14141 - Tornadoes Scott R. Lillibridge, M.D. Centers for...

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Tornadoes Scott R. Lillibridge, M.D. Centers for Disease Control & Prevention
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INTRODUCTION Background and Nature of the Problem Tornadoes are funnel-shaped wind storms that occur when masses of air with differing physical qualities (e.g., density, temperature, humidity and velocity) collide (1) . These violent rotating winds converge to form a vortex, which is usually narrow at the base and which gives a tornado its typical funnel-shaped appearance. Air and debris are actively drawn into the base of the vortex as the tornado moves across the ground, resulting in a path of destruction. Tornadoes in the northern hemisphere rotate counter clockwise, while those in the southern hemisphere rotate clockwise (2) .
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Winds associated with tornadoes can reach speeds in excess of 250 miles per hour (mph) (1,3) . Because weather conditions that create tornadoes may be present over a large geographic region, tornado outbreaks, defined as six or more tornadoes, may occur within a relatively short period of time (2) . For example, in 1974, an outbreak of 148 tornadoes throughout the eastern United States affected 13 states and resulted in approximately 300 deaths and 6,000 injuries (4) . The cost of property damage caused by a single outbreak can be in excess of 200 million dollars (5) .
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As shown in Table 11-1 , tornadoes are rated by the Fujita-Pearson Tornado Scale (F0 through F5) on the basis of the estimated wind speed of their vortices and the width and length of their paths (3,6) . Most tornadoes (60%) are considered weak (F0,F1), with wind speeds less than 113 mph; these have limited potential to cause injury or destroy property. However, 1-2% of all tornadoes are considered violent (F4-F5), with wind speeds greater than 206 mph; these tornadoes are highly destructive and account for more than 50% of all tornado-related deaths in the United States ( Table 11-2 ).  
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Because the force of a tornado is strongly associated with its potential to cause injury and death, the number of violent tornadoes (F4,F5) per area of land mass may provide a more accurate representation of a state's public health risk from tornadoes. States with high concentrations of F4 and F5 tornadoes include Oklahoma, Indiana, Iowa and Kansas (2) . Among all states, Florida has the highest concentration of tornadoes (tornadoes per 10,000 square miles) ( Table 11-3 ), although tornadoes in Florida tend to be weak (F0,F1) and thus to have limited public health impact.
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toward the northeast at an average ground speed of 40 mph (2) . However, tornadoes have been noted to move along the ground at velocities up to 75 mph, remain stationary, or even reverse course. The average length of a tornado is 4.4 miles and the average width of a tornado is 128 yards. Most tornadoes last for only a few short minutes; however, maximum strength tornadoes have been known to travel more than 200 miles ( Table 11-4 ), persist for hours, and span 3 miles in width
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This note was uploaded on 03/13/2012 for the course PHARM 300 taught by Professor Staff during the Fall '11 term at Rutgers.

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14141 - Tornadoes Scott R. Lillibridge, M.D. Centers for...

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