Beaufort Code

Beaufort Code is a system which uses letters and numbers to denote various weather types.  The tables below provide full details of Beaufort Code. At the end of this document is a section describing how to write and interpret Beaufort Code.

 

State of the Sky - Cloud cover
Letter Description
b Cloud cover: 0 to 2 oktas (0 - 25%)
bc Cloud cover: 3 to 5 oktas (26 - 74%)
c Cloud cover: 6 to 8 oktas (75 - 100%)
o Uniform thick layer of cloud completely covering the sky (100%)
Hydrometeors - Water and Ice
Letter Description
r Rain (drops of water >0.5mm diameter)
rx Freezing Rain (i.e. rain which freezes on contact with the ground and vegetation)
d Drizzle (drops of water <0.5mm diameter)
dx Freezing Drizzle (i.e. drizzle which freezes on contact with the ground and vegetation)
s Snow (ice crystals, often branched into 'flakes' in 'warmer conditions', temperatures >-5oC
h Snow Pellets (often referred to as 'soft hail', and typical of wintry showers, especially in coastal regions; white and spherical or conical, 2 to 5mm; fall only from cumulus and cumulonimbus clouds )
h Hail (transparent or opaque particles , usually spheroidal, but sometimes conical; over 5mm; can fall as larger aggregated lumps in heavy thunderstorms; most larger hailstones show evidence of concentric layering)
Small Hail (as above but consisting of snow pellets encased in a thin layer of clear ice, not easily crushable; under 5mm; very common in showery weather in Britain; falls only from cumulus and cumulonimbus clouds) Ice Pellets (spherical, conical or irregular transparent ice particles <=5mm; fall mainly from altostratus and nimbostratus clouds)
Diamond Dust (tiny ice crystals which fall from a clear sky; requires very cold weather; rare in Britain; common in polar regions)
sh Snow Grains (white opaque particles <1mm; fall usually from low stratus and stratocumulus clouds)
Visibility
Letters Description
f Fog (Visibility <1km)
fx Freezing Fog (water droplet fog, that freezes on contact with solid objects)
fe Wet Fog (damp fog which deposits a film of water on exposed solid surfaces)
fs Shallow Fog Patches (fog limited to a depth of 2 metres or less; if it is a continuous layer, the lower line of the symbol is continuous, but the Beaufort code stays the same)
m Mist (Visibility 1 to 2 km; there are a variety of definitions of mist; the British Met Office also require a relative humidity between 95 and 100%)
ks Drifting Snow (snow raised to heights below eye level - 1.8 metres; no overall reduction in visibility)
ks Blowing Snow (snow raised to a great height above the land surface causing severe reduction of visibility, e.g. a 'white-out'; strong winds needed; usually limited to upland areas in Britain)
w Dew (produced by night-time radiation cooling
w Advection Dew (caused by condensation on upright surfaces, usually after a cold spell as warm moist air blows against cold surfaces)
w White Dew (frozen dew drops, as opposed to hoar frost)
x Hoar Frost (the 'usual' white frost, produced by radiation cooling; takes the form of small needles, scales, feathers or fans)
x Advection Hoar Frost (forms in the same way as advection dew, but with temperatures still below freezing)
dr Drizzle and rain
rs Rain and snow (Sleet)
hs Hail and snow
hr Hail and rain
Miscellaneous
Letter Description
z Haze (the presence of microscopic particles in the air in sufficient quantities to give the sky an opalescent appearance; visibility is often reduced; most common in calm anticyclonic weather in summer in Britain, when pollen, dust and pollution contaminate the air)
tl Thunderstorm (thunder must be audible at the site before a thunderstorm can be recorded)
l Lightning (there are three common types: ground discharge - where the lightning strikes the ground; it is often in a ranched form, hence the name 'forked' lightning; cloud discharge - lightning that is within the cloud - its channel is often ndistinguishable and we normally refer to it as 'sheet' lightning; air discharge - often sub-horizontal, it runs from cloud to air outside the cloud; sometimes called 'streak' lightning; a fourth an rare form is ball lightning, which appears near the ground usually after a ground discharge; it varies between 10cms and 1 metre in diameter, 'floats' around for several seconds and usually dissipates with a violent explosion)
j Within Sight (used as a suffix for other phenomena, e.g. pj = shower within sight, but not over the recording station.
e Wet Air (wet air, but without rain falling)
y Dry Air (relative humidity < 60%)
u Ugly Threatening Sky (e.g. before the onset of a thunderstorm)
v Abnormally good visibility (e.g. over 50 miles)
p Shower (a relatively short period of precipitation; the type is indicated by additional letters or symbols)
g Gale (wind speed averaging between 34 and 47 knots for a period of 10 minutes or more)
G Storm (wind speed averaging over 47 knots for a period of 10 minutes or more)
q Squall (a strong wind that rises suddenly and lasts for at least a minute then dies away relatively quickly; an increase of 16 knots to a speed over 22 knots is required)
kq Line Squall (as above but occurring along the line of a cold front and accompanied by a roll shaped cloud with a horizontal axis and a sharp fall in temperature)

 

Beaufort Code Conventions

Assessing Intensity

We need to differentiate between, light/weak, moderate, heavy and violent/Severe events, such as rainfall, thunderstorms, lightning, (and to a lesser extent with fog), fog, etc.

