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THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS 87 value of 30 m/sec, and the cloud content is 4 g/m3, a hailstone can grow to about 7.5 cm in diameter before arriving at the ground, 25 minutes after being selected for such growth by having fortuitously attained a larger size than its neighbors. Figure 7.6 Schematic plan view of a tornadic thunderstorm at the surface. The heavy solid line encompasses radar echo. The wavelike gust front structure is depicted by a solid line and frontal symbols. Surface positions of the updraft are finely stippled; forward-flank downdraft (FFD) and rear-flank downdraft (RFD) are coarsely stippled; arrows represent associated streamlines (relative to the ground). Likely tornado locations are shown by encircled Ts. From Davies-Jones (1985). The rapid ascent of small particles in sufficiently strong updrafts gives insufficient time for the growth of any of them to large size. The development of hail in such strong updrafts may depend on the insertion into the updraft of hail embryos formed nearby and cycled into the updraft column by virtue of their descent from higher levels into horizontally convergent regions below. Major tornadoes are usually accompanied by large hailstones. However, we find that tornadoes are usually absent from the storm class that includes the most damaging hailstorms. Doppler-radar observations of the airflows show that the hailstorm updrafts are not so strong as updrafts in tornadic storms, but the updrafts in major hailstorms cover a substantially larger area. TORNADOES Major tornadoes are most often associated with thunderstorms of a type illustrated in cross section in Figure 7.6, and they are identified by a rapidly rotating funnel-shaped cloud that marks the condensation boundary of in-spiraling air at low altitudes undergoing adiabatic expansion and cooling. Tornadoes are most severe and least uncommon in the United States, but they occur occasionally in India, Australia and New Zealand, South Africa, Argentina, Japan, and several countries of western Europe. In Mississippi, the state in the United States most subject to tornado deaths and damage, statistics on annual damage and storm frequency suggest that about 1/1000 to 1/300 of the area is affected by tornadoes each year, with tornado winds of 50 m/sec or more and significant damage to structures. The maximum rate of pressure change may be between 50 and 100 mbars/sec, and the maximum wind of a major tornado is about 100 m/ sec. The frequency and intensity of these storms and the areas visited vary from year to year in association with irregular departures from seasonal norms of other quantities such as temperature and moisture. Radar data show that tornadoes start at middle levels (about 5 km), to the rear side of pre-existing cyclonic circulations about 3 to 10 km in diameter, and develop downward and upward on a time scale of about half an hour. At the ground, tornadoes appear on or near a boundary between rising warm moist air, within which the release of latent heat of condensation is the storm's principal source of energy, and air descending from middle levels where it is intrinsically cold, and cooled sensibly by the evaporation of precipitation into it. The most critical dynamical aspect of tornadoes involves the concentration of rotation within them. Various investigations during the past 10 years have established that two processes have direct importance in this concentration. The air's angular momentum is a consequence of the Earth's rotation and of various weather systems. The conservation of angular momentum accompanying horizontal convergence and ascent of air has long been appreciated. Such conservation is manifested, for example, by the increased rotation rate as a skater's arms are brought in from an outstretched position. The second process, more recently detailed, involves the rotation or twisting of horizontal vorticity is represented by the vertical variation of the horizontal wind (vertical wind shear), already cited as significant for severe-storm development through its role in facilitating removal of condensation products from the updraft. The twisting process is commonly effective on an