WHIRLWIND. A term specifically applied to small whirls or eddies of winds that continue only a few seconds, seldom longer than a minute; the diameter of the whirl is correspondingly small, and is made visible principally by dust, leaves, or light objects flying in the air. The most extensive dust whirls occur on hot, dry plains, as in India, Texas, and the eastern slope of the Rocky Mountains, and especially in the Desert of Sahara, where heaps of sand, sufficient to cover a caravan, are said to fall to the ground occasionally as the whirling simoom passes over. In the smaller whirls the rotation of the eddy may be either positive or negative, but in the larger whirls the direction, like that of the tornado, is always the same as that of the storms attending areas of low pressure. namely, from the west, by the south, to the east in the Northern Hemisphere. and the reverse in the
southern. Whirlwinds acquire different names according to the circumstances attending them. Thus the hot dry air over a dusty plain rising because of its high temperature gives rise to the dust whirl. The burning of prairies and for ests and the eruptions of gases front volcanoes cause the smoke whirls; the ascent of air drawn from the ocean up to the cloud gives rise to the waterspout, properly so called: the most violent ascending and rotating clouds constitute the tornado. Formerly hurricanes and typhoons were spoken of as whislwinds, but the present tend ency is to use those distinctive terms. Consult: Whirlwinds and Dust Storms of India (London. IS(0) ; Reye. Die Wirbelstiirme (Han over, 13301; Ferrel, On Cyclones, Tornadoes, and Waterspouts (Washington, 1 SS2