HAIL, HAILSTORM. The word hail, in English, is unfortunately used to denote two phenomena of apparently different origin. In French, we have the terms grele and gresii—the former of which is hail proper; the latter denotes the fine grains, like small shot, which often fall in winter, much more rarely in summer, and generally precede snow. The cause of the latter seems to be simply the freezing of rain-drops as they pass in their fall through a colder region of air than that where they originated. We know by balloon ascents and rations other methods of observation, that even in calm weather different strata of the atmosphere have extremely different temperatures, a stratum far under the freezing-point being often observed between two others com paratively warm.
But that true hail, though the process of its formation is not yet perfectly under stood, depends mainly upon the meeting of two nearly opposite currents of air—one hot and saturated with vapor, the other very cold—is rendered pretty certain by such facts as the following: A hailstorm is generally a merely local phenomenon. or at most, rav ages a belt of land of no great breadth, though it may be of considerable length. Hail storms occur in the greatest perfection in the warmest season, and at the warmest period of the day, and generally are most severe in the most tropical climates. A fall of hail generally precedes, sometimes accompanies, and rarely, if ever, follows a thunder-shower. A common idea, which has found its way, as many popular prejudices continually do, into scientific treatises, assigns electricity as the origin of hail. But all observation, rightly interpreted, seems to show that electricity and hail are results of the same com bination of causes.
When a mass of air, saturated with vapor, rising to a higher level, meets a cold one, there is, of course, instant condensation of vapor into ice by the cold due to expansion; at the same time, there is generally a rapid production of electricity, the effect of which upon such light masses as small hailstones is to give them in general rapid motion in various direct ions successively. These motions are in addition to the vortex motions, or eddies, caused in the air by the meeting of the rising and descending currents. The small ice-masses then moving in all directions impinge upon each other sometimes with great force, producing that peculiar rattling sound which almost invariably precedes a hail-shower. At the same time, by a well-known property of ice (HEGEL/awl:), the impinging masses are frozen together; and this process continues until the weight of the accumulated mass enables it to overcome the vortices and the electrical attractions.
when it falls as a larger or smaller hailstone. On examining such hailstones, which may have any size from that of a pea to that of a walnut, or even an orange, we at once recognize the composite ocharacter which might be expected from such a mode of aggre gation. Hailstones are reported to have fallen in tropical countries sometimes as large as a sheep, sometimes as large as an ox, or even an elephant! But it is probable that the aggregation in these cases was produced by regelation at the surface of the earth, when a series or large masses had impinged on each other, having fallen successively on the same spot. Whether this be the true explanation or no, it is certain that in British India, at the warmest season, hailstones have remained of considerable size for many days after their fall. 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 Jan., 1860. Here the stones were the size of half-bricks, and beat several of the crew off the rigging, doing serious injury.
We may conclude by a description (taken from Meat. de (' Acad. des Sciences, 1790) of one of the most disastrous hailstorms that has occurred in Europe for many years back. It illustrates very happily the greater part of what we have said about the origin of this meteor. This storm passed over parts of Holland and France in July, 1788. It traveled 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 meteor was from saw. to n.e. The length was at least a hundred leagues; but from other reports, it may be gathered that it really extended to nearly two hundred. It seems to have originated near the Pyrenees, and to have traveled at a mean rate of about sixteen and a half 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. The hailstones were generally of irregular form; the heaviest weighed about eight French ounces. This storm devastated 1039 parishes in France alone, and an official inquiry fixed the damage at about 24,000,000 francs—nearly a million of English money.