broad areas you might want to consider for detailed weather reports:

  1. Convective activity.
  2. Wintry weather.
  3. Severe winds.
  4. Other 'interesting' phenomena.

1. Convective activity

Note particularly the development of the cloud/s giving rise to the thunderstorm/tornado/waterspout/etc. What we are looking for is the rapidity of build of the cloud; its vertical extent in a noted time. Is it building directly upwards, or sheared to one side? Is it possible to determine whether you are observing a single or multi-cell complex? Is any rotation observed in the cloud elements etc? What was the wind regime, before, during and after the event? Note particularly the onset of notable gustiness, changes of wind in direction/speed as compared with onset of precipitation etc. Was a sea breeze front involved? This can sometimes be inferred by a change in humidity (it should 'feel' more humid), and there may be a line of precursor cumulus development, with a change in visibility and wind direction perhaps.

The type, duration and intensity of precipitation (abbr: ppn) should be noted; the Beaufort shorthand notation can be usefully employed for this.

If possible, amounts of liquid and/or solid ppn should be recorded, particularly accumulations of hail and snow, the diameter of hail, the appearance of hail (i.e. cloudy, translucent, mixed layers - these will define the various cloud regimes that the hail-stone has spent its life amongst.); the type of snow/pellets etc., and if possible, determine which portion of the cloud the ppn appeared to originate from. for advice on types of solid precipitation, see here.

In very severe rainfall events, its worth noting how water-butts and other containers fill up and to what extent. Whilst not being used to give a definitive rainfall amount, they can sometimes help to verify the order of magnitude of adjacent reports when the final report is written. Note the winds aloft prior to the storm onset. Severe convective storms form in an environment of marked vertical wind shear, both in direction and speed and this can sometimes be inferred from visual observations. Be careful to judge such motion against a static object though. Its easy to be deceived by relative motion of other clouds. You can usually position yourself in such a way as to have a tree, or corner of a building, or pylon or similar in the eyeline to provide the 'fixed' point.

Note the cloud features preceding the event, in particular any Altocumulus castellatus, floccus or towering cumulus development. Note the damage caused, flooding experienced etc. In particular, the period over which flood waters both rise, and subside, and the horizontal extent of the flood waters. Hail and wind gust damage reports are most useful, and the TORRO event reporting form is the best place to refer to for guidance on these aspects. see: http://www.torro.org.uk/

2. Wintry weather:

Although winter doesn't necessarily mean snow, that's the element that interests a great many people dipping in to the newsgroup. As an aside, although the UK radarnetwork can detect areas of precipitation very well, there are problems with detecting areas of rain versus snow, due to the narrow temperature/humidity band within which the phase-change occurs. The more reports of snow (or not-snow) are made, the better is the overall picture. Also, the more reports of snow lying are made the better as the 'synoptic/climatological' network doesn't always pick up the variability of nation-wide snow cover. A record of the times of onset/cessation of 'wintry precipitation', intensity, types etc. should be made, together with accumulations. This means not only noting the total depth of snow, but also the accumulation of fresh snow on top of old snow cover.

Is the snow drifting or blowing? Some types of snow will drift more that others - note whether snow is drifting after having earlier fallen, or is blowing around as it falls. In blizzard conditions the distinction will not always be clear. Note how old snow settles..its continuity (complete or patchy cover) and regularity (an even depth or irregular depth, with drifts)..its persistence from day to day.. try to note the depth/extent etc., at 0900 GMT as this is the reference time for snow cover for climatological reports, and your report can be integrated with other observations. Simply coming on line just after 0900GMT and reporting ....'I've got a snow cover of 2 cm', or ' all yesterdays snow has gone', can be a great help in gauging the overall picture.

Remember that snow can evaporate (sublimate) and settle, as well as melt, so get into the habit during a prolonged spell of snow-lying of always measuring the snow depth in the morning - don't assume its still the same depth just because its still there. Take three samples of undrifted snow that represent the 'general' picture, and average out, but also report notable departures from these readings. Garden canes etc., can sometimes be used as semi-permanent depth markers, provided of course the underlying soil isn't so frozen that you can't push the cane in the ground! In particular, dimensions of notable drifts will always be of interest. Note cases where heavy rain aids snow melt etc. Glazed ice - measure thickness - rapidity of accumulation - temperature fluctuations before and during event. The weather immediately preceding the event. Length of the event and consequences on traffic etc. Photographs of these events are always interesting.

3. Severe winds:

In particular, damage reports are most useful. Is the damage observed in a narrow/focused swathe, or widespread and indiscriminate. What is the scale of damage - i.e. a few minor branches, whole trees, chimney stacks down etc. The period during which the most damage was caused. Any special features .. twisting of branches, tree trunks etc. Other weather changes as damaging gusts occurred .. i.e. cold front/trough passage/squall line/cloud changes/ppn changes etc.

Wind observations are very important, especially when noting severe convective events. Even without expensive anemometers etc., just noting the direction and Beaufort Force of the wind can be interesting. (for advice on using the Beaufort wind scale, see: http://www.zetnet.co.uk/sigs/weather/Met_Codes/codes.htm ... and indeed this is the url for a complete introduction to many of the finer points of coding/de-coding etc.

4. Other 'interesting'phenomena.

These reports include such as optical phenomena, coloured precipitation, unusual objects falling in rain showers, unusual (or rarely seen) cloud types etc. As much detail as possible should be included, and if you are not sure what it is you are looking at, someone will do for sure! I have deliberately not gone into details here, as the subject is vast.

A general note on standards etc.: (thanks to Paul Bartlett for additional suggestions here) Some people contributing to the newsgroup will have access to some pretty sophisticated observing equipment which provide valuable record of weather events. However, if all you have is a plastic gauge from the garden centre, or a Six's max. and min on a north wall, don't be put off providing the information.
For reports of wind speed and direction: try using the 16 point compass (and remember that the wind comes from a direction, so a SSW wind comes from the SSW), and the Beaufort scale, as already mentioned above. So, a wind observation might be SSE/4 gusts 5; or, upon a cold frontal passage, SSE/3-4 becoming SW ocnl W 3 gusting 4 or 5.

Reports of temperature should be in degrees C. Don't try for 1/10's degree accuracy unless the thermometer allows it. Try for the nearest half of a degree, but say that that is the standard to which you are observing.

Atmospheric pressure (reduced/corrected to msl) in millibars/mb (or hecto Pascals if you prefer/hPa). Similarly with pressure change: the usual period to note a pressure change over in temperate latitudes is 3 hours, but hourly changes are valuable where possible. [ In subtropical/tropical regions, 24 hour changes are usually used, to eliminate the influence of diurnal changes. ]

Reports of rainfall (or melted snowfall) should be in mm. Dimensions of solid precipitation (e.g. hail), should be cm, as should snow depth.

Visibility (broadly how far you can see, though the strict definition is considerably longer and more complex), should be in metres up to and including 5000 m and km beyond that.

