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.