Clouds

 

A cloud forms when invisible water vapour turns into visible water drops. The warmer the air, the greater its capacity for holding water vapour. When air rises and cools it contracts and its capacity to hold vapour is reduced until it becomes saturated. Any further cooling results in a shedding of moisture as tiny water droplets, which form clouds. Clouds therefore indicate areas of rising, cooling air. Clear skies indicate areas of sinking, warming air.

Air masses sometimes rise quickly and vertically. When this happens they form one of two basic types of cloud – cumulus or stratus.

Cumulus clouds are brilliant white in those upper parts lit by the sun, dark grey in shaded areas. A cumulus cloud is constantly changing. From parts of it fresh towers of cloud rise, while other parts are caught in downdraughts of air and disappear by evaporation of their droplets.

Sometimes the top of the cloud rises high enough for its droplets to freeze into tiny ice crystals. These show in the sky as a frothy, dazzling white mass, falling diagonally in the direction the wind to form what is known as an anvil because of its shape. This type of cloud, known as a cumulonimbus or thunder cloud, can contain up to 50,000 tons of water and usually produces heavy rain or hail.

At other times air masses rise slowly and on a low gradient – often as low as one in 150. This is the case with a warm front, which is warm air gliding above a shallow wedge of cold air from more northern latitudes. As this warm air slowly rises and cools it forms the second basic type of cloud – stratus or sheet cloud. Light rain often falls from the thickest parts of this bank of cloud and in winter it tends to reach ground level as fog.

A cloud composed of small droplets reflects sunlight more than one consisting of large droplets. A silver lining occurs when sunlight behind a cloud composed of large droplets filters through its edges. A layer of strato-cumulus reflects 55-80 per cent of the sun’s energy. The amount reflected by other clouds depends on their density.

The highest clouds, known as noctilucent clouds, shine after dark on clear nights. They are found at heights above 50 miles above Earth and consist of ice-coated dust particles from outer space. But nearly all water vapour is found in the 6-9 miles nearest Earth. A slight amount of vapour is found at about 16 miles from Earth where ‘mother-of-pearl’ clouds can be seen on rare occasions.

© John Welford