268 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME During the colder months, when the temperature of the land areas is below that of the ocean, precipitation will take place along the coasts when the wind blows steadily from the water over the land, regardless of the barometric readings, because the moisture in the sea breezes is condensed when they reach cold areas. This is not true during the summer months, however; then the capacity for moisture of the onshore winds is increased, for their temperature is raised as they blow over the heated land areas. On the Pacific coast and from the Mississippi and Missouri valleys to the Atlantic coast, precipitation generally starts when the barometric pressure is falling, whereas in the Rocky Mountain region it seldom starts until the pressure begins to rise after a fall. In the summer the showers and thunderstorms in the eastern half of the United States usually come about the time the pressure turns from falling to rising. A pronounced barometric change indicates a change in weather. If you wish to try your hand at weather-forecasting, study the most recent weather map and see whether a cyclone is headed in your direction or not. Note that it will probably follow the direction of the isotherm (line of equal temperature) drawn through its center. Atmospheric pressure is continuously measured and automatically recorded by the aneroid barograph. Figures 87A and 87B are reproductions of a few records made by such an instrument. How to Read a Weather Map. Weather maps show the cyclones and anticyclones at a given time. By joining points of equal pressure, one obtains series of lines, called "isobars," which show the position of cyclones and anticyclones. The United States Weather Bureau now expresses atmospheric pressure in terms of millibars rather than inches or millimeters of mercury. One millibar is equivalent to 1000 dynes per square centimeter. The average pressure at sea level (standard pressure) is 1013.2 millibars. Isobars are drawn for every 2, 3, or 4 millibars according to the scale of the map used. Cyclones and anticyclones follow two more or less well-marked storm routes in the United States. Those coming from the Pacific Ocean move first toward the southeast, then usually swerve in the direction of the Great Lakes, and exit via the St. Lawrence Valley to the sea. The cyclones coming from the southwest travel over the Rocky Mountain region and then move to join those from the Pacific coast in their path to the Great Lakes and the St. Lawrence Valley. Sometimes the lows from the southwest bend to the south and traverse the Gulf states
WEATHER-FORECASTING 269 before turning to the Great Lakes. There are many exceptions to this general tendency, depending on the season and other factors. Cyclones travel variable distances; some are dissipated before they travel far, whereas others traverse entire continents. In February, 1925, a low was observed to travel completely around the earth, a distance of 21,379 miles, in about a month's time, before it finally broke up as it started on its second lap. Fig. 88. Weather map retouched to show lines of equal pressure and equal temperature. The heavy lines are lines of equal pressure, while the broken lines are lines of equal temperature. The shaded areas indicate rain. Note the relation between the areas of rain and the cyclones and anti-cyclones. Though it is uncertain just how cyclones are formed, they seem to involve the contact of large air masses from different sources. An average of about 112 cyclones affect the United States annually. Nearly 45 originate over the North Pacific, 20 over Northwest Canada, 10 over the Rocky Mountain Plateau, 12 over the Colorado Rockies, 15 over Texas and the Western Gulf region, 3 over the Ohio River Basin, and 7 over the Eastern Gulf states. Cyclones originating in the tropics are usually more violent than those in the other zones. Hurricanes and typhoons are tropical cyclones, 100 to 200 miles in diameter ; they originate over warm oceans near the boundary between the doldrums and the trade-wind zones. They move westward or northwestward as long as they remain in tropical latitudes.