Eva Laszlo, age 12, of Winnipeg, Manitoba, Canada, for her question:

How are different cloud types formed?

A cloud lives only so long and usually spends all of its life traveling. It changes its form as it goes and no other cloud was ever exactly like it. Below it, the solid earth gradually changes its face. The watery ocean is heaved by tides and tossed by stormy waves. But compared with the turbulent atmosphere, the earth and the ocean are very calm and quiet. This restless atmosphere forms the multitude of different clouds and keeps re forming them as long as they live.

Since no two clouds are exactly alike, it seems impossible to arrange them in different types. However, a workable system was devised to classify three major categories, plus a number of in between types. Cirrus clouds are those skimpy little curly white feathers, high in the sky. They are made from tiny crystals of frozen moisture. Most of them form when warm moist air rises aloft and becomes chilled. There the moisture freezes and strews its fine fragments of ice high in the sky to form a cirrus cloud.

Cumulus clouds look like piles of fluffy round white pillows. They are made from a misty mixture of air and minuscule droplets of water. Their story begins when the sun evaporates water from the land and sea, separating its molecules into gaseous molecules of vapor. If this moist air gets warm enough, it expands and rises and carries its vapor a mile or more above the ground. Up there it cools, but not down to freezing. Some of the vapor in this cooler air is changed into misty water droplets  ¬and a cumulus cloud is born.

Stratus clouds are flat, greyish layers that hang like veils or heavy curtains over the sky. They ton are mixtures of air and water droplets. But as a rule, they contain more moisture than a cumulus cloud. Their droplets are bigger and closer together. A stratus cloud may form from vapor when it rises and chills, as a cumulus cloud forms. But more often it forms from fragments of cirrus or cumulus clouds. This happens because the restless atmosphere changes the temperature and the winds and maybe mixes a helping of solid specks in the clouds.

Cirrus clouds form six or more miles above the ground because there the air is cold enough to freeze moisture into icy crystals. Cumulus and stratus clouds form lower down, where the air is not cold enough to freeze. But as the winds whisk a cloud along, it gradually sinks. When cirrus clouds sink down to the non freezing zone, their icy crystals melt into droplets of crater. If the droplets are fine, they may form cumulus clouds. Later, their droplets may group together and form larger drops. If the winds are calm, they may rest in flat stratus layers. When the drops are big enough and the air below is cool enough, these clouds shed rain.

Actually, every cloud has its own life story. It may be made and remade in many different ways. But it always is formed from moisture in the air and changed by changing factors in the atmosphere. The fickle winds blow it around and it changes its form as it meets masses of warmer or cooler air.