Rainfall is caused by the condensation of the water in air that is being lifted and cooled to its dew point. Rainfall tends to be associated with Atlantic depressions or with convection. The Atlantic lows are more vigorous in autumn and winter. In summer, convection caused by solar surface heating sometimes forms shower clouds and a large proportion of rain falls from showers and thunderstorms at this time of year. Rainfall caused this way is normally more intense than winter rainfall which tends to be more frontal with falls occurring over longer periods.
Atmospheric humidity is an important factor determining rainfall and the sea temperature largely controls this. A final factor which greatly affects the rainfall distribution is altitude. Moist air which is forced to ascend hills may be cooled to the dewpoint to produce cloud and rain. A map of rainfall therefore looks very like a topographic map.
In England and Wales the wettest places are in the Lake District, receiving an average of over 3000 mm of rain a year, whilst in the western Scottish mountains averages of over 4000 mm occur. Much of eastern England receives less than 700 mm per year and includes some of the driest areas in the country.
The course of mean monthly rainfall for 1971 - 2000 for 4 sites is shown below. There is a much more even distribution of rainfall throughout the year than in most other parts of the UK. This is mainly due to a combination of the 'rain-shadow' effect for winter Atlantic depressions produced by the high ground to the west and a higher frequency of convective rainfall in summer.
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Across most of the region there are, on average, about 30 rain days (rainfall greater than 1 mm) in winter (December to February) and less than 25 days in summer (June to August) with the highest averages being at the higher altitude of the Wolds.
Although rainfall is generally low, there have been some noteworthy severe storms. These include 25 August 1912 when over 100 mm was recorded in Norfolk causing damage to roads and bridges, with a maximum of 205 mm at Brundall, east of Norwich. On 1 September 1994, 147 mm was recorded in only a few hours at Ditchingham near Bungay in Suffolk, causing transport disruption and significant flooding. The highest recorded daily rainfall total in the UK was 279mm at Martinstown in Dorset on 18 July 1955.
The number of thunderstorms in a year can make a significant contribution to the total annual rainfall. They can occur at any time of year but are more frequent during the summer months. Over East Anglia, Lincolnshire and Humberside the average number of days of thunder per year is about 15 although there is considerable variability from year to year. For example, Cambridge recorded 2 days of thunder in 1997, 7 in 1998 and 22 in 1999.
In thunderstorms or heavy showers at any time of year the precipitation may be in the form of hail. The occurrence of hail tends to have a spring maximum, as in summer hail tends to melt before reaching the ground, although damaging hail is usually associated with summer thunderstorms. There are 6-8 days each year with hail over much of East Anglia, less elsewhere. In Lincolnshire on 25 August 2001 there were reports of hail the size of golf balls at Sleaford, Coningsby and Spilsby in the south of the county causing damage to aircraft, cars and greenhouses.
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