Precipitation / Rainfall
Overview
Very heavy rainfalls in the UK can arise from:
- 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.
- 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.
A list of the UK’s highest recorded rainfalls for each date of the year, up to 2007, can be found in the February 2009 issue of the International Journal of Meteorology,
available here. An updated list is in preparation.
A comprehensive chronology of historic flash floods for regions of England, Scotland and Wales can be
accessed here. Full details of this chronology are available in
this recent paper (Open Access) by David Archer and Hayley Fowler.
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.