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THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS 81 7 Thunderstorm Origins, Morphology, and Dynamics Edwin Kessler NOAA National Severe Storms Laboratory INTRODUCTION Thunderstorms involve rapid vertical rearrangement of deep air layers. Large processes promote and shape vertical and horizontal air motions, and processes within storms control development of rain, hail, and strong local winds. Espy (1841) first presented the concept that thunderstorm circulations are driven largely by the heat latent in water vapor, released during condensation. During 1946-1947, a U.S. federally funded project investigated thunderstorms in Ohio and in Florida. Findings of that project (Byers and Braham, 1949) were the first accurate description of thunderstorm details, and project methods were a principal part of the foundation of subsequent studies. A modern comprehensive and abundantly illustrated textbook was prepared by Ludlam (1980). Thunderstorm processes are presented in much detail, with many references, in Volume 2 of a recent three-volume treatise on thunderstorms (Kessler, 1985), and the lightning process is detailed elsewhere in this volume. VERTICAL CONVECTION AND THE GREENHOUSE EFFECT Vigorous vertical air currents and thunderstorms are a consequence of excessive warmth and moisture at low altitudes. In a global sense this stratification of properties stems largely from the radiative properties of atmospheric constituents. The atmosphere is largely transparent to solar radiation, which is concentrated in the yellow region of the spectrum. Most of the solar energy not reflected by clouds passes through the atmosphere unabsorbed and is converted to heat at the ground or in surface waters. The absorbing surface also radiates, but because its temperature is much lower than the Sun's terrestrial radiation is concentrated in infrared wavelengths. Atmospheric carbon dioxide, water vapor, and some other trace constituents are significant absorbers of much of the outgoing infrared emission, so this outgoing radiation is substantially blocked and further heats the air at low altitudes. The average temperature at low altitudes provides long-term equality between the rate at which heat is carried away and the rate at which it is received. Removal processes include some radiative losses through spectral windows between absorbing bands of carbon dioxide and water vapor and transport of heat by air parcels in motion. This latter process is known as convection. Heat gained at the surface is carried by convection to altitudes where the radiative process is more effective because there is less absorbing medium above. Figure 7.1 shows that, globally, thunderstorms are most frequent in low latitudes where a larger surplus of heat