Page:Handbook of Meteorology.djvu/164

 Let us take a low which is central in Illinois. The wind is blowing into it from all quadrants, to fill the updraught. The storm is preceded by a wind from an easterly quadrant and clears with one from a westerly quadrant, which is apt to settle in the northwest. Within the storm area the winds acquire a spiral motion, whirling upward contra-clockwise as they approach the updraught. The whirl brings warm and moist air from a southerly region to the easterly side of the low, where the air is colder and the temperature nearer to the dew-point—that is, to condensation. For this reason, most of the precipitation is on the east and south sides of the low. On the west side colder and drier air is blowing from the west and the northwest and, being colder, is drawn into the updraught to a less extent or perhaps, not at all. Westerly and northwesterly winds, therefore, usually are clearing winds.

Just as the trough of a wave is followed by a crest, so a low is pretty apt to be followed by a high; and cyclonic storms of the Alberta type are frequently followed by crests or waves of cold air from high latitudes. If a winter high pressure area lies over the northwestern part of North America, and a low forms anywhere in the vicinity of this area, a flow from the high to the low will naturally follow. This means that, in order to fill the low, the clearing northwest winds must also be descending currents; and, as a matter of fact, they flow along the surface, lifting the warm air above them. In their flow into lower latitudes and their descent, they, too, acquire a whirl. But the whirl is clockwise, or the reverse of the whirl of the cyclone; hence it is known in Weather Bureau cant as the “anticyclone.” The winter anticyclone, therefore, is usually a cold wave.

The high of the winter cold wave is an area of considerable pressure. Usually the barometer stands above 30.50 inches; occasionally it mounts nearly to 31.00 inches. For this reason the cold air spreads far south—sometimes carrying freezing weather far into Florida, to the detriment of the semitropical orchards. The southern part of Florida is the only part of the United States which escapes freezing weather.

In many respects, the cold wave is one of the most valuable health assets of the United States. Should the ground be covered with snow, so that gale winds pick up no dust, it brings the purest air that mortals on land ever breathe. Even if the