The air arrives empty
Why Is One Side of a Mountain Dry?
Air loses its water climbing one side, and there is nothing left for the other.
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Step 1 of 5
A mountain forces air to climb.
Wind cannot go through a range, so it goes over. That climb is not optional, and it happens to every parcel of air that arrives.
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Step 2 of 5
Rising air always cools.
Higher up the pressure is lower, so the air expands, and expanding air cools. About six degrees are lost for every kilometre of climb.
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Step 3 of 5
Cool air cannot hold its water.
Past the dew point the vapour condenses into cloud and falls as rain, and it all falls on the side facing the wind.
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Step 4 of 5
The descending air is dry and warms up.
Coming down the far side the air is compressed and warms again — but with nothing left to condense, so it arrives hot, dry and thirsty.
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Step 5 of 5
Which is why deserts sit behind mountains.
Many of the world's dry regions are simply downwind of a range that took the water first. The desert is a shadow cast by rain.
The dry descending wind has local names everywhere it happens — föhn in the Alps, chinook in the Rockies.
The short version
Air forced over a mountain cools and rains on the windward side, so it arrives dry and warm on the far side and leaves a desert.
Try it yourself
Look at a rainfall map of any mountainous country. The wet and dry bands run parallel to the ranges, not to the coast.
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