Cloud amounts can be made in oktas (eighths of sky covered) ...see  here for some advice on this, or more generally, the aviation cloud amount classification could be used, as under:

SKC: no cloud at all
FEW: 1 or 2 eighths/oktas of cloud
SCT: 3 or 4 eighths/oktas of cloud
BKN: 5 to 7 eighths/oktas of cloud
OVC: 8 eighths/oktas of cloud - i.e. complete overcast.

Cloud base will be difficult, and I won't go into detail on estimation of cloud bases. The Observers Handbook is your best guide. At present, the foot is still the 'standard' for cloud height observations, but many countries use metres so I suppose either is acceptable.

As to standards, as long as you tell people how you are measuring the variable you report, they can make their own judgement of relative accuracy. It would be a pity to lose data because you might feel its not of sufficient quality. Quite frankly, there is never enough data on current weather, and with the situation in many areas now where official 'eyeball' observations are too expensive to maintain, amateur observations, carefully made and reported, will once again come into their own.

And a final thought...don't be shy of saying you don't know what it is you are seeing. Either someone will pop up with the right answer, and we'll all learn by it, or it might genuinely be something 'new', and stimulate a discussion... which after all is what uk.sci.weather is all about.


Here are some specific notes regarding the observing and reporting of "dust-falls" after rainfall. I am grateful to Stephen Burt for this:.....

" Observing a fall of dust rain is not difficult, but it helps to have some ideas of what to look out for. A daily routine also helps (such as, in my case, the morning inspection of the raingauges) but even a few seconds careful examination of the car windscreen before driving to work can be worthwhile. If you own a raingauge, check the funnel daily for a ring of dust, often pale orange in colour and very fine in texture. The larger the funnel, the better: the largest falls even show muddy streaking. Most dustfalls are considerably less obvious than this, often with only the pale ring at the neck of the funnel visible.

Surprisingly perhaps, the actual rainwater sample collected may not be very obviously different from normal (at least visually). In my experience, dust rain is most obvious with small amounts of rain, certainly less than 1-2 mm, for otherwise much of the evidence is either washed away or so dilute as to be unrecognisable. Perhaps this is the fate of many falls of dust. Many of my observations of dust rain have been in overnight rainfall, possibly because my raingauge is normally checked only once per day at the morning observation.

Another excellent instrument for observing falls of dust is a car, preferably a clean one. Even on a car that is fairly dirty a heavy fall of dust will be very obvious as muddy runs on surfaces that are cleaned regularly (generally the windscreen). A clean car will collect a fall of dust extremely well; the aggravation of having just washed it should be balanced by the thrill of having recorded a fairly rare event! Of course, dust from considerably closer than the Sahara can build up on a car or in the raingauge funnel, particularly after a long dry spell. A regular wipe with a clean damp cloth solves that problem in the raingauge funnel.

Locally-deposited wind-borne dust (especially prevalent during hot, dry summers) is often coarse in texture. Beware of pollen in spring and early summer; light showers can bring down considerable amounts and can deposit a surprisingly yellow ring in the raingauge. Harvesting operations during late summer can stir up a lot of fairly fine dust, but common sense (not to mention a check on the synoptic situation) will usually enable an astute observer to ratify a possible sighting." (Extracted from the paper: Falls of dust rain within the British Isles,Weather 1991, 46, pp 347-353, by SHFJ Burt...text of above also published in uk.sci.weather newsgroup)

Instrument siting

The notes below have been written using replies to a survey in the newsgroup and using information extracted from the UK Met.Office handbooks dealing with instruments, and their siting, which in turn are based on internationally agreed standards published by the World Meteorological Organisation (WMO). The notes are only concerned with siting & exposure, not the instruments themselves, or the construction of the screen or a shield in the case of temperature measurements. For advice on these, contact a reputable supplier, or refer to the Observer's Handbook.The BBC Weather Centre web site also has some basic information regarding weather observations etc., and a visit to them would be useful.

Standards are set for a reason: data from many different sites around the world need to be compared one with another, in the knowledge that, as far as possible, the instruments used are exposed in the same way and subject to the same errors. This requirement is particularly important when trying to determine long-term trends in meteorological parameters, both using mean values, and with regard to extremes.

However, even when the WMO recommended standards cannot be met, it is natural that people will want to install weather monitoring kit to enhance their interest in the subject. This note therefore attempts to advise on what is, and what is not possible. At the end of the day it is for the individual user to determine whether the expense involved is worth the outcome. One thing must be made clear though: throwing money into expensive equipment will not improve the exposure!


Basic requirement: For synoptic and climatological meteorology, the temperature required is a representative one of the 'free air' conditions over as wide an area surrounding the observing point as possible, with an internationally agreed height (for the thermometer bulbs, sensors etc.) of 1.25 m above local ground level. A fixed height must be specified, because vertical temperature gradients can be intense: for example on a clear, calm night or around the middle of the day with strong solar heating.
The best site for a screen, or thermometer shield for a land station is therefore over level ground, freely exposed to the sun and wind, but not sheltered by buildings, trees, bushes etc. The temperature sensor must be shielded from direct sunshine (hence a screen or shield) and precipitation (or a dry bulb becomes a wet bulb), and there must be a good circulation of air around the bulb/sensor head. If you have a garden, then the 1.25m above ground level can usually be met with ease. What it usually problematic is gaining sufficient clearance from adjacent buildings, trees etc.

The screen/shield should be positioned over grass (or less preferably, but still acceptable, loose soil), but not compacted soil, tarmac or concrete, as these media absorb and radiate solar energy strongly, and affect the readings quite significantly.

If the garden is not suitable, or you have no garden, then consideration may be given to mounting a screen on a north facing wall. There is a problem in this case with possible contamination from heat energy emitted by the building itself. A practical compromise would be to use such a wall, but carry the screen/shield away from the wall on a bracket - this would allow a free airflow around the equipment. A distance of 20-25 cm for an unshielded sensor has been suggested, and this would certainly minimise any contamination from the walls. For a shielded (or screened) sensor, then 10 cm or so has been suggested as a useful distance. Even a north wall mounting needs watching around the summer solstice, particularly at more northern latitudes, as care needs to be taken to shield the thermometers/sensors from early morning and late evening sunshine with an unobstructed horizon to the northeast or northwest.

The roof is not considered suitable. Not only is the construction of such similar to a solid surface (e.g. tarmac or concrete), and therefore subject to the errors noted above, but a roof is obviously more than 1.25 m above ground level. However, it is worth noting that many of our current crop of weather centres, with London being a notable example, have for many years mounted thermometer screens at a considerable elevation above local ground/street level. If a roof location is all you have, use it, but bear in mind the limitations.


Basic requirement: Rainfall amounts are quoted as a depth of water that would result in any one location on a flat surface after a fall of rain, if there were no run-off, evaporation or percolation. The depth measured in a gauge is assumed to be representative over an area around the gauge, so it is necessary to eliminate as far as possible any local sources of error.

