Science Puzzle
Why Is One Side of the Mountain So Dry?
Fort William, on the west coast of Scotland, gets about 2,000 millimetres of rain a year. Edinburgh, on the east coast, gets about 950, less than half as much. Most of Britain's weather blows in from the Atlantic, from the west, and the Highlands lie between the two towns.
Why is the east so much drier?
The Answer
The mountains wring the rain out of the air. Damp Atlantic air is pushed up over the Highlands, cools and drops most of its water on the western slopes. By the time it comes down the other side, there's much less left.
Air pushed up a mountain moves into lower pressure, expands and cools. Cool air can't hold as much water vapour as warm air, so the vapour condenses into cloud, and the cloud rains or snows on the slopes facing the wind. Edinburgh isn't far from the sea at all; it's on the coast too. What it lacks is a direct supply of wet air.
On the far side, the air sinks again. Sinking air is squeezed by the rising pressure, warms up and can hold its remaining water as invisible vapour, so clouds tend to break up. The dry area downwind of a mountain range is called a rain shadow.
The effect is much stronger elsewhere. Death Valley in California, the hottest and driest place in North America, lies in the rain shadow of the Sierra Nevada mountains. In Britain the whole east is drier than the west for the same reason.
The principle: Rain shadow. Air forced up a mountain cools and drops its moisture as rain on the side facing the wind, then sinks and warms on the far side, leaving that side dry.