The American Cyclopædia (1879)/Hail
HAIL, the aqueous vapor of the atmosphere congealed in icy masses, called hailstones, and precipitated upon the earth. Hailstones vary in size and internal structure, from the homogeneous masses one eighth of an inch in diameter forming sleet, to the larger masses 3 in. in diameter, of beautiful crystalline structure, and to the still larger accretions of these masses sometimes a foot or more in diameter. The crystalline structure of most hailstones is remarkably distinct. The centre of the hail is a collection of semi-translucent granules or a spongy mass of snow and opaque ice; surrounding this nucleus is a more or less well defined radiated structure of crystals of ice; a large quantity of air is always enclosed within the interstices of the hailstone. Occasionally the stones are composed of concentric rings of ice and snow; when they consist of clear ice without the snowy nucleus, there is almost invariably found in the centre, in place of the nucleus, a cavity filled with condensed air; from experiments made in 1871 it has been shown that this bubble of enclosed air is subjected to a pressure of many atmospheres. Small bits of dust, leaves, and other foreign particles are occasionally found in the interior of the mass of a hailstone, and crystals of sulphur as well as ashes are particularly observed when the hail storm occurs in a volcanic region.
Different Forms of Hail.
Section of Hailstone, magnified.
The small hailstones that fall in storms of sleet are generally regarded as drops of water that have been frozen in their downward passage through layers of cold air; and their formation is therefore believed to be a different process from that attending the formation of larger hailstones. These latter occur in connection with a class of storms that are distinctively known as hail storms. — The velocity with which large hailstones fall to the ground is often so great that, taken in connection with their mass, they cause very serious devastation; instances are recorded of animals being destroyed in large numbers, and damage is frequently done to houses, forests, and crops. It is believed that the velocity is indeed usually much less than is due to bodies of their size and density, and several theories have been devised to account for this. Prof. Olmsted supposed that the true reason is found in the retardation occasioned by the nucleus continually taking up in its descent accessions of vapor, which immediately before was in a state of rest; it has however not yet been shown that there is any necessity for such an explanation, since we know too little concerning the altitude above the earth's surface, at which hail is formed. Hail storms occur most frequently in the spring and summer months, and in the warmest part of the day. Kaemtz has shown that in Germany and Switzerland 50 per cent. of these storms occur in the springtime. Wesselowski shows that in Russia 40 per cent. occur in summer, and 30 per cent. in spring. In the Netherlands and France 40 per cent. occur in spring. It is comparatively rare that hail storms take place between 9 P. M. and 7 A. M.; 60 per cent. of the storms in Germany occur between noon and 6 P. M. As regards the frequency of hail storms in various portions of the globe, it is generally believed that they rarely or never occur in the polar regions, and but little more frequently in those portions of the tropical regions whose climate is controlled by the neighboring ocean; thus they are recorded as of extreme rarity in the islands of Martinique, Mauritius, and in the lowlands of Java and Borneo. On the other hand, in the higher portions of Java, Borneo, and Santo Domingo, hail is by no means infrequent; in the island of Cuba, according to Poey, over 40 hail storms have been recorded since 1784. In the temperate zone, both in rolling and mountainous countries, hail storms are far more frequent. Of these countries, some, such as France, are peculiarly subject to severe hail storms, while again in every such land many localities are pointed out where it never hails. In general these latter localities are found to be decidedly higher than the average elevation of the surrounding country, or else decidedly lower; thus, according to Savigne, a mountain in the neighborhood of Clermont was during 23 years only once visited by hail, while the country about its neighborhood was frequently devastated. In Lithuania hail occurs on the hills more frequently than on the plains, while in Poland it occurs at the foot of the Carpathian mountains more frequently than in the lowlands. Leopold von Buch states that it never hails in regions where cretins are found; a generalization, however, that does not seem to be accepted by many, and the cause of which must, if the fact be granted, be looked for in some peculiarity common to the regions in question. — Among the special phenomena of hail storms may be mentioned the pauses that occur between successive falls of hail, which are well described by Kaemtz as observed by him. He says that at the beginning of the severest storm that he ever witnessed, there fell some large drops of rain; these soon ceased, and after a short interval there fell hailstones, shaped like beans, of one or two tenths of an inch in diameter; this ceased, and there followed rain, and after another pause fresh hail of two or three tenths of an inch diameter; again another pause, and a new fall of hail. Of these successive falls of hailstones, the first