There are many sources of error in rainfall assessment: evaporation, adhesion (sticking of the droplets to the side of the gauge), splash etc., but by far the greatest source of error is due to inadequate exposure. The 'ideal' location is one where objects which might disturb the airflow are some considerable distance away from the gauge, i.e. they are so far away that any perturbations of the wind-flow are so small as to be part of the general 'ground-effect' turbulent flow always present as air passes over the earth's surface.

The recommended standard is that the distance from surrounding objects should be not less than twice the height of such objects, and ideally at least four times. In most suburban gardens, even if the fences are low enough to just about site a gauge to these standards, surrounding trees, neighbour's bushes, and of course, the house (and adjacent buildings) usually are the largest objects, and cannot be realistically circumvented. Most hobby observers cannot meet the latter (4 times) requirement, but something approaching the twice-times-height standard is often attainable.

Official texts completely rule out mounting on a wall or roof (apex or flat), as these features cause marked eddies of wind which grossly distort the passage of falling rain across the mouth of the gauge. However, a flat roof might be a best approximation, if there are no adjacent buildings within the '2 x object height' footprint mentioned above.

The middle of a lawn is all that most of us have .. and provided that it is not grossly shaded, would present a reasonable guide, but unless the wind is very light, under-reading of rainfall (with respect to the 'standard' sites) must be expected. With time, it is usually possible to judge where in a garden is unduly sheltered, and careful note made to avoid these locations. Remember that the 'shadow' changes with wind direction. Try setting up identical collecting receptacles (e.g. the bottom half of plastic lemonade bottles) and note the variation in catch over a period of several months. Another method is to note at the start of any rainfall event which areas become wet first and which stay dry longest. Or perhaps a matrix of collectors, and take an average!

At the end of the day of course, you are measuring rainfall that is of significance to you. Indeed, in extreme rainfall events (such as notable local storms), any measurements are better than none; adjustments and allowances can be made for exposure, and even an 'official' gauge under extreme conditions has difficulty in capturing a 'true' measure of the event. The rain/snow that falls is the amount of rain (to a reasonable approximation) that has fallen in your garden, on your roof or whatever, and as such it is a meaningful record. For this reason alone, it is worth attempting such measurements - the only suggestion is that you don't spend huge amounts of money doing it!


Basic requirement: The wind speed and direction (see ** below) in the first 30 metres or so of the atmosphere varies rapidly with height, due to the varying frictional effect of the general 'surface roughness'. It is greatly affected by undulating ground, and by adjacent obstacles such as trees, bushes, buildings etc. This is a common experience - noted for example within built up areas, major shopping centres etc.

For synoptic and climatological work therefore, a 'standard' exposure is required. That standard is for the wind speed and direction over a level surface to be measured at a height of 10 m above ground level (agl). When these conditions cannot be met, it is permissible to raise the anemometer to give an effective height of 10 m, provided the obstructions are not large, and are distributed uniformly around the instrument site.

It will be immediately apparent, that in the common 'back-garden'/urban development situation, a considerable mast is needed to carry the anemometer clear of these 'ground effect' generating obstacles. For example, consider an outer-suburban garden with houses/trees of approx. height 6m in height, the recommended exposure height would be 6 m (obstruction) + 10 m (standard height)=16 metres. (That's around 50 feet!) This provides problems in maintenance of the sensor, and also there would possibly be planning and structural constraints. To be stable, such a structure would need to be well braced which will not be easy. When considering larger obstructions, such as large blocks of flats, or office blocks, then the sensors would need to be raised even more. An example: For an obstruction of some 15 m in height (a typical large building), which is about 75 m from the site of the intended anemometer site, then the wind vane/cups would need to be about 25 m above ground level.

If such conditions are beyond the scope of your pocket or what the neighbours will allow, then the best compromise would be a fitting a short height above the ridge of a house, provided always that adjacent buildings do not unduly affect the airflow at the sensor level.

[** Wind direction, when quoted in standard meteorological reports such as SYNOP and METAR, are given in terms of deviation from TRUE north (degT). This should be remembered when setting up a weather station that includes an anemometer: align the directional head such that the read-out gives degT - to do this you will need to allow for the local magnetic deviation of your compass from true north: many websites are available that will help with this subject.]

As will be appreciated from the above, the best advice we can give when 'standard' conditions cannot be met is to think seriously whether its worth the cost and effort. By all means mount a relatively inexpensive anemometer just above the roof level of your house etc., but treat this simply as a monitor of the conditions for your site. The reading you get will not be of use to compare with adjacent 'standard' instrumentation, or even with someone a few streets away with similar problems. However, it is a record at the point you have installed the anemometer head, and as such does provide interest. You will find though that the poorer the exposure, the greater the variability in wind direction.

And finally ... it is pertinent to note that there are occasions when limitations of exposure are a positive advantage. For example, a useful field study for students is to set up a series of temperature recording devices within a stand of trees, both horizontally and vertically. Readings from such an array would obviously be used for the study of the heat/humidity budget of the wood and any need to 'standardise' as above is not a factor, apart from ensuring of course that the thermometers or other devices are correctly calibrated and 'zero-referenced' for the range of values required. And there are specialised applications where the sensor site must depart from the WMO standards: for example, temperature and wind sensors set adjacent to a major motorway route are there precisely to monitor the disturbed airflow and heat characteristics consequent upon heavy traffic flow passing a couple of metres away. For these specialised applications, advice should be sought from the manufacturers of the equipment and other relevant authorities.

Observation timing

Systematic instrumental weather recording (e.g. using thermometers, rain-gauges etc.) began in the days when the only way such data could be obtained & logged was by someone reading & noting same. For this reason, the meteorological 'day' tended to be built around the 'human-activity' day. With most observers being available 7 to 9am local time, the climatological start/end for the day has settled in many European countries (though not all), to 0900 UTC (or to 0900 local/clock-time elsewhere). For those stations only noting maxima, minima, rainfall etc. once per day, the maximum temperature read at 0900UTC is credited to the previous day, on the assumption that on the majority of occasions, the highest value would have happened during that day's afternoon. The minimum is credited to the current day, on the assumption that, again for the most part, the lowest temperature would have occurred in the hour or so around local dawn - i.e. the day of reading the thermometer. Rainfall for this 09-09 period is apportioned to the day occupying the greatest part of the 24hr, that is it is 'thrown-back' to the previous day's date. This means of course that a severe thunderstorm producing a lot of rain in the period after midnight will have it's rainfall credited to the previous day!

A better way of noting (and recording) these data is to further divide the 'climatological' day into two periods: 0900 to 2100 & 2100 to 0900 UTC. This of course means that thermometers and rain-gauges have to be read, reset/emptied at 2100UTC. However, as with the 09-09 method, anomalies will occur. For example, if a major air-mass change occurs after 2100UTC (e.g. cold to warm in winter), then the highest temperature in the 24hr period may occur overnight, yet the logged maximum will be that recorded 0900 to 2100. And rainfall will still be credited to the date at the start of each period, i.e. the 21 - 09 UTC rainfall (read at 0900UTC) is again 'thrown-back' and added to the 09-21 total.

