Precipitation (including rain, snow, sleet, hail and frost) can vary annually from:
- more than 2250 mm on the Cairngorm summits, to
- less than 900 mm in the straths of the Dee and Spey.
These larger valleys appear to show a greater ‘rainshadow’ effect than the narrower mountain glens. Locally, the higher the altitude, the greater the rainfall.
During the summer, higher temperatures over the summits can cause convective thunder and lightning storms, with associated heavy, localised, rainfall and flash floods – more so than in the west Highlands.
The Cairngorms National Park is traditionally the snowiest part of Britain, with snow falling in any month, although not generally in August. However, recent research suggests a relative decrease in snowfall in the Park and north-east Scotland compared with the western Highlands of Scotland.
Snowfalls occur when cold moist air is brought either by cold Arctic air approaching Scotland from the north or by polar continental air from the east or north-east. This air is often associated with an anticyclone extending over Scandinavia, and is generally very cold, bringing heavy falls of dry snow to north-east Scotland and the Cairngorms National Park.
Snowfalls may also occur when Arctic or polar maritime air approaches from between the north and west. This is often associated with a low-pressure area over Iceland and an anticyclone west of Britain. In this case, the north and west of Scotland receives heavy snowfall and the east less. The snow is wetter, heavier, and melts faster.
During recent winters (late 1980s through the 1990s) the westerly-type weather has prevailed and suggests an explanation for the relative lack of snowfalls in the Park and eastern Grampians.
Strong winds on the plateau cause severe scouring and drifting of snow. The depositing of snow in sheltered hollows and corries lead to cornice formation and semi-permanent snow patches, lasting through the summer, e.g. Garbh Choire Mor (Braeriach) and Ciste Mhearaid (Cairn Gorm). The overloading of slopes during snowstorms also leads to avalanches.
Sublimation, where snow evaporates directly to vapour without first melting to water, occurs on the Cairngorms plateau on warm, winter days.
The result of reverse sublimation is hoarfrost. It commonly occurs when temperatures fall below freezing. A mist of super-cooled water droplets forms across the hills, and where it touches colder surfaces (rocks, trig points, fence wires) forms frost feathers (hoarfrost) on the windward side.