How can a mountain range create a desert?

A mountain range can create a desert by wringing the rain out of the air before it reaches the other side. Wet wind blowing in from the sea hits the mountains and is forced upward. As it rises, it expands and cools, and its moisture condenses into clouds and rain. If the range is tall and wide enough, most of that water falls on the windward slope. When the air finally tumbles down the far side, it is squeezed and warmed, and it soaks up moisture instead of giving it. The dry land behind the mountains is called a rain shadow.

The contrast can be startling. In New Zealand, the western side of the Southern Alps catches about 6,300 to 8,900 millimetres of precipitation a year, enough to feed large glaciers. Barely 50 kilometres east of the peaks, yearly rainfall drops below 760 millimetres. In Washington State, the town of Aberdeen, on the wet side of the Olympic Mountains, gets nearly 2,200 millimetres of rain a year, while Sequim, in their shadow, gets only 250 to 380.

Rain shadows make some of the driest places on Earth. Death Valley, behind two Californian mountain ranges, is the driest place in North America. Chile's Atacama Desert is squeezed between two ranges that block moisture from both sides, and it averages around 15 millimetres of rain a year. Its soil is so lifeless that NASA uses it to test instruments for future Mars missions.