A further complication arises from the requirement of those providing information for use in SYNOP bulletins. As there is no 'main' SYNOP at 0900UTC, in Europe the night minimum (18 to 06) is reported in the 2-group of the 333 section of the 0600 SYNOP. (Note that during the long winter nights, the actual minimum could well occur after 0600UTC.) The day maximum (06-18) is reported in the 1-group of the same section at 1800 UTC. The rainfall (or melted snowfall) at these (and other times), is recorded using the 6-group in the main SYNOP (see here).

With the advent of high-quality electronic systems, it is very easy to log data according to the 'normal' day, 00-24 UTC, and users of such find it cumbersome to comply with the above scheme(s). This FAQ makes no comment upon this debate, except to note that all methods of recording climatological data throw up anomalies, and there is a large body of historical data tied to the 09-09, or 09-21-09 standard. The important point, especially when noting extreme temperatures and notable rainfall, is to annotate your own records (and report same where possible), so that researchers in years to come can pick out the deviations from the 'assumed' diurnal cycle of temperature, or the 'noteworthy' precipitation event.

Setting up a weather station

In addition to the hints, tips & general advice in the earlier parts of this Section (above), many of the manufacturers in the Suppliers Reference list will supply advice on the installation of equipment.

For some basic ideas of how to start-up, see the following sites (I haven't given the full urls because they keep changing and it is difficult to remain up-to-date with same):

The Met Office: (follow links for the Education Section .. some very useful ideas for the hobbyist, and also advice if you want to make observations to climatological standards.)

and direct to the section relating on advice on weather recording, reporting etc.,

The Royal Meteorological Society: (follow links for Publications: the Society encourage meteorology at all levels, and publish some leaflets on weather observing which can be obtained either free, or for a small sum.)
[ and don't forget that other national meteorological societies will have similar information.]

The BBC Weather Centre (follow links for 'Weatherwise' .. or use the Search engine; a useful site to help the beginner in all aspects of observing and understanding the weather.)

Don't forget though: throwing a large amount of money at the subject won't improve your understanding. Read up on the basics, use your senses to observe the changing 'sky-scape', note the weather (rain, hail, snow etc.), and just get used to deciding from which direction the wind is coming. For all of this, you don't need fancy equipment - that can be built up later. Even then, start in a small way if your budget is limited: some thermometers and rain-gauges from garden centres are quite good, and many department stores and catalogue shops sell 'all-in-one' desk weather stations which can stimulate interest. Other suppliers are listed in the Suppliers Reference list.


Codes, coding, plotting etc.


Information on codes and coding

Try Dave Wheeler's site - a wealth of information on the SYNOP code - at: http://www.zetnet.co.uk/sigs/weather/Met_Codes/codes.htm
The site also hosts a display of the 'ww' symbols for present weather. Dave was awarded the MBE in the Queen's Birthday Honours in 2002 for services to meteorology on Fair Isle.

For the 'new' automatic weather types output from the latest generation of AWS (they use a different present weather code), & a plotting matrix of said code figures see here.

Information to decode a METAR

There is a FAQ entry devoted to decoding METAR reports here.

Amongst other useful information, and links to other sites, data sources etc., this site has a decode page relating to the METAR and TAF code: http://www.ukweather.freeserve.co.uk ... then follow the route from the 'Aviation' button to 'Weather Codes Explained'.

Snow groups on SYNOP & METAR reports

SYNOP reports (i.e. 5-figure numeric codes exchanged internationally).
Snow depths (& state of ground when snow covered) are reported in the group 4E'sss in the sub-section with a start indicator '333'. The frequency of report varies from country to country - in the UK, reports are hourly if conditions are relevant.

E' records the state of snow cover in the following code:
0: Ground predominantly covered by ice.
1: Compact or wet snow (with or without ice) covering less than one-half of the ground.
2: As for 1 (above), covering at least one-half of ground, but not completely.
3: Even layer of compact or wet snow covering the ground completely.
4: Uneven layer of compact or wet snow covering ground completely.
5: Loose dry snow covering less than one-half of the ground.
6: As for 5 (above), covering at least one-half of ground, but not completely.
7: Even layer of loose dry snow covering ground completely.
8: Uneven layer of loose dry snow covering ground completely.
9: Snow covering ground completely; deep drifts.

sss records the depth of snow:
001 1cm
002 2cm
003 3cm (etc.) ...

996 996cm
997 Less than 0.5cm
998 Snow cover not continuous
999 Measurement impossible or inaccurate.

METAR reports carry information relating to the state of ice/snow ON THE RUNWAY only, in a group properly called a 'runway state' report, but often referred to as a 'SNOWTAM'. The group should follow the TREND group. These groups have little relevance to general snow conditions in the vicinity of the airfield & care must be exercised when looking at these.
The SNOWTAM (from the NOTAM - 'Notice to airmen' root) group takes the following format:


nn: runway designator (50 added to indicate 'right' runways; 88=all runways; 99=repeat of previous report)
C: Type of deposit (0=clear/dry; 1=damp; 2=wet/puddles; 3=rime or frost covered; 4=dry snow; 5=wet snow; 6=slush; 7=ice; 8=compacted or rolled snow; 9=frozen ruts or ridges; /=type of deposit not reported)
L: Extent of runway contamination (1=10% or less; 2=11 to 25%; 5=26-50%; 9=51-100%; /=extent not reported)
RD: Depth of deposit (note: millimetres NOT cm) 00=less than 1mm; 01=1mm etc. through to 90=90mm; 91=not used; 92=10 cm; 93=15 cm .. then 5 cm steps to 97=35 cm; 98=40 cm or more; 99=runway(s) not operational due to snow or runway clearance. //=not measurable or not significant.

When the runway (nn=specific runway or 88=all runways) is/are declared operational, the group has the four letters "CLRD" within it, with the braking action xx encoded.
xx: Friction coefficient/braking action.

Rainfall groups in SYNOP reports

Rainfall (or melted snowfall) is reported in SYNOP reports in a group: 6RRRt r, which should come immediately after the 5 (pressure tendency) group in the main section of the SYNOP, and before the 7 (weather group) if reported. (However, a few countries, mainly in South America, place this group in the 333 section .. this can be confusing, see below.)

