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.
-
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.
-
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.
-
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.
-
Step 4 of 5
The descending air is dry and warms up.
It reaches the far side warmer than it started, and completely empty. 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.
-
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.