Page:Popular Science Monthly Volume 23.djvu/769

Rh wind in advance of it will be easterly, blowing off an area of high barometer into the low; and the barometer will continue to fall. Toward the north of the storm, the wind will be north; and to the south of it the wind will blow from the south, frequently causing what are known on the Western plains as "sand-storms"; while, if the storm occurs during the winter season, the wind and snow in the northern portion is called a "blizzard." In the rear of the storm the wind will be westerly, shifting to northwest, frequently blowing a gale; the barometer will rise and the temperature will fall, sometimes rapidly, and clearing weather will follow.

Thus it will be observed that the true axis is the "storm-center," and that the storm revolves on this axis as it moves forward in an easterly, northeasterly, or southeasterly direction. This is the true cyclone. It may be only a few hundred miles in diameter, or its influence may be felt from British America to the Gulf of Mexico. It may be accompanied by what are known as local showers or storms; or the rains may be general, and of several days' duration. The greatest precipitation is frequently in advance of the storm-center, and may be either rain or snow, depending upon the season of the year and the temperature. During the spring and summer months the area of low pressure is usually accompanied by precipitation in the form of local showers, or thunder-storms, of more or less severity, in which case the strongest air-currents move with the local storm-cloud, and are known as simply high winds, sometimes approaching the severity of a hurricane, but seldom extending over any considerable area of territory.

The true cyclone usually travels from the Rocky Mountains to the Atlantic coast, in nearly a straight line. Sometimes the distance is traversed in four days and nights, exceeding the speed of a railway express-train; but frequently the progress is slow, and the time required much longer, depending largely upon the atmospheric conditions met with in the vicinity of the Great Lakes.

Frequently, while the wind may be blowing from the southeast or northeast, or between these points, the storm is approaching from an opposite direction, and pushes out its cloudy streamers, or "feelers," hundreds of miles in advance. There is, then, no such thing as a "northeast storm," as far as the interior of this continent is concerned.

The origin and movement of continental cyclones being understood, we naturally turn, next, to the investigation of tornadoes. These seem to occur most frequently in the Western States, and are usually confined to the territory between latitude 35° and 45° north, and longitude 10° and 25° west. They are purely local in effect, although their cause may be remote; always depend upon and frequently accompany the meteorological conditions developed during the