National Academies Press: OpenBook

The Earth's Electrical Environment (1986)

Chapter: Conditioning the Air for Overturning

Suggested Citation:"Conditioning the Air for Overturning." National Research Council. 1986. The Earth's Electrical Environment. Washington, DC: The National Academies Press. doi: 10.17226/898.
Page 82
Suggested Citation:"Conditioning the Air for Overturning." National Research Council. 1986. The Earth's Electrical Environment. Washington, DC: The National Academies Press. doi: 10.17226/898.
Page 83

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THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS 82 (due to radiative imbalance) is transported aloft by convection. Figure 7.1 Annual number of thunderstorm days. Adapted from World Meteorological Organization (1953). The process of vertical convection within an air mass contrasts with a global process driven by horizontal temperature differences on a very large scale. Thus, equatorial regions are warmer than polar regions, and there are various circulation regimes that function to reduce this temperature contrast. These circulations, primarily horizontal, associate with some vertical air motions as well, and they tend sometimes to enhance and sometimes to diminish the conditions that favor the intense vertical circulations of thunderstorms. THE SIZE OF THUNDERSTORMS About 85 percent of the atmospheric mass is contained in the lowest 13 km—this is the layer that generally participates in the vertical circulation of a typical thunderstorm. The typical horizontal dimension of a thunderstorm is similar to its vertical dimension. On the other hand, the storms of winter, essentially embodying horizontal air motions, respond to variations of temperature in the horizontal plane extending over hundreds or even thousands of kilometers; these storms are correspondingly larger horizontally than vertically. The vertical circulation of air in a thunderstorm is a result of density differences. When the air is relatively warm, it is relatively expanded and weighs less than a parcel of the same size in its surroundings. The warm parcel is subject to pressure forces; these vary with height according to the weight of air in the large environment, and they are not fully balanced by the warm parcel weight. The imbalance is represented by a net upward force, Archimedean buoyancy, which causes the warm parcel to rise. As it rises, the air ahead of it must move out of the way. The greater the horizontal extent of a rising parcel, the farther the displaced air must move and the more resistance there is to the parcel's motion. Very large buoyant parcels cannot rise rapidly because of the large volume of air that must be simultaneously displaced horizontally. Thus, vertical motions are faster in smaller rising parcels. On the other hand, the smallest buoyant parcels tend to be more eroded by diffusive processes operative on their boundaries. In the final analysis, more definite conclusions as to scale rest largely on empirically determined characteristics of diffusion and consideration of equations of three-dimensional motion. Figure 7.2 shows a zone of severe thunderstorms extending hundreds of miles across the central United States. The large horizontal extent of the zone is related to the large scale of a disturbance engendered primarily by horizontal temperature contrasts. Along the zone, numerous boundaries between individual thunderstorms are evident. They mark the dimensions of local storms driven primarily by vertical temperature contrasts, locally enhanced by processes intrinsic to the larger-scale disturbance. Conditioning the Air for Overturning A proclivity for overturning is enhanced as the lower atmosphere is warmed. This occurs daily after sunrise, and afternoons are often marked by puffy clouds indicative of rising thermal currents. Seasonally, temperature rises as day length increases, and as the Sun rises higher in the sky, so spring and early summer usually present more frequent and more vigorous showers and thunderstorms than does fall. In other words, the thundery weather that tends to be frequent around the vernal

THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS 83 equinox in mid-latitudes is stimulated by the relative coolness aloft that remains from winter. Figure 7.2 Photograph in visible light from synchronous Meteorological Satellite SMS-1, 6 May 1975, 1405 CST. A major squall line harboring tornadoes lies across the central United States. Lakes Superior, Huron, and Michigan are in upper right. Photo courtesy of NOAA, National Environmental Satellite Service. A rising air parcel encounters decreasing air pressure and expands. This expansion represents work done on the environment and is accompanied by a decline in temperature. A rising parcel continues to rise as long as it remains warmer than its environment—this means that the ambient temperature must decline with height as fast as or faster than the temperature of the rising parcel declines as its altitude increases. A shower or thunderstorm occurs only when there is a sufficient decline of ambient temperature with height in a deep layer. There would be no rain without moisture in the air,

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This latest addition to the Studies in Geophysics series explores in scientific detail the phenomenon of lightning, cloud, and thunderstorm electricity, and global and regional electrical processes. Consisting of 16 papers by outstanding experts in a number of fields, this volume compiles and reviews many recent advances in such research areas as meteorology, chemistry, electrical engineering, and physics and projects how new knowledge could be applied to benefit mankind.

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