The Atmosphere and the Weather

storms accompanying low pressure centers associated with fronts are some-times called "frontal cyclones." References to cyclones are often misunderstood; they usually evoke thoughts of violent weather. There is some justification for such a reaction: Over the central United States, tornadoes are commonly called cyclones; in Southeast Asia, Australia, and Mexico, the name is applied to the intense tropical storms known as hurricanes in the United States. These are not improper uses of the name, although most cyclones over the earth are centers of low pressure that do not contain threatening weather. The middle latitudes of the earth experience, at regular intervals, the passage of cyclones with their associated clouds and precipitation. On weather satellite photographs, cyclones sometimes have very distinctive patterns (Figure 3-7). Some places see more of them than others do because there are favored regions for the formation of cyclones. Cyclones most often develop along well-established, nearly stationary fronts. In winter, cold air from the Arctic commonly sweeps southward.

                                         

over North America behind a cold front, pushing warm air ahead of it. Usually the Rocky Mountains act as a barrier that stops the air from moving westward. As a result, the front separating the polar air over the Great Plains from warmer air becomes stationary along the eastern foothills of the Rocky Mountains. Many cyclones develop along this front, particularly over Colorado (Figure 3-8). On a weather map, a new cyclone first appears on the front as a wavelike wiggle that coincides with an area of falling atmospheric pressure. In some cases, the wave amplitude increases and pressure continues to decrease. Clouds appear, which thicken, become widespread, and even-tually yield rain or snow. As all this is going on, the cyclone usually moves more or less eastward under the influence of the major west wind currents dominating the middle latitudes. In winter, polar air often pushes southward until the front becomes stationary in the form of a giant arc along the Gulf coast and up the eastern seaboard. Cyclones frequently develop along this front and move north-ward or northeastward. Many of them originate over the Gulf. Another favorite spawning ground is just off Cape Hatteras, North Carolina.