Mary Lou Troman, age 11, of Lethbridge, Alberta, Canada, for her question:

HOW ARE CLOUDS FORMED?

Wherever you are, you can watch the clouds in .at least a small section of the sky. And cloud watching is a fascinating hobby. For example, no two clouds are exactly alike  what's more, they change from moment to moment. They seem to do as they choose. But when you learn a little about them, you know that they form and reform themselves because they must obey certain rules and regulations.

All the clouds on high are made from a foggy mixture of air and misty moisture. The basic ingredient in nature's cloud¬ making recipe is water vapor, which the sun evaporates from the wet and moist surfaces of the earth. This invisible vapor mingles with the other invisible gases of the air.

A certain amount of water vapor is present, even in the driest air above the arid deserts. So cloud making is possible everywhere above the earth.

However, certain weather conditions are necessary to complete the recipe. These conditions depend almost entirely on temperature and on the amount of vapor that happens to be present in an air mass.

This is so because of a rule that warm air is allowed to contain more water vapor than cool air. The rule is called relative humidity. For example, saturated air has a relative humidity of 100 percent. It contains its limit of water vapor. With a slight change in temperature, some of its gaseous water is converted into liquid droplets  and a cloud begins to form.

Most cloud formations occur when the sun warms patches of the land and sea. The air gets its warmth from the surface. Warm air expands and rises aloft, where things are cooler. Now suppose this rising air is saturated with vapor. As it rises, it chills  then the air has more vapor than permitted at this cooler temperature.

This is when the surplus vapor changes to misty moisture and clouds form high above the ground. Other clouds form when cool moist air is warmed as it blows over a mountain and when mild, damp air blows from the sea onto the cooler continents.    

For a time, a fluffy new cloud can float aloft because its separate droplets are very small and light. But its total water weighs many tons and gradually it sinks. Meantime winds and temperatures change its condition. Dust and cool winds may change its droplets into heavy raindrops. Sometimes a warm air mass evaporates the liquid droplets and its clouds change back into invisible water vapor.