n. A violent squall or whirlwind of small extent.n. Specifically— On the west coast of Africa, from Cape Verd to the equator, a squall of great intensity and of short duration, occurring during the summer months, but most frequently and with greatest violence at the beginning and end of the rainy season. On the western part of the coast, near Sierra Leone, these squalls come from easterly points, and blow off shore; while on the eastern part of the coast, near the mouth of the Niger, they occasionally blow on shore, partly because of a variation in the direction of the squall, and partly because of a different trend of the coast. The squall is marked by peculiar, dense, arched masses of dark cloud, furious gusts of wind, vivid lightning, deafening thunder, and torrents of rain; it produces a slight rise in the barometer and a fall of temperature amounting on the average to 9° Fahr. Similar squalls in other tropical regions are usually known by the name of arched squalls, but are sometimes also called tornadoes. The principal period when these squalls occur (namely, at the change of the seasons or of the monsoons) is that in which great quantities of vapor-laden air are stopped by a land wind, and accumulate near the coast, producing a hot, sultry, unstable state of the atmosphere. The tornado is the overturning process by which the atmosphere regains its stability. The wind ordinarily turns through two or three points during its progress, but in general a complete cyclonic motion is not established.n. In the United States, east of the 100th meridian, a whirlwind of small radius and of highly destructive violence, usually seen as a whirling funnel pendent from a mass of black cloud, occurring most frequently in the southeast quadrant of an area of low pressure several hundred miles from its center, and having a rapid progressive movement, generally toward the northeast. The principal condition precedent to the formation of a tornado, just as for a thunder-storm, is an unstable state of the atmosphere. In the tornado a whirling motion from right to left, of tremendous energy, is generated in a mass of clouds, and is often maintained for several hours, while in the ordinary thunder-storm a complete cyclonic motion probably seldom becomes established. Tornadoes generally arise just after the hottest part of the day, when the atmosphere has its maximum instability; the months of greatest frequency are April, May, June, and July. The destruction in a tornado may be caused either by the surface wind which is forced in on all sides to feed the ascending current of the tornado-funnel, or by the gyrating winds of the funnel itself when sufficiently low to come within the reach of buildings; in the latter case no structure, however strongly built, is apparently able to withstand the wind's enormous force.
