266 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME San Francisco, Jacksonville, Kansas City, and Washington, U. C, at each of which they are charted and forecasts made for that district from the charts. Reports are also received from ships at sea, and other reports are interchanged with Mexico and Canada. At headquarters highly trained men receive and decode the messages. They then chart this information. The next step is a physical interpretation of the present state of the weather. On the basis of these conclusions and past experience, the Fig. 86. The aneroid barograph. (Courtesy of the U. S. Weather Bureau.) weather to be expected for the next twenty-four to thirty-six hours is forecast. The forecasts of the Weather Bureau are widely distributed from different district headquarters by telegraph, mail, and telephone; the newspapers, radio short wave, and the teletype telegraph machine are also widely used to broadcast coming weather conditions. Cyclones and Anticyclones Are Charted in Weather-forecasting. Areas of relatively high barometric pressure, called "anticyclones," and areas of relatively low pressure, called "cyclones," pass over the United States from westerly toward easterly directions, in succession, with speeds varying from about twenty miles per hour in the summer to thirty miles per hour or more in the winter. These so-called highs and lows cover territories ranging up to more than a thousand miles in diameter. Highs generally are accompanied by fair weather, though not always, whereas lows usually represent more or less stormy con-
WEATHER-FORECASTING 267 ditions. Dry air is more dense than moist air; the average molecular weight of dry air is 29, while the molecular weight of water vapor is 18. The precipitation of moisture is typical of storms. In the northern hemisphere the winds blow spirally outward in a clockwise direction around anticyclone areas and spirally inward in a counterclockwise direction around cyclone areas; the directions of wind circulation are opposite in the southern hemisphere. Fig. 87A. An atmospheric pressure record for the week, June 6-12, 1932, at Stockton, California. The weather was fair all week. Monday I Tufidau I Wednciday IVednttday f/ Thunday / Friday / Saturday I Sunday / . . I iij^Bt « « « .OH'? « f. i loril; 4 6 I iofi « I I ,0^2 « » t I..II'? . « t loOit . 6 « lolT; 4 t i .o ifl ? . t i lolft 4 t'i -lliiflllMi-t!lln^''W Fig. 87B. An atmospheric pressure record for the week, January 11-17, 1932, at Stockton, California. The weather was stormy during this week. In general, a storm is approaching when the barometer indicates falling air pressure. The more rapidly the air pressure is falling, the more rapidly is the storm approaching and the more severe it is likely to be. The indications afforded by the wind and barometer are the best guides now available for determining future weather conditions. In most regions winds from the east and a falling barometric pressure usually indicate foul weather, and winds shifting to the west indicate clearing and fair weather. The south winds usually bring warmth, the north winds bring cold, the east winds in the middle latitudes indicate a storm approaching from the westward; and the west winds show that the storm has already passed eastward.