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 early winter and bring most of the rain that falls in these seasons. In summer, convection caused by solar surface heating sometimes forms shower clouds.
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.
Much of Eastern Scotland is sheltered from the rain-bearing westerly winds. This shelter reaches its greatest potential along the coasts of East Lothian, Fife and the Moray Firth and these areas receive less than 700 mm of rainfall in an average year. In contrast, the wettest area is the southern Grampians where the average annual rainfall is over 1500 mm. These values can be compared with annual totals around 500 mm in the driest parts of eastern England and over 4000 mm in the western Scottish Highlands.
Rainfall is generally well-distributed throughout the year. The frequency of Atlantic depressions is normally greatest during the autumn and winter but, unlike other parts of the UK, Scotland tends to remain under their influence for much of the summer too. The wettest months tend to be in autumn and early winter, whereas late winter and spring is normally the driest part of the year. The course of mean monthly rainfall for 1971-2000 for 4 sites is shown below.
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Over much of Eastern Scotland, 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 winter (December to February), there are about 30 wet days on average along the coasts of East Lothian and Fife, rising to over 55 days in the Grampian mountains. In summer (June to August) the East Lothian and Fife coasts have about 27 wet days and the Grampian Mountains over 40 days.
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. One example was the severe flooding in the Tay valley in mid-January 1993, caused by a combination of thawing snow and heavy rain. At Ardtalnaig, on Loch Tay, snow was 36 cm deep on the 12th and followed by 115 mm of rainfall from the 14th to 16th. By the 17th, the River Tay at Perth had risen to its highest level since 1814. Another example occurred in late October 2002 when a deep depression brought strong winds and 70-80 mm of rain, resulting in widespread flooding with disruption to road and rail transport across the Borders, Lothians and Tayside. Falls of about 75 mm of rain in 48 hours in November 2002 also resulted in flooding across NE Scotland, including Elgin where the River Lossie burst its banks.
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