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 depressions are more vigorous in autumn and winter and most of the rain which falls in those seasons is from this source. However, in autumn and winter the islands and western coastal areas are also prone to convective showers caused by the relatively warm sea. In spring and summer, convection caused by heating of the land can form showers inland in the region.
A further factor which greatly affects the rainfall distribution is altitude. Moist air which 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 very similar to a topographic map.
Average annual rainfall totals range from less than 1000 mm in the upper Clyde valley and along the coasts of Ayrshire and Dumfries and Galloway to over 3500 mm over the higher parts of the west Highlands, approaching the maximum values found in the UK (over 4000 mm further north). These averages can be compared to annual totals around 500 mm typical of the driest parts of Eastern England.
Rainfall is generally well-distributed throughout the year but there is a marked seasonal variation. The frequency of Atlantic depressions is normally greatest during the winter but, unlike other areas of the UK, Scotland tends to remain under their influence for much of the summer too. Autumn and early winter are the wettest seasons, especially from October to January, and spring and early summer is normally the driest part of the year, especially from April to June. The average monthly rainfall for 4 sites is shown below.
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The numbers of days with rainfall totals of 1 mm or more ('wet days') tend to follow a similar pattern to the monthly rainfall totals. In coastal areas from Ayrshire southwards about 45 wet days is the norm in each of the autumn (September to November) and winter (December to February) seasons, rising to over 55 days over the higher ground. In each of spring and summer, there are about 35 wet days on the coasts and over 40 over high ground.
Periods of prolonged rainfall can lead to widespread flooding, especially in winter and early spring when soils are usually near saturation and snowmelt can be a contributing factor. An example was 10-11 December 1994 when a slow-moving frontal zone resulted in unusually high rainfall totals in the Glasgow-Irvine areas. For example, at Paisley, 88 mm was recorded on 10th with about 133 mm over the 2 days. Flooding occurred in the River Irvine catchment and there was disruption to rail travel in Glasgow. The highest Scottish daily rainfall total (238 mm) was recorded in the region at Sloy, Loch Lomond on 17 January 1974. This may be compared with the highest daily UK total of 279 mm recorded at Martinstown in Dorset on 18 July 1955.
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