CYCLONE. The word was first coined by H. Piddington in the Sailors' Horn-book (1855) for tropical revolving storms, to indicate the combined circular and centripetal movement, which was once thought to be characteristic of all central systems of low pressure. (Gr. KUKAWP, whirling, from K(.KXos, the coil of a snake.) It is now used not only for the tropical revolving storms of the Indian ocean but also for any atmospheric system where the pressure is lowest at the centre. The winds in consequence tend to blow towards the centre, but are diverted by the earth's rotation, and circle round the centre in a counter-clockwise direc tion in the Northern Hemisphere, and the reverse in the Southern Hemisphere. The whole system has a motion of translation, being usually associated with the great wind-drifts. Thus the direc tion of movement of cyclones over the British Isles is usually from south-west to north-east, though they may remain stationary for a time or move in other directions. Well-formed cyclones are usually accompanied by gales and bad weather. They are fre quently referred to as "lows" or "depressions" on weather charts. The convention that all cyclones are regions of upward convec tion whereas anticyclones (q.v.) are regions of downward con vection can no longer be accepted.
See Shaw, Manual of Meteorology (part 4, 1919) and vol. i., ch. xiv. (1926).