RRR=rainfall total in mm, 990=trace, 991 to 995=0.1 to 0.5mm, /// not recorded for any reason.
tr=period over which rainfall recorded in the following code: (ending at the time of the report)
1=6 hours; 2=12 hours; 3=18 hours; 4=24 hours; 5=1 hour; 6=2 hours; 7=3 hours; 8=9 hours; 9=15 hours; /=accumulated rainfall in a period notified nationally. For example, Blocks 41 & 42 (India & nearby island groups), report accumulated rainfall since 0300 UTC.
Note that this group may also be used in the '333' (supplementary) section to indicate rainfall for other periods: for example, in the main SYNOP, a station might have a group 60422, indicating 42mm has fallen in the past 12hr, but in the '333' section, the group 60067 would indicate that 6mm of that total has fallen in the past 3hr.

In addition, many countries report 24hr total precipitation in the group 7RRRR, in the 'supplementary' (or '333') section of the SYNOP: this is agreed regionally - European countries reporting (mainly) at 06Z, but elsewhere at other 'main' SYNOP hours, sometimes every 6 hours. The rainfall/melted snowfall is in mm & tenths, so 70467 in the '333' section would indicate 46.7mm has been recorded in the past 24hr.

Wind reports carried in SYNOP reports

The mean wind direction and speed is carried in the SYNOP report in the ' Nddff ' group (for the structure of the SYNOP, consult Dave Wheeler's excellent guide: http://www.zetnet.co.uk/sigs/weather/Met_Codes/codes.htm )

The wind direction is that FROM which the wind is blowing (e.g. if dd=27, then the wind is blowing from the west; if dd=36, the wind is coming from the north etc.)
The wind speed is averaged over 10 minutes (but see below), and is given in units of either knots (kn) or metres per second (m/s).
You need to inspect the group that comes at the head of a collective of observations in the form YYGGiw (where YYGG = month-date & hour-time);
if iw=1, then the wind speeds are given in m/s (double to convert roughly to knots), and if iw=4, then the values are in knots.

Re: FM 12 (SYNOP), FM 13 (SHIP), FM 14 (SYNOP MOBIL)
Regulation: " The mean direction and speed of the wind over the 10-minute period immediately preceding the observation shall be reported for ddff. However, when the 10-minute period includes a discontinuity in the wind characteristics, only data obtained after the discontinuity shall be used for reporting the mean values, and hence the period in these circumstances shall be correspondingly reduced. "

If recording/logging equipment is not available, then it's much looser, but the same principal (should) apply ... you take an estimate at the beginning of the 10 mins, one at the end and use a 'mean', or simply make a Beaufort estimate at ob-time and convert to appropriate value.

Thus, for practical purposes, it should be assumed that 10 minutes prior to observation time is the 'standard' for reports in the main section of a SYNOP report (i.e. in the 'Nddff' section).
However .... (life isn't that simple) .... in the United States, and presumably territories where they have an influence in the provision of equipment, setting up services etc., the period is 2 minutes. I'm informed that for the US (Block 72) specifically, the mean wind direction & speed (ddff) is derived from the METAR report, which is known to be a 2 minute mean.

... when we move away from the main section of the SYNOP, there is some variability allowed for in the 'additional' data section as under:

Code 3778 ('9-group' sections)
910ff Highest gust during the 10-minute period immediately preceding the observation.
911ff Highest gust .... (see note below*)
912ff Highest mean wind speed .... (see note below*)
913ff Mean wind speed .... (see note below*)
914ff Lowest mean wind speed .... (see note below*)

*Note: " ... during the period covered by W1W2 (i.e. past weather) in group 7wwW1W2, unless a different period of reference is indicated by group 907tt; or during the 10-minute period immediately preceding the time of observation indicated by group 904tt "
where tt =
00 at observation time
01 .. 09 units of 6 minutes before observation (i.e. 02=12 minutes before etc.)
10 1 hr before observation (the most often used in this context)
11 .. 60 continuing the scheme of units of 6 minutes before observation. (i.e. 39=3hr 54mins)
61 6 to 7 hrs
62 7 to 8 hrs etc. ... up to
66 11 to 12 hrs
67 12 to 18hr
68 more than 18hr
69 time unknown
70 began during observation
71 ended during observation

Sunshine groups carried in SYNOP reports from automatic stations

Where Weather Stations are suitably equipped, and are reporting in the SYNOP code for inclusion in international bulletins, then "sunshine" duration (mainly using radiation sensors) is carried in the following groups:
At 0600Z ONLY .... In Section 333: the group 55SSS shows the duration of sunshine, in tenths of an hour, for the 24 hours of the previous day.
At ALL hours .... In Section 333, the group 553SS is reported, showing the sunshine total for that hour in tenths of an hour.

The local software integrates direct solar radiation received by the AWS sensor in order to estimate the sunshine duration. For day-to-day work, the data are accurate and timely, and will in time form the standard database for bright sunshine records. The sensors respond quickly and accurately to sunshine, and eliminate the problems experienced with the Campbell-Stokes recorder (CSR). This latter uses a glass sphere to focus the rays of sunlight on a card, which is charred upon strong heating - much as you would focus sunlight by a magnifying glass to try and light a fire. There are well-known problems with the CSR instrument, in particular with intermittent, short-duration sunshine, and the subjectivity used in estimating the burn-lengths. The figures obtained from the AWS and CSR units are obtained & processed in totally different ways and care must be applied when comparing the two sets of records. (Also see 'Sunshine recording' in the Glossary)


Code 4680

FM 12-IX Ext. SYNOP code form

In reports from fully automatic stations (THAT ARE EQUIPPED WITH THE APPROPRIATE SENSORS), a separate 'present weather' code table is used (wawa): After the station number group (IIiii), comes a group which contains information on whether or not 'weather' groups are included, and which code table is used. The important figure is the second figure in the group [ iRiXhVV ] as under:

 iX  Type of station  7 ('weather')- group
 1  Manned  Included
 2  Manned  Not-included (no sig.Wx to report)
 3  Manned  Not-included (data not available)
 4  Automatic  Included using 'old' wwWW present weather coding
 5  Automatic  Not-included (no sig. Wx to report)
 6  Automatic  Not-included (data not available)
 7  Automatic  Included using 'new' wawa Code table (as below)


A scanned display of the the plotting convention for this code type will be found here.

 01-09  Description
 01  Clouds generally dissolving or becoming less developed (during past hour)
 02  State of sky generally unchanged
 03  Clouds forming or developing (during past hour)
 04  Haze, smoke or suspended dust (vis >=1.0km)
 05  [as 04 but vis < 1.0km ]
 06  -
 07  -
 08  -
 09  -

 10-19  Description
 10  Mist
 11  Diamond dust (not yet used)
 12  Distant lightning (not yet used)
 13  -
 14  -
 15  -
 16  -
 17  -
 18  Squalls
 19  -

Code 20-26: used to report precipitation, fog (or ice fog) or thunderstorm at the station in the past hour, but not at time of observation. 27-29 appear to be current weather!

