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 therefore looks similar to a topographic map.
The exposure of NW England to westerly maritime air masses and the presence of extensive areas of high ground mean that the region has some of the wettest places in the UK. The higher parts of the Lake District are particularly wet, with an average of over 3200mm of rain each year. In contrast, the reputedly wet city of Manchester averages only 860mm and the more sheltered areas of Cheshire and the Eden valley in Cumbria are even drier with less than 800mm per year. These areas benefit from the 'rain shadow' effect of the high ground of N Wales and the Lake District respectively. The annual averages for the Isle of Man range from about 900mm on the coast to 1800mm around Snaefell. These values can be compared with annual totals of about 500mm in the drier parts of eastern England and over 4000mm in the western Scottish Highlands.
The course of mean monthly rainfall for 1971 - 2000 for 4 sites is shown below. Whilst rainfall is generally well-distributed through the year, there is a seasonal pattern. The driest season is spring whilst there is an autumn/winter maximum, when the Atlantic depressions are at their most vigorous. This contrast is most pronounced in the wetter upland areas.
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Over much of the region, 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 in winter (December-February), 50-60 days is the norm but this decreases to 40-45 days in summer (June-August). In the drier areas of Cheshire and Merseyside, 35-40 days in winter and about 30 days in summer are typical.
Periods of prolonged rainfall can lead to widespread flooding, especially in winter and early spring when soils are usually near saturation. The autumn of 2000 was particularly wet, with over twice the normal rainfall in much of the Pennines, Greater Manchester and Cheshire. A recent example was the heavy rainfall in early January 2005 that led to widespread and severe flooding in Carlisle, considered to be the worst to affect the city since 1822. During this period, 227 mm of rainfall were recorded at Shap, Cumbria in 72 hours.
Thunderstorms are most likely to occur from May to September, reaching their peak in July and August, In the Isle of Man thunder occurs on about 5 days per year, on average, while on the mainland the number is 8 to 10 days. The heaviest falls of rain in the UK are often associated with summer thunderstorms. An example was the 95.9 mm that fell at Manchester-Ringway in 11 hours on 5/6 August 1981.
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