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 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 to the dew point to produce cloud and rain. A map of average annual rainfall therefore looks similar to a topographic map.
The average annual rainfall exceeds 1500 mm in the higher parts of the Pennines. There is a decrease as the land falls eastwards, such that the east coast is one of the driest parts of the UK with less than 600 mm in places such as Tees-side and the Northumbrian coast. Relatively low averages are also found in the Vale of York. In contrast, the higher ground of the North York Moors results in averages of over 1000 mm in places. These values can be compared with annual totals around 500 mm in parts of eastern England and over 4000 mm 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. At the lower sites and towards the coast the distribution is more even, with showery rainfall in summer contributing as much as the autumn/winter depressions.
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Over much of the region, the number of days with rainfall totals of 1mm or more ('wet days') tends to follow a pattern similar to the monthly rainfall totals. In the higher parts in winter (December-February), 45-50 days is the norm but this decreases to about 35 days in summer (June-August). In the drier areas closer to the coast, about 30 days in winter and about 25 days in summer are typical.
Periods of prolonged rainfall are often associated with east or NE winds on the northern flank of depressions passing to the south of the area. Examples of such periods include 15-16 July 1973 when Sheffield and surrounding areas received over 100 mm.
Hail and thunder are less frequent in NE England than in areas further south and most places can expect only 5-6 days with thunder each year. These are most likely to occur from May to September, reaching their peak in July and August. The heaviest falls of rain in the UK are often associated with these summer thunderstorms. An example was a storm on 10 August 2003 at Carlton-in-Cleveland, N Yorkshire when 47 mm fell in just 12 minutes, with the highest rainfall rates over 5-10 minutes ever recorded in the UK. Another was the storm that struck the western edge of the North York Moors on 19 June 2005, when almost 60 mm fell in 1 hour at Hawnby and there was considerable flooding in Ryedale.
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