Tracie Huch, age 11, of Tucson, Ariz., for her question:

HOW DOES A CLOUD FORM?

Clouds in the sky form from water that has either evaporated from lakes, oceans or rivers, or from moist soil and plants. The evaporated water Is called water vapor, and it expands and cools as it raises into the air.

Air can only hold a certain amount of water vapor at any given temperature and when the temperature drops, some of the water vapor begins to condense, or change into a liquid.

Water vapor can rise to form clouds in several ways. When the sun warms the ground, the air next to the ground is heated. Because warm air is lighter than the same volume of cold air, the warm air rises. This rise of warm air is called a convection current and this method of cloud formation is called convection. Clouds can also be formed when warm, moist air moves up the side of a mountain. As it is lifted and cools by expansion, clouds are formed.