νεφέλη

Clouds in the Earth atmosphere are made of water. Almost everywhere (except deserts) there is some water in the air. When warm air rises to higher layers of the atmosphere, its temperature drops causing the gas molecules to get closer together and the water drops in between them to condense. The agglomerations of water drops are less transparent than the mere gas of the air. As they block sunshine they appear white (and not transparent) in the sky. The more material they agglomerate, the more does the white fade to brighter or darker shades of gray.

Caused by different circulations of air around them, clouds will have different shapes: some types of wind at high altitude shape lens-shaped clouds (foehn clouds) and others cause the clouds to grow to tremendously high altitudes (thunderstorm clouds).