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THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS 84 but moisture represents energy as wellâa large amount is latent in water vapor and enters the atmosphere as surface waters evaporate. Most of the air that harbors thunderstorms is conditioned during days or weeks over warm seas. During the westward journey of an air mass in the North Atlantic trade-wind zone, for example, the depth of the moist layer increases daily and is often about 2000 m thick when the air turns northward and enters the United States. The low-altitude moist layer may be surmounted by a very dry layer. This stratification is produced even as heat and moisture are supplied from below, while weak subsidence, associated with the same subtropical high-pressure area that engenders the trade winds, maintains dry air at middle levels, protected from incursions of moisture from below by a temperature inversion. Dry air aloft may also be produced by other processes. For example, precipitation that accompanies ascent of air on the windward side of a mountain range leaves the air drier on the leeward side. A warm temperature and large quantity of water vapor in an air mass at low altitudes and only a small amount of water vapor aloft, is a significant combination (known as a convectively unstable condition) whose potency is most realized when a large-scale disturbance causes generalized horizontal convergence and rising air motion. In such a case, the moist air at low altitudes cools less rapidly than the air above because the lowlevel air gains the heat latent in the water vapor as that vapor condenses. A result is that the temperature decline with height increases, i.e., the lapse rate becomes much steeper. Then rising parcels within the air mass and overturning within the air mass can become a violent process. CONDENSATION AND PRECIPITATION IN RELATION TO AIR MOTION As an ascending air parcel cools below the temperature at which its vapor is saturated, condensation occurs on nuclei, usually motes of sea salt, nitrates, or sulfates, numbering typically about 1000/cm3 over land. There is an initial selection or activation process whose details depend on the nature and number of the nuclei and on the ascent rate, reflected in extent of vapor supersaturation. Following activation, only the selected nuclei continue to grow and the balance evaporate. Owing to differences in initial size and composition, growth of the different nuclei proceeds at different rates. If ice coexists with supercooled liquid water, i.e., liquid at temperature below the normal freezing point, there is growth of the ice particles by deposition of vapor diffused from the liquid particles, which tend to evaporate. Brownian motion, turbulence, and differential fall speeds among the particles all contribute to coalescence and further broadening of the size distribution. After some tens of minutes, unless reduced in size by evaporation in dry air mixed into the cloud from its environment, some of the particles have appreciable fall speeds. Whether ice or liquid, they now begin to sweep out the smaller particles in their paths and fall ever more rapidly to the ground. Convective precipitation is typically intermittent. Its fluctuating character at a place is related not only to the passage of discrete shower cells overhead but also to a dual effect of the condensation process. The condensation process releases latent heat, which tends to reduce air density and stimulate the updraft, but the condensation process also burdens the rising air with the weight of the thousandfold-denser condensate particles, which may be water or ice. Unless the updraft speed is markedly stronger than the fall speed of precipitation particles, precipitation accumulates in the updraft until the positive effects of thermal buoyancy are quite overwhelmed. Thus an initial updraft usually becomes a downdraft after about 30 minutes, as indicated in Figure 7.3. The downdraft can be quite strong, especially when the weight of precipitation is augmented by a cooling that accompanies evaporation of cloud and precipitation into dry air mixed into the cloud at middle levels. Such downdrafts necessarily become divergent horizontally as they near the Earth's surface, where they represent a significant threat to aviation. In some severe storms, the weight of accumulated condensation products contributes to a splitting process that leads to separate storms, moving to the left and right of the direction of the vertically averaged wind. In the northern hemisphere, the rightward-moving storms are more likely to harbor tornadoes; damaging hail is often found in left movers. Figure 7.3 Three stages in the life cycle of an ordinary thunderstorm cell, as observed by the Thunderstorm Project. After Byers and Braham (1949).