A mountain can affect air traveling across it. Wind can push moist air upward. Moist means containing water vapor, water in its gas form. Rising air expands and cools. Vapor may condense, change into liquid drops, and precipitation may fall.

Precipitation is water falling from clouds, such as rain or snow. After losing moisture, air passes over the mountain. On the other side it can sink and warm. That side may receive much less precipitation.

This dry area is called a rain shadow. Our invented place records compare precipitation amounts over many years. They let you test whether heat or sand alone identifies a desert. They do not reproduce air crossing a mountain. Use the diagram to follow that explanation, and the records to separate useful evidence from appearance.

Desert pictures often show sand beneath a hot Sun. But deserts are identified by dryness, not by sand or heat alone. Some have rocky ground. Others are cold for much or all of the year.

Rain shadows are one way places become dry. Large patterns of air movement can also bring descending dry air. That can make it difficult for clouds and rain to form. Different deserts have different combinations of conditions.

One rainy day does not show that a place is wet overall. A dry afternoon does not make a place a desert. You need observations across time. Before naming a place from a photograph, ask what you know about its precipitation. Sand, rocks or snow may catch your eye, but they cannot answer that question by themselves.

together · About 10 minutes; estimate.

You need

• Paper and pencil

Try it

1. Draw three invented places: hot sandy A, cold rocky B, warm green C.
2. Choose which could be a desert before seeing the next clue.
3. Ask an adult for the long-record clue about each place.
4. Match each precipitation clue to its place and revise your choice.
5. Explain which clue changed your idea and what real measurements you would still want.