TABLE OF CONTENTS Earle Williams
Clouds in the earth s atmosphere are composed of water droplets and ice crystals. Clouds are commonly white in appearance because these liquid and solid particles are large relative to the wavelengths of visible light, and so no selective scattering occurs to colour the cloud. Owing to the abundance of cloud condensation nuclei, clouds appear whenever the air becomes locally supersaturated in water vapour. This supersaturation condition is most often achieved by a lifting process in which air parcels subsaturated with respect to water vapour cool by adiabatic expansion. The lifting process is usually caused by the heating of air near the earth s surface, which is itself warmed by sunlight. The warmed air parcels become buoyant relative to their surroundings and rise. A second mechanism for lifting depends on the forced ascent of air by horizontal pressure gradient forces. Regardless of the lifting mechanism, the altitude at which the supersaturation condition is achieved in the rising air parcel and cloud begins to form is the lifted condensation level (LCL).
In many circumstances where clouds are frequent occurrences, the LCL is within 1000 metres of the earth s surface, and substantially lower than the altitude of the 0 C isotherm, typically 4000 5000 metres above ground. As a consequence, the great majority of clouds form at temperatures above freezing and consist entirely of liquid droplets. The weight of the evidence shows that such clouds are not strongly electrified and rarely, if ever, produce lightning.