  • Weak/Light: a subscripted suffixed 'o' is used, e.g. ro, so, do mean light rain, snow and drizzle respectively. For very light precipitation a double 'o' may be used, e.g. soo  This, however, is non-standard.
  • Moderate: simple lower case letters, e.g. r, s, d, f, tl
  • Heavy: capital letters, e.g. R, S, D, H, F,TL
  • Violent/Severe: a subscripted suffixed '2' is used, following a capital letter e.g. R2 indicates torrential rain.

The same procedure is used for rain, hail, etc.

Assessing Continuity

If a phenomenon is continuous, the code is simply repeated, e.g. rr represents continuous rain, soso represents continuous light snow.  If a phenomena is intermittent (i.e. broken by intervals less than an hour long) the prefix 'i' is used, e.g. iro means intermittent light rain.

Shower Length

The difference between a lengthy shower and a period is rather subtle.  By definition showers can only fall from convective cloud (cumulus and cumulonimbus), and are usually broken by sunny spells or clear interludes.  There is no standard way of differentiating between short, medium and lengthy showers.  However, an easily applied non-standard solution is to use the same suffixes as for intensity, to refer to length, e.g.

  • poR = short shower of heavy rain.
  • pooroo = fleeting shower of very light rain, e.g. just a few drops, not enough to completely wet the ground.
  • Pro = lengthy short of light rain.

Shower Frequency

This is another non-standard convention.  Official staffed weather stations would actually record the precise times of showers.  For those of us who have other jobs to do the following method may suffice:

  • (f)poRH = frequent short showers of heavy rain and hail
  • (o)pro = occasional showers of light rain

Ensure that the brackets are used, otherwise 'f' means 'fog' and 'o' means 'overcast'.

Code Order

When several phenomena occur together they are recorded in the following order:

  • State of sky
  • Thunderstorm
  • Precipitation
  • Atmospheric obscurity (visibility)
  • Other phenomena
  • Example: cTLrzu = cloudy with a heavy thunderstorm and moderate rain, haze present and an ugly threatening sky.

Coding a Sequence of Weather

Beaufort Code can be used for a current weather status report, i.e. for a fixed point in time, or it can be used to record weather throughout the day, to show the sequence of weather events.  Where it is used to represent a sequence a comma can be used to separate events, e.g.

  • Code: b, bc, c, oiro, ororo, orr, cRR, bc(o)pR
  • Translation: cloud cover less than 2 oktas increasing to over 6 oktas, eventually 8 oktas with a dull featureless sky and intermittent light rain, becoming continuous, moderate and heavy, with a more differentiated cloudy sky of 8 oktas, followed by a cloud cover of 3 to 5 oktas and occasional showers of heavy rain

Most observers will add additional information in code sequences, often in brackets, e.g. the time of certain events, descriptions of phenomena which have no eaufort Code (the international symbols may be used), number of discharges in a thunderstorm, types of cloud present, etc. Curly brackets are used to indicate sunrise } and sunset {, e.g.
RR } c = continuous heavy rain before sunrise, followed by cloudy dry weather after sunrise
Cloud cover and types of cloud are also embedded in the Beaufort Code strings, inside square brackets, e.g. [50Cu(con)], which would indicate a cover of 50% Cumulus congestus.

There is a separate download available covering cloud codes.

Thunderstorms

The details of thunderstorms are included inside a double set of backslashes, e.g.
\\[0130-0215][33cc25cg]\\ which means the storm lasted from 0130 to 0215 GMT/UTC and consisted of 33 inter-cloud discharges (usually appearing as sheet lightning, especially at night) and 25 cloud to ground (or vice-versa) discharges.

Snow Lying

Snow lying (over 50% ground cover) at any point in the day is indicated by the usual Beaufort letters, plus a double superscripted asterisk, e.g.
C**[3cm]

The average snow depth is indicated inside square brackets using centimetres or millimetres

Fronts

Fronts are indicated by a double set of capital letters:
  • WF warm front
  • CF cold front
  • OF occluded front
  • SF stationary front

which are colour coded with the F subscripted in downloaded excel spreadsheets, but not in queries running from the website, which simply return two capital letters.