Whirlwinds and Waterspouts. Whirlwinds differ essentially in nature from the storms of wind described in the articles STORMS. Individual whirlwinds seldom continue longer than a minute at any place, and sometimes only a few seconds; their breadth varies from a few yards to nearly a quarter of a mile, and during their short continuance, the changes of the wind are sudden and violent. If a fall of the barometer is recorded it is virtually instantaneous, and is immediately followed by a rise equally as sudden as the previous fall. The direction of the eddy of the whirlwinds, especially when the diameter is very small, differs from the rotation of winds in a storm in that it may take place either way—right to left, or left to right—according to the direction of the stronger of the two winds which give rise to the whirlwind. Whirlwinds often originate within the tropics during the hot season, especially in flat sandy deserts; since these becoming unequally heated by the sun give rise to ascending columns of heated air and descending currents of cooler air contiguous to them, which result in eddies that draw up with them large clouds of dust; and the whole is borne forward by the wind that may happen to be blowing at the time. This is the origin of the dust whirlwinds of India, locally known as ‘dust devils,’ which have been admirably described and illustrated by P. F. H. Baddeley. These dust-storms are frequent in dry, hot regions; and in the case of the Simoom (q.v.), which may be regarded as a succession of such whirlwinds, they appear on a scale of the most appalling grandeur.
Extensive fires, such as the burning of the prairie in America and the bush in Australia, and volcanic eruptions also cause whirlwinds by the ascending and descending currents of air they occasion; and these, as well as the whirlwinds already mentioned, are generally accompanied with heavy rains, hail, and electrical displays. Whirlwinds are also of frequent occurrence in France and many other countries, doing great damage to vineyards and other crops; but in Great Britain they seldom occur.
Waterspouts are whirlwinds occurring on the sea and other sheets of water. When fully formed they appear as tall pillars of cloud stretching from the sea to the sky, whirling round their axes, and exhibiting the progressive movement of the whole mass precisely as in the case of the dust-whirlwind. The sea at the base of the whirling vortices is thrown into violent commotion, resembling the surface of water in rapid ebullition. It is a popular fallacy that the water of the sea is sucked up in a solid mass by waterspouts, it being only the spray from the broken waves which is carried up. Observations of the rain-gauge conclusively prove this.
What are sometimes called ‘waterspouts on land’ are quite distinct phenomena. They are merely heavy falls of rain of a very local character, and may or may not be accompanied with whirling winds. They generally occur during thunderstorms, or in the vicinity of thunderstorms, being quite analogous to severe hailstorms, from which they differ only in point of temperature, the heavy drops being probably no more than melted hailstones. Also all the moisture that falls is the result of condensation; whereas, in the true waterspout, the rain is mixed with spray which has been caught up from the broken waves, carried aloft by the ascending currents of the whirlwind, and ultimately precipitated with the rain.