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Unformatted text preview: This document last updated on 20-Oct-2011 EENS 2040 Natural Disasters Tulane University Prof. Stephen A. Nelson Exceptional Weather - Thunderstorms, Tornadoes, Nor'easters, & Drought Thunderstorms Thunderstorms occur anywhere that warm moist air has absorbed enough heat to make the air less dense than the surrounding air. This commonly occurs along cold fronts, but can occur in other places as well, particularly where daytime heating forms hot air near the Earth's surface. As the warm moist air rises it begins to cool and water begins to condense into tiny droplets that form clouds. Condensation of the water droplets in the clouds releases the latent heat of evaporation, adding heat to the rising air, thus decreasing its density and allowing it to rise to higher levels in the atmosphere. This rising air, called an updraft, starts to build clouds to heights of up to 6 km. Further rising and cooling within the clouds causes more condensation, as well as the formation of ice crystals which release further latent heat and build cloud heights up to 12 km. Eventually the water droplets and ice crystals in the clouds become so large that they can no longer be supported by the uprising air mass, and they begin to fall forming rapid downdrafts on the leading edge of the cloud. In the mature stage of thunderstorm development updrafts and downdrafts operate side by side within the cloud. This is the most dangerous stage of a thunderstorm because of the high winds accompanying the downdrafts, the heavy rain, as well Exceptional Weather - Tornadoes 10/20/2011 Page 1 of 10 as thunder, lightning, and possible hail and tornado development. Eventually the cloud reaches the dissipating stage as the downdrafts drag in so much cool dry air that it prevents further updrafts of warm moist air. With lack of updrafts of warm moist air, the cloud begins to dissipate and eventually it stops raining. Thunderstorms can form as single cells, with only one cloud mass, or as multiple cells, with several clouds moving along a similar path. Although thunderstorms can occur nearly everywhere, they show an unequal distribution through the United States. Areas that receive the highest number of thunderstorms are areas where warm moist air moves northward from the Gulf of Mexico. As seen in the figure to the right, areas in southwestern Florida have over 100 days per year with a thunderstorm. Hail - Hail is a rain of semi-spherical, concentrically layered ice balls that are dropped from some thunderstorms. Hail rarely kills people, but it does heavy damage to agriculture, roofs, and automobiles. The conditions necessary to form hail during a thunderstorm are: z Large thunderstorms with high cloud tops formed from hot moist rising air....
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