Stratus Clouds: The word stratus comes from the Latin word that means "to spread out." Stratus clouds are horizontal, layered clouds that stretch out across the sky like a blanket. Sometimes a layer of warm, moist air passes over a layer of cool air. Stratus clouds often form at the boundary where these layers meet. Where two such layers of air meet, the warm air is cooled. If the warm air is cooled below its dew point, the excess water vapor condenses to form a blanket - like layer of stratus clouds. If the layers of air are very large, the stratus clouds may extend for many kilometers across the sky.
Cumulus Clouds: The word cumulus comes from the Latin word for a heap or a pile. Cumulus clouds are puffy in appearance. They look like large cotton balls. Cumulus clouds usually form when warm, moist air is forced upward. As this air rises, it is cooled. If it is cooled below its dew - point temperature, condensation will occur. The size of a cumulus cloud depends on the force of the upward movement of air and the amount of moisture in the air. The largest cumulus clouds are caused by very strong upward movements of warm, moist air. The clouds that produce heavy thunderstorms in summer are a form of cumulus clouds called cumulonimbus. Cumulonimbus clouds may extend upward for hundreds of meters.
Cirrus Clouds: Cirrus clouds are a third general type of cloud. The word cirrus comes from the Latin word for a tuft or curl of hair. Cirrus clouds are very wispy and feathery looking. They form only at high altitudes, about 7 km above the earth's surface. Cirrus clouds are composed of ice crystals and are so thin that sunlight can pass right through them.