This page will make more sense to you if you have come from the page on heat transfer.

To review:

Warm air rises.

Cold air sinks.

The smoke that comes from a cigarette rises because it is being carried by hot air.

Now let's add to this:

 

As air grows warmer, it picks up water. Warm air can hold water.

                Air that is growing cooler releases water.

Here warm air in a room has come in contact with a glass of ice water.

The moisture in the chilled air has condensed out, leaving drops of water on the outside of the glass.

As the sun rises, the land and water on a planet begins to warm up. The land warms more quickly than the water, and the air above the land warms up too.

Warm air rises.

As the warm air rises, it pulls moist air landward from above the sea. This warm, moist air also begins to rise.

Water falls from cooling air.

As the moist air rises, it begins to get cooler. The water begins to fall out as rain. This provides favorable conditions for forest growth.

The air continues to rise higher and higher up the mountains. Rainfall lessens as much of the moisture is left behind.

The highest parts of the mountains probably do not get much water, but there may be some showers or snow.

Warm air continues to rise from lower elevations, pushing the high cool air upward. Eventually this air rises above the mountain tops and begins to flow downward.

Cold air sinks.

As this air flows downward, it warms up somewhat.

Warm air picks up moisture.

Well, there's not a lot of moisture on this side of the mountains, but the air will pick up some of it. There is not enough to make a good, wet rain.

This side of the mountain is dry. There may be grass, semi-arid shrubs, and desert on this side of the mountains. Dry areas like this are called the rain shadows.

If you look at a map of the earth, you will see rain shadows behind the Andes in South America and behind the Sierra Mountains in California. Can you find other deserts that have formed this way?