British Weather Extremes
EXTREMES//PRECIPITATION

Precipitation / Rainfall

Overview

Very heavy rainfalls in the UK 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 the UK especially from mid-April to mid-October. Such rainfall may affect a large area (sometimes with convection embedded in a larger area of cyclonic/frontal rain) such as, for instance, on 10th July 1968 when exceptional daily falls occurred in a broad swath from east Devon to Lincolnshire and 19-20 July 2007 when peak totals across the southwest Midlands. Exceptional convective rainfall can also be associated with very localised but intense thunderstorms, often involving a ‘train echo’ of storm cells. Examples include at Hampstead on 14th August 1975 (see Journal Meteorology, 1(1), 1975 6-8), in Calderdale, West Yorkshire on 19th May 1989 (see Weather 44, 1989, 438-46), Boscastle, Cornwall on 16 August 2004 (see special issue of Journal Meteorology 29, Number 293), at Ottery St Mary on 30 October 2008 and around Coverack (Cornwall) on 17 July 2017. Some other examples of extreme point deluges are discussed in Journal Meteorology, 13 (129), 1988, 167-70.
Several case studies are presented in the TORRO 40th Anniversary Book (Doe, 2016, Ed: Chapter 14, Mason, Brown, Webb and Doe, 2016). A method of classifying heavy short-duration rainfalls was presented 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 re-label 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 European Severe Weather Database (ESWD) uses a similar graph based classification but with a single ‘Extreme Rainfall’ category.

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) and the Ottery St Mary storm of 30 October 2008 highlight the continuing need for a dense network of rain gauges throughout the country to complement radar coverage.


Highest daily rainfalls for each month of the year

Month mm inches Location Region Date
January 238.4 9.39 Loch Sloy main adit Strathclyde 17 January 1974
February 196.6 7.74 Ben Nevis Highland 6 February 1894
185.5 7.30 Kinlochhourn Highland 5 February 1989
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 239.9 9.44 East Wretham Norfolk 16 August 2020
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 Quoich Highland 11 October 1916
November 253.0 9.96 Seathwaite Cumbria 19 November 2009
December 264.4 10.41 Thirlmere Cumbria 5 December 2015

Note: * = Approximate value.
A listing of the highest daily rainfalls recorded in each British county was published in Journal Meteorology, 12(122), 1987, 263-6.


Extreme short-duration rainfall events, since 1870

Duration (mins) Rain (mm) Location Region Date
5 32* Preston Lancashire 10 August 1893
10 45 Carlton-in-Cleveland North Yorkshire 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
30 80 Eskdalemuir Dumfries & Galloway 26 June 1953
45 97 Orra Beg Co Antrim 1 August 1980
60 110* Wheatley Oxfordshire 9 August 1910
90 117 Middle Knoll Lancashire 8 August 1967
90 111 Miserden Gloucestershire 10 August 1970
120 193* Walshaw Dean Lodge West Yorkshire 19 May 1989
120 155 Hewenden Reservoir West Yorkshire 11 June 1956
120 131 Knockholt Kent 5 September 1958
155 169 Hampstead Greater London 14 August 1975
180 178 Horncastle Lincolnshire 7 October 1960
240 197 Otterham Cornwall 16 August 2004

Note: * = Approximate value.

Mason, J; Brown, P. R; Webb, J. D. C; Doe, R. K (2016). Extreme Rainfall and Flash Floods in the United Kingdom and Ireland: Synoptic Patterns and Selected Case Studies. In: Extreme Weather: Forty Years of the Tornado and Storm Research Organisation (TORRO), Chapter 14. Doe, R. K (Ed). John Wiley and Sons: Chichester, UK, 261-282.