Page:Popular Science Monthly Volume 53.djvu/327

Rh ." The heavy, arrow-tipped lines show the actual direction of the wind as it was observed on this occasion. It will be seen that the winds flow spirally outward from the highs in the direction of motion of the hands of a watch, while they blow from the high spirally inward toward the center of the low in the opposite direction to that of the watch's hands. These same directions will always be found about a well-defined high or a low. A severe storm and a low will always be found together, and the law of the winds about a low enables navigators to judge of the direction of the center of the storm and to steer away from it in time to avoid disaster. The lows are called cyclones, because of the inward direction of their winds; and the highs are known as anticyclones, because the winds resulting from them flow outward.

The highs are caused by descending currents from the upper air due to increased density which results from cooling by radiation of heat from the upper air. The lows are caused by the air absorbing heat from large areas of the earth's surface where the sun has acted strongly. The heated air ascends and flows outward at the upper levels, reducing the barometric pressure. If the air is moist.



when it rises under the action of heat, its pressure lessens until the moisture is condensed; and the liberated latent heat serves further to heat the air and increase the upward flow. The condensed moisture usually appears as clouds or rain.

The highs are found to enter the United States from only two points. In the winter they usually originate in Alberta to the north