The plane of meeting between these two currents is more or less inclined northward in the northern hemisphere, for the reason just stated; and the lower end of the plane, or the space of air between these two currents where they meet on the surface of the earth,[1] constitutes the centre line or area proper of the storm, and the region of lowest barometer. The horizontal plane beneath this inclined plane[2]
Fig. 1.—Atmospheric Currents.
is the geographical extent of the region affected by the storm and the region of low barometer. The place where the currents meet is constantly changing with the changing seasons, following the sun northward in summer and southward in winter. These changes of locality do not, however, take place in one continuous movement of the atmosphere; but with successive oscillations, like the waves of a rising tide, each succeeding wave advancing farther and receding less than the one before it, until its most northern or southern limit is reached as represented by the numbers 1 and 2 in the diagram when the oscillations in the opposite direction again begin. Whenever the lower end of the plane of meeting between the two opposing currents at B oscillates or passes over any place on the surface of the earth, it will cause storm or change of weather there a change of wind, of temperature, and of atmospheric pressure.
The inclination of the plane of meeting, or the slope of the tropi-
