Page:Popular Science Monthly Volume 9.djvu/328

306 much more steep on the southward than on the northward side of this area; from which it follows that the rain-area is much less on the southward than on the northward side of a progressive storm.

All the atmospheric changes and phenomena above stated result from the same general cause, but under different conditions and circumstances. This cause is the meeting of the polar and tropical currents in their movements northward and southward, to restore a disturbed equilibrium in the atmosphere toward the equator or the poles.

Applying this theory in brief explanation of the facts stated in connection with Ballot's law, we find the area of lowest barometer at the place where the two currents meet on the surface of the earth. It is produced by the obliquely upward movement of the tropical current over the polar current, and by its rising more or less vertically in the vicinity of contact, after its horizontal progress northward has been checked by encountering the polar current. This oblique and upward movement of the tropical current diminishes the atmospheric pressure there, as shown by the barometer, and produces that depressing calm which is always felt by persons in any locality where this meeting of currents takes place, or over which its area moves or oscillates during the continuance of a storm. The elongated, elliptical shape of this area is accounted for by the fact that it is the narrow space between the two currents where they meet, and extends eastward and westward between them. It is rounded at the ends or margins of the currents, where the wind, in accordance with Ballot's law, blows inward toward the centre line of contact, which is also the centre line of lowest barometer. And, as the two currents force each other backward and forward during a storm, they necessarily carry along the elliptical space between them, and thus its movements in the direction of its shorter axis are accounted for.

The rain-area, or that of low barometer, which surrounds the elliptical region of lowest barometer where the currents meet on the surface, as just explained, extends horizontally beneath the plane of meeting, which is inclined northward. It is produced chiefly by the oblique and upward movement of the tropical current over the polar.

The gradients, or different degrees of pressure within the rain-area, are caused by the same upward movement of the tropical current over the polar, in connection with the constantly-varying heights or depths of both polar and tropical air, which are vertically above the space beneath the inclined plane from the region of lowest to that of highest barometer northward; and the steeper or more abrupt gradients southward are explained by the fact that when the tropical current meets the polar current it is suddenly checked, and while a portion of it moves obliquely over the polar current, as stated, another portion of it rises, more or less vertically, for some distance around the vicinity of contact, and the pressure is thus more suddenly diminished on the southward side of this area of low barometer than on