British Weather Extremes: Rainfall / Precipitation

Very heavy rainfalls in Britain can arise from:

  1. Prolonged orographically-enhanced frontal precipitation. This most frequently occurs in hilly western areas of Britain in very moist airstreams from the Atlantic Ocean. Such heavy rainfalls may occur at any time of the year, but are especially frequent during the autumn and winter months when south-westerly winds are dominant.
  2. Convective (thundery) rainfall which may give rise to very high daily rainfalls in any part of Britain especially from mid-April to mid-October. Such rainfall may affect a large area such as, for instance, on 10th July 1968 when exceptional daily falls occurred in a broad swath from east Devon to Lincolnshire. Exceptional convective rainfall can also be associated with very localised but intense thunderstorms, as, for example, at Hampstead on 14th August 1975 (see Journal Meteorology, 1(1), 1975 6-8) or more recently in Calderdale, West Yorkshire on 19th May 1989 (see Weather 44, 1989, 438-46). Some other examples of extreme point deluges are discussed in Journal Meteorology, 13 (129), 1988, 167-70.

A method of classifying heavy short-duration rainfalls was devised by Ernest Bilham in the 1935 edition of British Rainfall. He used a simple graph to classify such falls into three categories; noteworthy, remarkable, and very rare; each were related to their approximate frequency of occurrence (see Journal Meteorology 12(122), 1987, 268). Although more recent studies have used a classification based on regional variations in rainfall the Bilham formula remains the simplest and has the advantage of assuming that the impact of a flash flood is related to local topography and drainage, rather than to how frequently an extreme rainstorm is recorded. It is more realistic to relabel Bilham's three categories as: 'noteworthy', 'remarkable' and 'extreme', thus avoiding the misleading implication that such events have any regular return period. This is consistent with TORRO's classifications of tornadoes and damaging hailstorms, which are based on severity rather than estimated frequency.

The problems associated with verifying extreme rainfall events such as the Calderdale cloudburst of 19th May 1989 (when 193 mm of rain was reported in two hours) highlight the continuing need for a dense network of raingauges throughout the country to complement radar coverage.

HIGHEST DAILY RAINFALLS FOR EACH MONTH OF THE YEAR

             mm     inches

JANUARY      238.4  9.39    Loch Sloy main adit    Strathclyde    17 January 1974

FEBRUARY     196.6  7.74    Ben Nevis              Highland        6 February 1894

MARCH        164.3  6.47    Glen Etive             Highland       26 March 1968

APRIL        182.1  7.17    Seathwaite             Cumbria        22 April 1970

MAY          172.2  6.78    Seathwaite             Cumbria         8 May 1884

             193*   7.60*   Walshaw Dean Lodge     West Yorkshire 19 May 1989

JUNE         242.8  9.56    Bruton                 Somerset       28 June 1917

JULY         279.4 11.00    Martinstown            Dorset         18 July 1955

AUGUST       238.8  9.40    Cannington             Somerset       18 August 1924

SEPTEMBER    190.7  7.51    West Stourmouth        Kent           20 September 1973

OCTOBER      208.3  8.20    Loch Avoich            Highland       11 October 1916

NOVEMBER     211.1  8.31    Lluest Wen Reservoir   Rhondda        11 November 1929

DECEMBER     199.1  7.84    Dalness                Highland       17 December 1966




Note: A listing of the highest daily rainfalls recorded in each British county was

published in Journal Meteorology, 12(122), 1987, 263-6.

SOME EXTREME RAINFALLS FOR SPECIFIED SHORT DURATIONS IN THE BRITISH ISLES 1870 TO DATE



DURATION IN    AMOUNT OF      LOCATION       COUNTY                 DATE    

MINUTES        RAIN (mm)                                    

5              32*            Preston        Lancashire             10 August 1893

10             45             Carlton-in-Cleveland (North Yorks)    10 August 2003

12             51             Wisbech        Cambridgeshire         27 June 1970

15             56             Bolton         Greater Manchester     18 July 1964

20             63             Sidcup         Kent                    5 September 1958

20             63             Hindolveston   Norfolk                11 July 1959

25             67             Pershore       Worcestershire         11 June 1970

30             80             Eskdalemuir    Dumfries & Galloway    26 June 1953

45             97             Orra Beg       Antrim                  1 August 1980

60             92             Maidenhead     Berkshire              12 July 1901

75             102            Wisley         Surrey                 16 July 1947

75             95             Ilkley         North Yorkshire        12 July 1900

90             117            Dunsop Valley  Lancashire              8 August 1967

90             111            Miserden       Gloucestershire        10 June 1970

100            116            West Wickham   Greater London         22 July 1934

105            116            Sevenoaks      Kent                   25 June 1980

120            131            Knockholt      Kent                    5 September 1958

120            155            Hewenden Reservoir   West Yorkshire   11 June 1956

120            193*           Walshaw Dean Lodge   West Yorkshire   19 May 1989

155            169            Hampstead      Greater London         14 August 1975

180            178            Horncastle     Lincolnshire            7 October 1960

* Approximate value