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 rainfall is 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.
Rainfall in Northern Ireland varies widely, with the wettest places being in the Sperrin, Antrim and Mourne Mountains. The highest areas have average annual totals of about 1600 mm, which is about half that of the English Lake District or the western Highlands of Scotland. In addition to topographic effects, greater exposure to rain-bearing winds off the Atlantic results in higher averages in the more western counties of Fermanagh, Londonderry and Tyrone. The wettest places are in the upland area around Killeter Forest in the extreme west of County Tyrone, where the annual average reaches about 1950 mm. The driest places are further east - around Strangford Lough and close to the east coast, and near to the southern and eastern shores of Lough Neagh - where the annual totals are just under 800 mm.
The seasonal variation of rainfall in Northern Ireland is less marked in the drier southern and eastern areas than in the wetter areas, but in all areas the wettest months are between October and January. This is partly a reflection of the high frequency of winter Atlantic depressions and the relatively low frequency of summer thunderstorms in Northern Ireland. For example, at Armagh, thunder occurs on an average of less than 4 days a year, compared with 15 to 20 days at many places in England. Only in a few locations, mainly away from the coast, does the frequency of thunder exceed 5 days a year.
The course of mean monthly rainfall for 1971-2000 for 4 sites is shown below. The pattern of rainfall is similar at each, with the months October to January the wettest and the late spring and early summer months the driest.
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Over much of Northern Ireland, the number of days with a rainfall total of 1mm or more ('wet days') tends to follow a pattern similar to the monthly rainfall totals. In the higher parts, over 55 days is the norm in winter (December to February) and over 45 days in summer (June to August). In the driest areas around Lough Neagh and eastwards to Strangford Lough, less than 45 days in winter and about 35 days in summer are typical.
The combination of close proximity to active weather systems arriving from the Atlantic and the extensive areas of upland can lead to notable daily and monthly falls. The highest fall in a day was 158.9mm at Tollymore Forest (County Down) on 31 October 1968. Periods of prolonged rainfall can lead to widespread flooding. For example, autumn 2000 was the wettest for over 100 years with several flooding episodes and included a fall of 167 mm at Silent Valley (County Down) over 48 hours in early November.
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