 20-29  Description
 20  Fog
 21  Precipitation
 22  Drizzle (non-freezing) or snow grains
 23  Rain (non-freezing)
 24  Snow
 25  Freezing drizzle or rain
 26  Thunderstorm (with/without ppn) (not yet used)
 27  Blowing/Drifting snow/sand (not yet used)
 28  Blowing/Drifting snow/sand: vis >=1.0km (not yet used)
 29  Blowing/Drifting snow/sand: vis < 1.0km (not yet used)

 30-39  Description
 30  Fog
 31  Fog/Ice Fog in patches
 32  Fog/Ice Fog - thinning in past hour
 33  Fog/Ice Fog - no change
 34  Fog/Ice Fog - set in, or thickened in past hour
 35  Fog - depositing rime
 36  -
 37  -
 38  -
 39  -

 40-49  Description
 40  Precipitation (PPN)
 41  PPN - slight/moderate
 42  PPN - heavy
 43  Liquid PPN - slight/moderate
 44  Liquid PPN - heavy
 45  Solid PPN - slight/moderate
 46  Solid PPN - heavy
 47  Freezing PPN - slight/moderate
 48  Freezing PPN - heavy
 49  -

 50-59  Description
 50  Drizzle
 51  Drizzle, not freezing - slight
 52  Drizzle, not freezing - moderate
 53  Drizzle, not freezing - heavy
 54  Drizzle, freezing - slight
 55  Drizzle, freezing - moderate
 56  Drizzle, freezing - heavy
 57  Drizzle & rain mixed - slight
 58  Drizzle & rain mixed - moderate/heavy
 59  -

 60-69  Description
 60  Rain
 61  Rain, not freezing - slight
 62  Rain, not freezing - moderate
 63  Rain, not freezing - heavy
 64  Freezing rain - slight
 65  Freezing rain - moderate
 66  Freezing rain - heavy
 67  Rain/drizzle & snow mixed - slight
 68  Rain/drizzle & snow mixed - moderate/heavy
 69  -

 70-79  Description
 70  Snow
 71  Snow - slight
 72  Snow - moderate
 73  Snow - heavy
 74  Ice Pellets - slight
 75  Ice Pellets - moderate
 76  Ice Pellets - heavy
 77  Snow grains
 78  Ice crystals
 79  -

 80-89  Description
 80  Intermittent PPN
 81  Intermittent rain - slight
 82  Intermittent rain - moderate
 83  Intermittent rain - heavy
 84  Intermittent rain - violent
 85  Intermittent snow - slight
 86  Intermittent snow - moderate
 87  Intermittent snow - heavy
 88  -
 89  Hail (not yet used)

 90-99  Description
 90  Thunderstorm (TS)(not yet used)
 91  TS, slight/moderate - no PPN (not yet used)
 92  TS, slight/moderate - rain/snow showers (not yet used)
 93  TS, slight/moderate - hail (not yet used)
 94  TS, heavy - no PPN (not yet used)
 95  TS, heavy - rain/snow showers (not yet used)
 96  TS, heavy - hail (not yet used)
 97  -
 98  -
 99  Tornado !!! (not yet used)


NB: This is not an official publicising of the WMO code form. It is purely intended for interested amateurs who want to be aware of the relevant code forms. If you have a professional requirement to use these codes, you should refer to the appropriate WMO publications for the official listing, or contact your national meteorological service for more details. I will not be responsible for any omissions, errors etc.

Code 4680 Plotting Matrix

(For the CURRENT/'old-style' symbols, go to Dave Wheeler's site)

Plotting matrix of Code 4680

METAR Decode

This page is designed to help Internet users decode a METAR (Meteorological Aviation Report). It will not necessarily cover the finer points of coding (though I have tried to cover many such), and therefore should not be quoted as the 'final authority': users wishing to know the correct procedures for coding, usage etc., should consult their national 'meteorological authority' for guidance, or the appropriate ICAO or WMO regulations.

The code-form follows the pattern:

In the list below, I have tried to build up from the 'basic' version (via CAVOK) to the more complex issues. I have colour-coded as follows:

   always included in a report (provided station so equipped).
   always included, unless for a CAVOK report.
   added/inserted when necessary, as required by coding convention.
   optional: added as available - often to national or regional (ICAO) rules.

Although the basic code-form should always be recognisable, many nations have adapted the METAR (or merged / integrated with their own national form of same) to produce hybrids. Where possible, I have tried to indicate these variants. Military users in particular will have extra information - e.g. Colour States. Also note that there is an increasing use of automatic weather stations to provided some or all of the data when airfields are closed, or an observer is not available for some reason. In these cases the word AUTO is inserted after the date/time group - other variations follow and are listed in the appropriate sections below. However, in general, if there is a failure of all/some of the systems with these units, then the appropriate element will be replaced by solidi in varying numbers.


EGKK 021950Z 24015KT 8000 FEW008 SCT012 BKN020 18/15 Q1007
EGKK 021350Z 17009KT 8000 NSC 23/12 Q1018

EGKK .. airfield identifier according to the ICAO listing. This one is Gatwick airport.

021950Z .. date/time of report, in this case 2nd day of month, at 1950 GMT (or UTC).
[in the second example, 1350 GMT]

24015KT .. 'surface' wind direction (usually at 10 metres, but some take as low as 2 metres) and mean speed: wind blowing from 240deg True, with an averaged speed (over 10 minutes usually, but some use 1 or 2 minute sampling periods) of 15 knots (KT); some countries use metres per second (MPS) or kilometres per hour (KPH). If the wind direction is 'variable', then the direction is replaced by VRB; dead-calm would be 00000KT.
[in the second example wind direction from 170deg, speed 9 kt]

8000 .. horizontal prevailing visibility** representative of the airfield in metres up to 9 km (9000): if the prevailing visibility (as defined below) is 10 km or more, then this group is given as 9999 & if < 50 metres, it is coded as 0000.
[ ** the 'prevailing visibility' is defined as the value that is reached or exceeded over at least 50% of the horizon (contiguous or in fragments), or within at least half of the airfield / airport surface. See examples below for the reporting of significant variations from this value.
** prior to November 2004, the convention long adopted (except in the US & Canada), was to report the lowest visibility as the primary value, with 'better' values appended to defined rules. Note, however, that this use of 'prevailing visibility' is only applicable to the METAR code; SYNOP visibility reports continue to show the lowest visibility figure.

FEW008 SCT012 BKN020 .. amounts and height-of-base of clouds over/in near vicinity of airfield. SKC (some use CLR) =  no cloud; FEW = 1 or 2 eighths cover; SCT = 3 or 4 eighths cover; BKN = 5, 6 or 7 eighths cover & OVC = 8/8 cover. Heights are given in 100's of feet above airfield level, thus 008 = 800ft, 012 = 1200ft, 020 = 2000ft etc.
[ When there is more than one layer of cloud, the convention for inclusion of cloud groups is ....
(a): the lowest layer
(b): the next highest layer, covering 3 oktas or more of the sky (SCT or more)
(c): the next highest layer, covering 5 oktas or more of the sky (BKN or more)
(d): any CB not already included by these rules - the group being inserted in 'natural' height order.]

