Page:Popular Science Monthly Volume 9.djvu/324

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from twelve to twenty-four hours, or longer. Sometimes these clouds, before reaching the zenith, will recede and disappear beneath the southern horizon. This indicates a backward oscillation of the southern current, caused by the greater resistance of the polar current. But in such case the stratus clouds will reappear next day, or sooner, and uniting and, becoming denser, they will advance over the zenith, and cover the whole heavens, discharging rain, snow, or sleet, according to the thermal conditions present.

PSM V09 D324 Cumulo stratus clouds.jpg
Fig. 4.—Cumulo-Stratus Clouds.

Thus, by observing the clouds, a northeast or winter storm may always be predicted from one to three days beforehand, while the barometer shows no change until the stratus clouds from the south have reached and passed over the zenith, when it begins to fall; but the thermometer indicates no change.

At this stage of the storm the wind from the north rises and blows more violently, while the clouds move northward against the wind, and the rain or snow, driven by the prevailing wind, comes down obliquely from the north. After some time the direction of the wind changes, and there is a calm. The air is warmer, the thermometer rises suddenly, the barometer has reached its lowest point, and the rain or snow falls vertically. This calm continues for a longer or shorter time, and the wind gradually changes until it comes from nearly or quite the opposite quarter from which it came at the beginning of the storm, and blows more powerfully than before. The