possessed only a slight coating of ice over the snowy nucleus; the second class were partially surrounded with a thicker layer of ice; and the last hailstones were generally rounded masses one third of an inch in diameter. In all cases he found the kernel not transparent, while the surrounding ice was so in a high degree. Another very general peculiarity of hail storms consists in the fact that the central portion of the region passed over by the storm is almost entirely free from the fall of hailstones, which on the other hand are almost invariably found in two or more belts parallel to the track of the storm centre, and some distance therefrom. Thus in the storm of July 3, 1788, which passed from France in a northeasterly direction into Holland, the storm central track was about 500 m. in length, and was traversed in less than nine hours; over the central track, to a breadth of 6 m. on either side, no hail fell, but heavy rain; on either side of this region, to a distance of 5 or 10 m., the country was visited with hail of the most destructive kind, by which property valued at more than $5,000,000 was destroyed; rain also fell over a district stretching far beyond the belts of hail. — Perhaps the most frequent accompaniment of hail, and the most prominent peculiarity of the hail storm, is found in the discharges of electricity, which are usually but not always remarkably severe. While numerous thunder storms occur without attending hail, it is on the other hand generally the case that hail storms are also thunder storms. Ordinary thunder storms of a moderate degree of severity, as well as tornadoes, waterspouts, sand or dust storms, whirlwinds, and hail storms, have many points of similarity, and may be said to pass by insensible shadings from one to the other. Peltier enumerates 116 tornadoes or trombes, of which 14 were accompanied by hail. Reye, in his work on Wirbelstürme, enumerates 33 tornadoes that occurred in America, of which only three are noted as having been accompanied by hail. Hail storms, and indeed all that class of disturbances just enumerated, have a local character, and it is believed that in general their paths are related to the larger areas of low barometer that move over the surface of the earth; they are more numerous and more intense at those times when the barometric pressure is diminishing in advance of some extensive region of low pressure; they may in fact be said to be the precursors of, or to intiate, some more general atmospheric disturbance. — Our knowledge of the operations going on in the interior of a hail storm has been materially increased of late years, though still far from being complete; and the theories of Volta, Olmsted, &c., may be said to possess now only a historical interest. According to Volta, atmospheric electricity plays a very important part in the formation of hail, the snowy nucleus being alternately attracted and repelled by two layers of clouds charged by opposite electricities, and in the mean time continually adding to its size, until its weight brings it down to the earth. This theory may be regarded as distinct from the earlier electrical theories of Musschenbroek, Monge, &c.; and notwithstanding its many defects, it seems to have been very widely accepted, especially in France and Germany, during the latter part of the 18th century. Montbeillard was led in 1776 by its consideration to propose the use of Franklin's lightning rod as a protection against hail storms; a proposition that has been very widely adopted in France, but it is believed without producing the desired effect, although a popular and almost superstitious belief prevails in that country in regard to its efficiency. Leopold von Buch maintained that the water was frozen by very rapid evaporation from the surface of each drop; a hypothesis concerning which Kaemtz remarks that even if it were possible thus to convert rain drops into hailstones, this method of formation would not accord with the ordinary saturated condition of the atmosphere in the cloud region. The hypothesis that uprising currents of moist warm air, by their mixture with higher currents of very cold dry air, thereby give rise to the formation of hailstones, seems to have been first propounded by Muncke, and has, in a more or less modified form, been favored and even adopted by prominent meteorologists in Europe, and has been developed independently by Olmsted in America. More recent writers, as Peslin (1866) and Reye (1864 and 1872), have developed the consequences of the principle first announced in meteorology by Espy, that storms (including in that term every phase of atmospheric disturbance) owe their energy to the condensation of aqueous vapor caused by the cooling consequent on the internal work performed in the ascension of moist air to elevated regions of the atmosphere. It is demonstrated by these writers that the rapid ascent of the moist air found near the surface of the ground on a warm summer's day is attended with such a rapid cooling that a portion of the vapor must necessarily be condensed, either as drops of water or flakes of snow and crystals of ice. According to Reye, who in this respect is but a disciple of Espy, the phenomena of cyclones and hurricanes, of waterspouts and whirlwinds, of thunder storms and hail storms, can all be developed as the consequences of a single simple law of the mechanical theory of heat, namely, the condensation of atmospheric aqueous vapor contained in uprising currents of air.