PITSFORD HALL WEATHER STATION

PAST WEATHER CODING - BEAUFORT LETTERS

In common with most climatological stations, Pitsford Hall has adopted the Beaufort system of recording past weather events. This system comprises a number of letters which summarise significant aspects of the weather. This 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.

Beaufort letters for past weather appear in the Daily Weather Report issued by this station at 0900 hours and the regularly updated pages of this site. The letters, as written in the register (Metform3100), appear in groups, each group separated by a coma and representing different weather sequences. For ease of data entry into the spreadsheets used for the compilation of weather reports, we have placed these letter groupings as separate rows within the column corresponding to the appropriate day. The order of letters in each grouping follows the convention:

state of sky
thunderstorm (if present)
precipitation (if present)
atmospheric obscurity (if present)
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.

STATE OF SKY

THUNDERSTORM/ALLIED PHENOMENA

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

Intensity:

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 = r_o, slight shower of rain and snow = pr_os_o 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, a subscript 2 is used, 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.

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 2mm 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 intensity 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: tl_oR. 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: tl_o.

Continuity:

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..... ir_o, 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..... r_or_o, 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; d_or_o= 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:

cs_os_o, cr_os_o , S , cs_o , c

Note that each group of letters is separated by a comma.

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

Continuous observations of atmospheric obscurity are not made at Pitsford Hall, other than the remarks detailed under 'visibility' at the time of morning observation. Thus, where these entries appear in the past weather record they generally relate to significant changes in visibility during the day.

(%) 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.

OTHER PHENOMENA, ETC.

* 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. 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;

*** 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."

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.