How are clouds formed? Classify them.
Clouds are formed by the condensation of water vapour in free air at considerable elevations. They consist of a mass of minute water droplets or tiny crystals of ice.
Clouds are classified into four types based on their height, expanse, density and transparency: (i) Cirrus – thin, feathery, white clouds formed at high altitudes (8,000-12,000m); (ii) Cumulus – cotton wool-like clouds formed at 4,000-7,000m with a flat base; (iii) Stratus – layered clouds covering large portions of the sky; (iv) Nimbus – black or dark gray, dense and opaque clouds formed at middle levels or near the surface.
Explanation
The textbook defines clouds as masses of minute water droplets or tiny ice crystals formed by condensation of water vapour at considerable elevations. The classification into four types (Cirrus, Cumulus, Stratus, Nimbus) is based on height, expanse, density and transparency. Each type has distinct characteristics mentioned in the text - Cirrus at high altitudes with feathery appearance, Cumulus looking like cotton wool, Stratus as layered clouds, and Nimbus as dark, dense clouds.