The appearance of hail clouds seems to be distinguished from other stormy clouds by a very remarkable shadowing. Their edges present a multitude of indentations, and their surfaces disclose here and there immense irregular projections. Arago has seen hail-clouds cover with a thick veil the whole extent of a valley, at a time when the neighbouring hills enjoyed a fine sky and an agreeable temperature.
Hailstones of similar forms are produced at similar levels. They are smaller on the tops of mountains than in the neighbouring plains. If the temperature or the wind alter, the figures of the hailstones become immediately changed. Hailstones
of the form of a six-sided pyramid have been known to change, on the wind changing to the north-east, to convex lenses, so transparent and nicely formed, that they magnified objects without distorting them. Some hailstones are globular, others elongated, and others armed with different points.
In the centres of hailstones small flakes of spungy snow are frequently found, and this usually is the only opaque point in them. Sometimes the surface is covered with dust, like fine flour, and is something between hail and snow. This never falls during summer in southerly countries. In the Andes hailstones from five to seven lines in diameter are sometimes formed of layers of different degrees of transparency, so as to permit rings of ice to be separated from them with a very slight blow. In Orkney, hailstones have fallen as finely polished as marbles, of a greyish white colour, not unlike fragments of light-coloured marble. Hailstones are often so hard and elastic, that those which fall on the stones rebound without breaking to the height of several yards; and they have been known to be projected from a cloud almost horizontally, and with such velocity
as to pierce glass windows with a clear round hole.
On the 7th May, 1822, some remarkable hailstones fell at Bonn, on the Rhine. Their general size was about an inch and a half in diameter, and their weight 300 grains. When picked up whole, which was not always the case, their general outline was elliptical, with a white, or nearly opaque spot in the centre, about which were arranged concentric layers, increasing in transparency to the outside. Some of them exhibited a beautiful star-like and fibrous arrangement, the result of rows of air bubbles dispersed in different radii. The figures at the head of this chapter show the external and internal appearances of these hailstones.
The smaller figures represent pyramidal hail, common in France, and occasionally in Great Britain.
Brown hailstones have been noticed. Humboldt saw hail fall of the colour of blood.
On the 15th July, 1808, Howard noticed, in Gloucestershire, hailstones from three to nine inches in circumference; appearing like fragments of a vast plate of ice which had been broken in its descent to the earth.
On the 4th June, 1814, Dr. Crookshank noticed, in North America, hailstones of from thirteen to fifteen inches in circumference. They seemed to consist of numerous smaller stones fused together.