Hail (O. Eng. hægl and hagol,1 cf. the cognate Teutonic hagel, as in German, Dutch, Swedish, &c.; the Gr. κάχληξ, pebble, is probably allied), the name for rounded masses or single pellets of ice falling from the clouds in a shower. True hail has a concentric structure caused by the frozen particles of moisture first descending into a warm cloud, whence they are carried upwards on an ascending current of heated air into a cold stratum where the fresh coating of water vapour deposited in the cloud is frozen. The hailstone descends again, receives a fresh coating, is carried up once more, refrozen, and again descends. Thus the hailstone grows until the current is no longer strong enough to support it when it falls to the ground. At times masses of hail are frozen together, and a very sudden cooling will sometimes result in the formation of ragged masses of ice that fall with disastrous results. Hail must be distinguished from the frozen snow, “soft-hail” or “graupel,” that often falls at the rear of a spring cyclone, since true hail is almost entirely a summer phenomenon, and falls most frequently in thunderstorms which are produced under the conditions that are favourable to the formation of hail, i.e. great heat, a still atmosphere, the production of strong local convection currents in consequence, and the passage of a cold upper drift.
1 “Hail,” a call of greeting or salutation, a shout to attract attention, must, of course, be distinguished. This word represents the Old Norwegian heill, prosperity, cognate with O. Eng. hāl, whence “hale,” “whole,” and hæl, whence “health,” “heal.”
