Carol Stewart, age 12, of Sioux City, Iowa, for her question:

What is fog?

Fogs and clouds are made from the same misty material and caused by the same weather conditions. In fact, a fog is just a cloud on the ground. It forms when warm, vapor filled air is chilled    perhaps by a cool night or an icy wind. All air contains a percentage of gaseous water molecules. But every air sample can hold so much of this vapor and no more, depending upon its temperature. When warm air that is saturated with vapor is chilled, it has more vapor than it can hold. The surplus vapor is changed into liquid moisture    perhaps into a misty fog.

The misty material in a fog is made of miniature droplets of liquid water, each one small enough to float suspended in the air. The droplets form when vapor filled air is chilled to a point where it has more gaseous water molecules than it can hold. Favorite fog forming regions are moist valleys and mountain slopes. Along the coasts, warm air soaks up lots of vapor and the nights often turn very cool. These conditions cause foggy patches along the shores.