Rainfall is caused by the condensation of the water in air that is being lifted and cooled below its dew point. Rainfall tends to be associated with Atlantic depressions or with convection. The Atlantic Lows are more vigorous in autumn and winter and bring most of the rain that falls in these seasons. In summer, convection caused by solar surface heating sometimes forms shower clouds and a large proportion of rain falls from showers and thunderstorms then. The additional heat from the London urban area can encourage such shower development in summer.
A further factor that greatly affects the rainfall distribution is altitude. Moist air that is forced to ascend hills may be cooled below the dew point to produce cloud and rain. A map of average annual rainfall therefore looks similar to a topographic map.
Much of Southern England is relatively distant from the route of many Atlantic depressions and towards the north-east of the region there is increasing shelter from rain-bearing SW winds. This shelter reaches its greatest potential around the Thames Estuary. The wettest areas are therefore the South Downs and the higher parts of Dorset, with an average of over 950 mm per year. In contrast, the Thames Valley, London and the north Kent coast normally receive less than 650 mm of rain per year. These values can be compared with annual totals around 500 mm in the driest parts of eastern England and over 4000 mm in the western Scottish Highlands.
Rainfall is generally well-distributed throughout the year but with an autumn/early winter maximum that is more pronounced in counties bordering the English Channel. Further north, in London and the Thames Valley, there are also significant amounts in the summer associated with showery, convective rainfall. The course of mean monthly rainfall for 1971 - 2000 for 4 sites is shown below.
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Over much of Southern England, the number of days with rainfall totals of 1 mm or more ('wet days') tends to follow a pattern similar to the monthly rainfall totals. In winter (December to February), there are 35 to 40 wet days on average over the Downs and the higher parts of the west, decreasing to less than 30 days around the Thames Estuary. In summer (June to August) there are about 25 wet days, with the North Downs and western areas being most prone.
Periods of prolonged rainfall can lead to widespread flooding, especially in winter and early spring when soils are usually near saturation. An example was the widespread inundation of the Thames flood plain in early January 2003, following well above average rainfall the previous autumn and significant rain in late December 2002. Flows on the middle reaches of the Thames were the highest since March 1947. Heavy rainfall in October 2000 also resulted in severe, localised flooding, with Sussex being particularly hard hit including Uckfield where homes and businesses were inundated when the River Uck burst its banks. Falls included 134 mm in 24 hours on 11 October at Plumpton.
Southern England is susceptible to summer thunderstorms, especially at inland locations. The associated high intensity rainfall can also result in flooding, but this is usually short-lived. Noteworthy examples include the storm at Hampstead (Greater London) on 14 August 1975, when 169 mm fell in 2.5 hours, and one at Bracknell (Berkshire) on 7 May 2000 when 65 mm occurred in 1 hour. The intense thunderstorms that broke out during the afternoon and evening of the 18 July 1955 resulted in rainfalls of over 175 mm in south Dorset. The highest was a remarkable 279.4 mm at Martinstown, near Dorchester and this is the highest daily fall ever recorded in the UK.
Conversely, the region can be subject to dry periods that place demands upon water supplies and require conservation measures such as summer hosepipe bans. If a period with below average rainfall includes winter months as well as the high-demand summer months, then conditions can become severe as the winter is the normal recharge time not only for reservoirs but the chalk aquifers upon which much of the region relies for water supplies. Examples include the period November 2004 to February 2006, when about 75% of the normal rainfall occurred over the area, making it the driest such period since 1932/34.
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