the growth of a thunderstorm

the growth of a thunderstorm is impeded by a stable layer sometimes a temperature inversion—that serves as a lid over the cloud top. Asia thunderstorm matures, a draftsman develops adjacent to the updraft core. Both drafts arc composed of churning high-speed air resembling a tumbling stream of rushing water. This pattern of air motion accounts for the turbulence: experienced by an airplane flying through a thunderstorm. The drafts may carry an airplane up or down while eddies in air velocity buffet it and, in extreme cases, muse structural damage. Most thunderstorms pass through their lifetimes in less than an hour, yielding a few lightning strokes and rain. Provided no one is struck by lightning, these storms do a great deal more good than harm. The rain is usually very beneficial, particularly to farms. The cool air that precedes an arriving thunderstorm on a hot summer day is the result of air from high altitudes being cooled by the evaporation of water drops; the air descends to the ground and spreads out, mostly in the direction of the storm motion. Some thunderstorms, because of their large sizes and long duration, arc called "supermodel" thunderstorms. They can last for many hours and have strung, persistent updrafts and downdrafts. They commonly occur as parts of a squall line, a line of thunderstorms, often oriented northeast-southwest, that sweeps across the Great Plains ahead of an advancing cold front. Super cell storms an produce violent weather. They are the sources of tornadoes and hail. As these storms move over the flatland of Kansas, Nebraska, Iowa, Illinois, and surrounding states, they sometimes lay down swaths of hail that devastate wheat, corn, and soybeans (Figure 3-16). In an average year, hail damage to agriculture in the United States amounts to more than 5700 million. Hailstorms also cause widespread damage in many other parts of the world: the fruit orchards of northern Italy, the grapevines of the Caucasus region of the Soviet Union, the tea plantations in Kenya, the farmlands of South Africa and Argentina. In India, 246 people were killed during a severe hailstorm near New Delhi in April 1888.

                                           

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.