Page:Popular Science Monthly Volume 9.djvu/325

Rh barometer now rises again, but is not as high as before the storm, because it is in the tropical current which has reached the locality. If, now, the wind from the south, which has prevailed and driven back the northern current, continues in the same direction until the entire atmospheric area of the storm passes over the zenith northward, and the sky clears up from the south or southwest, as is generally the case in early autumn or late spring, then the next storm or change of weather will come from the north. But if the wind changes its direction again before the storm is over, as is mostly the case in mid-winter, and blows from the north, as it did at the beginning, until the entire atmospheric area of the storm is carried backward over the zenith, and the sky clears from the north, then the next storm or change of weather will come from the south, as described above. In this case the polar current has prevailed, the air is colder, the thermometer falls, the barometer rises higher than in the other case, and the atmospheric conditions existing before the storm are gradually reestablished.

—Before a progressive summer storm, the air is usually warm and sultry, the sky cloudless but somewhat dim, and a light southerly breeze is blowing. Suddenly the sound of distant rumbling thunder is heard, and large masses of dark cumulus clouds rise and arrange themselves on a long bank of stratus clouds in the northern or northwestern horizon. This is the cumulo-stratus combination of clouds which is the herald of a polar or progressive summer storm. Soon the south wind increases in violence, and drives clouds of dust before it. The thunder, rolls, and lightning flashes more frequently. The clouds bank up higher and higher, and advance more slowly, until at last they become stationary. These are the ordinary indications of a violent progressive summer storm, which sometimes ends in a tornado.

Like a winter storm, it is produced by the meeting and conflict of the polar and tropical currents under greater differences of temperature and other conditions, and is therefore attended with more violent and complex phenomena than those of a winter storm. The changes of wind, and of the barometer and thermometer, during its development at any locality, are similar to those of a winter storm in its return, oscillation southward; that is, these changes occur in a reverse order to those of a winter storm during the regular progress of the tropical current northward, in the same order as during its oscillation southward.

In most cases of this kind of summer storms, after the clouds have remained stationary for some time, discharged their rain and restored the disturbed equilibrium of the atmosphere, the polar current which produced it by moving southward oscillates back to the north again, and the storm at this locality is over—although similar phenomena and changes will be occasioned by it later at other localities over which it sweeps in its oscillation northward.