Forecasts, mountains and rain

Imagine moist air from the sea blowing toward a mountain range. The mountain forces the air to rise. As it climbs, the pressure around it falls, so it spreads out and cools. Higher up the air is also thinner. Cooler air cannot hold as much water vapour, so the vapour condenses into cloud droplets and rain or snow falls on the windward side of the mountain. By the time the air goes over the top it has lost much of its water. It sinks on the other side, warms up and is dry. So the far side gets little rain, and we call this area the rain shadow. Many dry lands lie in a rain shadow. A desert usually gets about 250 mm of rain a year or less.