cloud

cloud Masses of water particles or ice crystals suspended in the lower atmosphere. Clouds are formed when water from the Earth's surface becomes vapour through evaporation. As the water vapour rises, it cools and condenses around microscopic salt and dust particles, forming droplets. There are ten different classifications of clouds: cirrus are high (above 6000m/20,000ft), white and thread-like. Cirrocumulus are also high clouds, but are often thin sheets. Cirrostratus are white, almost transparent, sheets. Altocumulus are greyish-white globular clouds found between 2400m (8000ft) and 6000m (20,000ft). Altostratus are grey/blue and streaky, and often cover the whole sky. Nimbostratus are low, thick and dark, and shed rain or snow. Stratocumulus are masses of white, grey or dark cloud. Stratus are low-lying and grey. Cumulus are white and fluffy-looking. Cumulonimbus are towering, dark clouds that often produce thunderstorms. Their bases almost touch the ground and extend upward to 23,000m (75,000ft). By day, clouds reflect the rays of the Sun back into the atmosphere, keeping the ground cool. At night, clouds trap and re-radiate heat rising from the Earth, keeping surface temperatures warm. See also fog; hydrological cycle