Hail, Hailstorm. The word hail in English is used to denote two phenomena of quite different origin and formation. These have in recent years been distinguished as hard hail, or true hail; and soft hail, which denotes the fine, light grains, like small shot, that frequently fall in winter, much more rarely in summer, and are generally a precursor of snow. Soft hail is the grésil of the French, and the graupel of the Germans. The theory of the formation of soft hail has yet to be formulated. True hail is round, hard, compact, and formed of either clear or granular ice, the hailstones being often found when broken across to be composed of alternate layers of these two states of ice. It has a well-marked diurnal period, 80 per cent. of the whole number of hailstorms occurring in the six hours from 10 A.M. to 4 P.M., and only 8 per cent. in the fourteen hours from 6 P.M. to 8 A.M. The essential point to be noted in the diurnal period of hail is that the maximum period of hailstorms is two hours earlier than the maximum period of Thunderstorms (q.v.). The maximum period for thunderstorms is when the ascending current from the heated surface of the earth is at its greatest strength for the day; but the maximum period for hail occurs two hours before the ascending current has fully established itself, or it occurs at the time when atmospheric temperature and vapour diminish with the height at a much greater rate than the normal. In the higher latitudes the fall of hail may be regarded as restricted to the warmer months of the year; in countries where the summer is practically rainless no hail falls; and where the rainfall is small and at rare intervals very few cases of hail occur.
Hail is connected with whirlwinds, more or less developed; and it is when the hailstorm is an attendant on a tornado or on a great thunderstorm that it assumes its most destructive form, carrying devastation through a narrow belt of land usually of considerable length.
The theory of the formation of hail has been given by Ferrel in his Meteorological Researches for the Use of the Coast Pilot, part ii. p. 85. The vapour carried upwards by the vortical gyrations of the tornado is, below a certain height, condensed into cloud and rain; but above that height into snow. Now when the raindrops formed below are carried higher up into the cold snow regions by the powerful ascending currents of the tornado, and are kept suspended there a little while, they become frozen into clear hard hail. If these hailstones be now thrown quite outside the gyrations of the tornado, they fall to the earth as a shower of compact homogeneous hailstones of clear ice of ordinary size. But should they be caught in the descent and carried in towards the vortex by the inflowing aerial current on all sides, they are again rapidly carried aloft into the freezing region. A number of such revolutions of ascent and descent may be made before they ultimately fall to the earth. While high up in the snow region the hailstones receive a coating of snow; but while in the region lower down, where rain, yet unfrozen, is carried up, they receive a coating of solid ice. In this way alternate coatings of ice and snow are received, and the number of each sort indicates the number of ascents and descents performed before the hailstone falls to the ground. When the nucleus is compact snow, as it usually is, the hailstone has its origin high up in the snow region as a small ball of snow or soft hail.
From a well-known property of ice (regelation), the impinging hailstones are frequently frozen together not only in their course through the air, but also at the surface of the earth, giving rise occasionally to hailstones of larger dimensions. A curious instance of the fall of large hail, or rather ice-masses, occurred on one of Her Majesty's ships off the Cape in January 1860, when the stones were the size of half-bricks, and beat several of the crew off the rigging, doing serious injury. More than once in the summer of 1889 hailstones proved unusually destructive on the continent of Europe; in Moravia, for instance, where many stones fell as big as a man's fist, and weighing 3 lb., a number of people were killed in the fields, and many more were injured.
A description (taken from Mém. de l'Acad. des Sciences, 1790) of a most disastrous hailstorm may be here added. This storm passed over parts of Holland and France in July 1788. It travelled simultaneously along two lines nearly parallel—the eastern one had a breadth of from half a league to five leagues, the western of from three to five leagues. The space between was visited only by heavy rain; its breadth varied from three to five and a half leagues. At the outer border of each there was also heavy rain, but we are not told how far it extended. The general direction of the storm was from south-west to north-east. The length was at least a hundred leagues, probably two hundred. It seems to have originated near the Pyrenees, and to have travelled at a mean rate of about 16½ leagues per hour towards the Baltic, where it was lost sight of. The hail only fell for about seven and a half minutes at any one place, and the heaviest hailstones weighed about 9 ounces. This storm devastated 1039 parishes in France alone, doing damage to the extent of nearly a million of English money.