When fog or heavy snow is occurring, and it is not possible to determine cloud structure, then these groups are replaced by VVhhh or VV///, where either the vertical visibility can be determined (hhh) in hundreds of feet, or impossible to determine (///)

NSC: increasingly (outside North America), cloud information above 5000 ft / 1500m (or the highest minimum sector altitude, whichever is greater) is being omitted from METAR reports, unless it is 'significant' e.g. when CB or TCU are observed (i.e. the bases of these latter may be above 5000 ft / 1500 m). You will therefore see such replacing the cloud groups: note carefully, that 'NSC' (no significant cloud), means just that .... it does NOT mean no cloud at all!
NCD: in reports from automatic stations (e.g. 'AUTO' obs.), then you may see this abbreviation used to mean .. "no cloud detected". It is important to understand that such means exactly that - there is no cloud below 5000 ft / 1500 m overhead the cloud sensor: there may indeed be cloud floating around well below this level - e.g. 2 oktas of stratus over the airfield approach, or 1 okta cumulus associated with a nearby shower. Also, because of the coding convention now used, there may be cloud higher up, but it will not now be reported under these rules.
[ The formerly-used abbreviation 'SKC', meaning sky clear of cloud, should no longer be used under any circumstances. ]

18/15 .. air and dew-point temperatures (screened), both generally around 1.25m above station level, in degC. Negative values preceded by 'M', thus M02/M05 would indicate air temperature minus 2degC, and dew-point minus 5degC.
[in the second example, air temperature 23 and dew point 12 degC]

Q1007 .. atmospheric (i.e. mean sea level) pressure/QNH, in whole mbar (or hPa). In North America in particular (and associated reporting stations), then this is reported in inches of mercury multiplied by 100, thus A2997 = 29.97 inches. The value in mbar may then be appended to the end of the report as SLPppp, where ppp = QNH in whole mbar, with the 'thousand & hundred' figures missed off where necessary: thus SLP987 would be 998.7 mbar, SLP030 would be 1003.0 mbar.
[in the second example, QNH=1018]

 METAR:-  "CAVOK version"

EGKK 132020Z 22013KT CAVOK 18/15 Q1016

Provided the visibility is >= 10 km, AND the height of the lowest cloud (any amount) is >=5000 ft (or highest minimum sector altitude) AND there are no cumulonimbus clouds (CB, any height) within sight AND there is no significant weather (see list below), then the visibility and cloud part of the standard METAR is replaced by CAVOK (say "cav-oh-kay": 'Ceiling And Visibility OK'). (not used by certain countries, e.g. the United States)

 METAR:-  with additional WIND information

EGKK 312355Z 24028G42KT 210V280 9999 SCT018 12/06 Q0984

24028G42KT.. if over the period that the mean wind is assessed (1, 2 or 10 minutes), the 'peak' gust is greater than the mean by 10KT or more (or equivalent in MPS or KPH), then the gust is appended as 'Gff' .. in above, gust is 42 knots. Note carefully when comparing with SYNOP data, the period of the gust is only for the past 10 minutes at most, NOT the past hour, 3-hours, 6-hours etc., as in SYNOP. For this reason, METAR gusts do not give the true picture of peak winds for any one synoptic situation.

210V280 .. if over the period of observation the wind direction is varying between defined limits, and is in excess of a pre-set level (generally 3 knots or more - national variations), then the 'outer limits' of variation are given, in a clockwise direction: .. in this example between 210degT and 280degT.

 METAR:-  Example of WIND SHEAR report

EDDS 120820Z 24028G45KT 210V280 9999 SCT018 12/06 Q0984 WS RWY28

Some (not all) airfields add wind shear information to the end of the METAR when above a certain threshold: this example would be interpreted as " critical wind shear has been exceeded on approach to runway 28 ".

 METAR:-  with additional VISIBILITY information

EGZZ 231020Z 02006KT 4000 0900NE R27/0600U R32/0150D PRFG OVC007 12/11 Q1028

4000 .. 0900NE .. The first figure given is the 'prevailing visibility', which can be regarded as the 'best' figure that can be applied to at least 50% of the horizon (contiguously or otherwise). So, for example, if the visibility varies from 8km down to 4000m for at least half of the visible horizon, the prevailing visibility is 4000m. It is important to note that the visibility may be lower than this figure elsewhere, but for deviations to be reported, they must obey certain rules: if the visibility in one particular direction is less than 1500m or is less than half of the prevailing figure, then the lowest visibility observed (900m in above) is reported, with the direction shown (NE). If the lowest value applies in several directions, then the 'most operationally significant' direction is given. If the visibility is fluctuating wildly (e.g. rapid shower transistion), then only the lowest visibility is reported. Where the observation is a fully-automated one (e.g. an 'AUTO' ob.), then no variation with direction of visibility is usually possible, and the letter-group NDV is appended to the visibility value.

R27/0600U .. R32/0150D.. The visibility given above is not necessarily the most useful indicator of what a pilot would actually 'see' along a particular runway. To try and overcome this limitation, for airfields so equipped, the 'Runway Visual Range' (RVR) is given when general visibility is poor. In this example, the RVR along runway 27 is 600metres, and along runway 32 it is 150metres. In addition, the letters U, D & N are sometimes used to denote respectively increasing (Up), decreasing (Down) and unchanged (Nochange) RVR values since last report. If the RVR is less than 50 metres, then the group is coded as M0050. If the RVR is more than 2000 m (but general visibility is poor), then the group is coded as P2000: however, many systems only have a maximum operating limit of 1500 m, so you will then seen P1500 in these cases. There are other variants - see current regulations for exact style, meaning etc.

 METAR:-  with additional WEATHER information

EGKK 111150Z 24018KT 8000 -RA SCT012 OVC015 12/10 Q0984 RETS

Weather information follows the format:
Intensity .. Description .. Precipitation .. Obscuration .. Other
This table sets out the full list. (See note 4 below).

- Light MI shallow DZ drizzle BR mist (see note 5) PO well developed dust / sand whirls
Moderate (no symbol) PR partial (e.g. fog bank - see note 1 below) RA rain FG fog SQ squalls
+ Heavy BC patches (see note 1 below) SN snow FU smoke (see note 5) FC funnel clouds, inc tornadoes / waterspouts
VC Vicinity (see note 3 below) DR low drifting (less than 2 m height) SG snow grains VA volcanic ash SS sandstorm
  BL blowing (2 m or more agl; i.e. affecting visibility significantly) IC ice crystals DU widespread dust haze (see note 5) DS duststorm
  SH showers PL ice pellets SA sand  
  TS thunderstorm GR hail HZ haze (see note 5)  
  FZ freezing (see note 2 below) GS small hail PY spray  
    UP unknown

.. up to three weather groups can be included.
.. the individual categories are used from left-to-right in the table above, when more than one applies: thus ... -SHRA light shower of rain; +TSRA heavy thunderstorm with rain; DZ BR moderate drizzle and mist; SN moderate snow; VCPO dust/sand whirls in vicinity (but not over airfield); -SHRAPL FC light shower of rain, with ice pellets and funnel cloud observed.

