A weather system with higher pressure in the centre than in its surroundings. An anticyclone is characterised on a weather chart by completely closed isobars, roughly circular in form, spreading out from the central high pressure area. The presence of an anticyclone system usually results in fine, cloudless weather.

Altostratus (As)

One of the 10 characteristic cloud types (or cloud genera) recognised by the International Cloud Atlas. Its name comes from altum (height) and stratus (spread out). It appears as a greyish or bluish sheet of cloud, partly or totally covering the sky, but with parts sufficiently thin to at least vaguely reveal the sun.