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.
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 looks similar to a topographic map. However, because most rainfall arrives from the west with Atlantic depressions, much of the West Midlands lies in the 'rain shadow' of the Welsh mountains. This makes the region relatively dry, with the effect enhanced locally by the Severn and Wye valleys.
The wettest areas in the Midlands, with an average of over 800 mm per year, are along the Welsh border, in the Cotswolds and, especially, in the Peak District; the highest altitudes exceed 1000 mm. In contrast, the more sheltered areas of the South and East Midlands are the driest with less than 600 mm per year in parts of Northamptonshire, the lower Trent valley and the Avon valley. These values can be compared with annual totals around 500 mm in the drier parts of eastern England and over 4000 mm in the western Scottish Highlands.
Rainfall is generally well-distributed through the year, but the wettest month varies across the region. In the wetter upland areas of the north and west, there is a pronounced winter maximum when the Atlantic depressions are at their most vigorous. In contrast, the East and South Midlands tend to have a more even distribution through the year, with summer amounts there 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 the Midlands, 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 the higher parts of the west and north in winter (December - February), 40 to 45 days is the norm but this decreases to near 30 days in summer (June - August). In the drier east and south, 30 to 35 days in winter and 20 to 25 days in summer are typical. This seasonal change reflects the tendency for summer rainfall to occur over shorter periods because of more convective activity and less frontal rainfall.
Periods of prolonged rainfall can lead to widespread flooding, especially in winter and early spring when soils are usually near saturation. The Severn valley is particularly prone, since it drains extensive upland areas in mid-Wales. At Easter 1998 a stationary band of heavy rain that stretched across the Midlands from Worcester to Peterborough resulted in floods in which 5 people died and 1000's were evacuated from their homes. Late October and early November 2000 also saw severe flooding, particularly in areas bordering the Rivers Severn and Trent, following an exceptionally wet autumn with over twice the normal rainfall. On 20 July 2007, up to 18 hours of rainfall resulted in many places in the south Midlands receiving their highest daily rainfall on record. Many rivers burst their banks, including the lower Severn, upper Thames and their tributaries. Thousands of homes and businesses were flooded and there was severe road and rail transport disruption across a wide area.
Thunderstorms are most likely to occur from May to September, reaching their peak in July and August, with eastern areas among the most prone in the UK. High intensity rainfall is often associated with summer showers and thunderstorms, rates of 100 mm/hr or more being possible for short periods. A notable example that affected Birmingham was the storm of 14 July 1982, with about 35 mm falling in 20 minutes and peak intensities of over 250 mm/hour at Edgbaston, causing extensive flooding. In another storm on 24 July 1994, also in Birmingham, about 21 mm fell in less than 1 hour and 15 mm diameter hailstones fell.
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