Note 1: 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!
Note 2: When used with FG, DZ, RA etc., the qualifier 'FZ' is now used to mean BOTH fog (drizzle, rain) depositing rime AND fog (drizzle, rain) occurring with an air-temperature below zero deg.C; this latter may or may not be depositing rime-ice. This definition is therefore not the same as that for the SYNOP code (but I wonder how carefully the distinction is going to be maintained!)
Note 3: 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. )
Note 4: in observations from fully-automated units (e.g. 'AUTO' obs.), then if present weather cannot be assessed due to failure (or otherwise) of the system, then the present weather will be given at //. If the system is functioning correctly but there is no 'significant' weather, then as for manual observations, the 'weather' group is blank.
Note 5: the present weather mist (BR), dust (DU), smoke (FU) and haze (HZ) should only be encoded when the prevailing visibility is 5000 metres or less (though in practice this rule is not often followed).

.. RExx.. significant 'recent' weather may be added after the pressure group, using the list below.

 Thunderstorm  RETS  Freezing Rain  REFZRA
 Freezing Drizzle  REFZDZ  Moderate/heavy rain  RERA
 Moderate/heavy snow  RESN  Moderate/heavy small hail  REGS
 Moderate/heavy snow pellets  REGS  Moderate/heavy ice pellets  REPL
 Moderate/heavy hail  REGR  Moderate/heavy snow grains  RESG
 Moderate/heavy rain showers  RESHRA  Moderate/heavy snow showers  RESHSN
 Moderate/heavy small hail shower  RESHGS  Moderate/heavy snow pellet shower  RESHGS
 Moderate/heavy ice pellet shower  RESHPL  Moderate/heavy hail shower  RESHGR
 Moderate/heavy ice crystals  REIC  Moderate/heavy blowing snow (visibility significantly reduced)  REBLSN
 Sandstorm  RESS  Dust Storm  REDS
 Funnel Cloud  REFC  Volcanic Ash  REVA
 Unidentified precipitation (AUTO obs. only)  REUP    

 METAR:-  with additional CLOUD information

ENBR 232350Z 32017G36KT 270V010 9999 FEW009 SCT018TCU 02/M03 Q0992 WS RWY28

SCT018TCU .. this group indicates 3 or 4 eighths of cloud with base 1800ft, cloud type 'towering Cumulus' (TCU); the only other cloud type allowed (in the official METAR code) is Cumulonimbus (CB). However, some national services use additional types: CBMAM .. Cumulonimbus mammatus (implying turbulent air in the vicinity); ACC .. Altocumulus castellatus (medium level vigorous instability); CLD .. standing lenticular or rotor clouds.
Where the observation is an 'AUTO' ob, and as such cannot determine such variations, then the group /// is added after each cloud group.

 METAR:-  with TREND appended

EGKK 151550Z 24018KT 8000 -RA SCT012 0VC015 12/11 Q0984 RERA TEMPO 3000 RA BKN008

For some major airports (and many military airfields with forecasters attached), a TREND forecast is added to airfield weather reports, usually covering the following 2 hours. If there is no significant change expected (as defined in both international and national regulations), then "NOSIG" is added. Otherwise, the TREND will indicate the expected change in wind direction/speed, visibility, significant weather and cloud base using conventions applied to the TAF code. Some abbreviations you might see: BECMG = becoming (with time groups); TEMPO = temporarily; NSW = no significant weather; AT = at (time); TL = until (time); NSC = no significant cloud.

 METAR:-  additional (non-standard) variants

Some nations (e.g. US, Australia), add more information to the end of the METAR. This list below is not exhaustive but covers some of the more usual inclusions:
RMK .. additional remarks are added after this group.
Volcanic eruptions .. plain language, to include name of volcano, lat/long or direction/distance, date/time of eruption, size/description of ash cloud etc.
Funnel cloud .. format: Type B(hh)mm LOC: type can be one of TORNADO, FUNNEL CLOUD, WATERSPOUT; B(hh)mm: beginning time (can also be used to show end time/E(hh)mm). hh is the hour of the sighting, which can be left out if redundant. mm is the minute of occurrence; LOC: location or direction of movement.
Type of automatic station .. A01 - without precipitation sensor; A02 - with precipitation sensor.
Peak wind .. format: PK WND dddff(f)/hh)mm: ddd wind direction; ff(f) wind speed in knots; (hh)mm time at which wind speed occurred, with hh being left out if redundant.
Wind shift .. format: WSHIFT (hh)mm [FROPA]: (hh)mm time wind shift occurred; FROPA added if windshift is at frontal passage; again, hh is left off if redundant.
Visibility .. format: TWR VIS vvvvv - shows ATC tower visibility; SFC VIS vvvvv shows surface visibility; VIS lllllVuuuuu - shows variable visibility, lower to upper; VIS [DIR] - sector visibility with additional direction (e.g. VIS S 1 1/2 .. visibility to south 1 and a half miles).
Lightning .. format: Frequency LTG(type) [LOC]: frequency can be one of OCNL - less than 1 flash per minute, FRQ - 1 to 6 flashes per minute & CONS - more than 6 flashes per minute; CG: cloud-to-ground, IC: in-cloud, CC: cloud-to-cloud, CA: cloud-to-air.
Tstttsddd .. the actual temperature and dew-point temperature in degrees and tenths (C), with "s" indicating the 'sign' of temperature: 0 for positive, and 1 for negative: thus T00081016 would be interpreted as air temperature +00.8degC and dew-point temperature -01.6degC.
1sTxTxTx 2sTxTxTx .. as for the SYNOP code, where TxTxTx is the maximum temperature and TnTnTn is the minimum temperature - but the period is the last 6 hours. Code 's' indicates the sign of temperature, 0 for at or above zero degC, 1 for below.
4sTxTxTxsTnTnTn .. 24hr maximum and minimum temperature, with s indicating sign as before.
Prrrr.. hourly precipitation amount.
6rrrr.. 3 and 6 hour precipitation amounts (3hr at intermediate hours, 6hr at 'main' hours).
7rrrr.. 24hr precipitation amount.
4/sss.. snow depth on ground (not runway).
CIGhhh.. ceiling in hundreds of feet (hhh), generally the lowest layer with 5 oktas or more, but USAF bases have variations on this.

Colour states: METAR reports from military airfields operated by the RAF, some USAF and others may have a 'colour-code' appended (usually only when ATC is open), which describes the airfield 'fitness': these run from BLU best, through WHT GRN YLO (1 and 2), AMB and RED. The colour is based on the lowest cloud base (usually 3 oktas or more cover, but some use 5 oktas) and the horizontal 'MET' visibility. BLACK is also used, for airfield closed for non-weather reasons.