Look at the following. It shows a cross-section (横截面) of a mountain and the land on each side of it. The sea is on the west side of the mountain.
In January, February and March, warm wet winds blow strongly across the sea towards (朝) the mountain. The winds pick up moisture (湿气) as they blow across the sea. When clouds reach the mountain, they rise and become cooler. The higher they have to go, the cooler they become. The moisture in the clouds turns to rain. As a result, the clouds drop rain on the west side of the mountain. Some of the clouds pass over the mountain, but there is little rain in them.
In July and August, strong winds blow from G towards the mountain. They pass over a wide desert, where it is always very hot. The winds become hot and dry. They rise when they meet the mountain. They do not form clouds because there is no moisture in the hot air.
The diagram shows why there are wonderful farms on one side of the mountain, but there is a desert on the other side.
