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 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 below the dew point to produce cloud and rain. A map of average annual rainfall therefore looks similar to a topographic map.
Much of Northern Scotland is exposed to the rain-bearing westerly winds, particularly the Western Isles and the west coast. As a result, most of the western half of the region has an average annual rainfall of at least 1700 mm. The highest average annual rainfalls occur over the higher, west-facing slopes, with the wettest area being to the northwest of Fort William (over 4000 mm per year). Over the lower lying islands the average is less than about 1600 mm whilst near the Moray Firth, to the lee of the mountains, it is only about 700 mm. These values can be compared with annual totals around 500 mm in the driest parts of eastern England.
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. In the western and northern areas there is an autumn/early winter maximum, whereas places close to the Moray Firth tend to have a more even distribution through the year. Late spring and early summer 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 Northern Scotland, 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 winter (December to February), there are fewer than 40 wet days on average close to the Moray Firth, rising to over 60 days in much of the western half of the region and in Shetland. In summer (June to August) the Moray Firth area has about 30 wet days and the western areas over 45 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. An example was 5-6 February 1989, when prolonged rainfall occurred over the western Highlands including the headwaters of rivers such as the Ness and Spey. In a 48 hour period over 200 mm of rain fell, with a new Scottish record for a 2-day total of 306 mm at Kinloch Hourn. This, combined with high tides, led to considerable flooding in the Inverness area and the destruction of a railway bridge over the River Ness that had stood since 1862. The Inverness area was also flooded on 7-8 September 2002 after heavy thunderstorms. These produced 84 mm of rainfall at Allanfearn, east of Inverness, of which 61 mm fell in 5 hours and 31 mm in 1 hour.
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