Page:Popular Science Monthly Volume 53.djvu/326

310 blows from each side, and a descending current of air near parallel 32 of each hemisphere, from which the air flows north and south.

Friction between the surface of the earth and the atmosphere tends to carry the air with the earth in its rotation. As the velocity of the earth's surface is nothing at the poles and increases toward the



equator, those winds which blow toward the equator will lag behind and have a westerly direction, and those that blow toward the poles will retain their greater velocity of the lower latitudes and travel faster than the more northern parallels, resulting in an eastward direction over the earth's surface.

In the temperate zone of the northern hemisphere there is a general northeasterly drift of the surface atmosphere at a fairly regular rate of motion, and this causes our storms almost without exception to travel from west to east. This eastward tendency of all atmospheric disturbances is the basis of all predictions.

In the great ocean of air, as in the ocean of water, there are constantly occurring waves and hollows, or areas where the air is piled to an unusual height, showing increased pressure on the barometer, and areas where the height and pressure are less than the normal. These high and low areas, or "highs" and "lows," as they are technically known, travel in a general northeasterly direction. In Fig. 2 is shown an actual case of a low between two highs in the United States, and it is extremely interesting to notice the laws of the winds around them. The finer, oval lines are used to connect all points having equal barometric pressure. They are known as "