2. TORNADOES 



Status of Tornado Research 



Tornadoes are among the smallest 

 in horizontal extent of the atmos- 

 phere's whirling winds, but they are 

 the most locally destructive. Al- 

 though they are occasionally reported 

 from many places, it is only in the 

 United States that very intense tor- 

 nadoes occur frequently. A typical 

 intense tornado accompanies an 

 otherwise severe thunderstorm, lasts 

 about 20 minutes, and damages an 

 area a quarter of a mile wide along 

 a 10-mile path toward the northeast. 

 The maximum winds (never accu- 

 rately measured) are probably be- 

 tween 175 and 250 miles per hour, 

 but damage is caused as much by a 

 sudden drop of pressure, amounting 

 in extreme cases to about 0.1 of the 

 total atmospheric pressure, or 200 

 pounds per square foot. Especially 

 when structures are poorly vented, 

 roofs and walls are moved outward 

 by the higher pressure within; then, 

 as their moorings are weakened, they 

 are carried off horizontally by the 

 wind. 



During the past 15 years, about 

 125 persons have been killed an- 

 nually by tornadoes. Average prop- 

 erty damage has been about $75 

 million. These figures may be com- 

 pared with estimated losses owing to 

 lightning, hail, and hurricanes as 

 shown in Figure V-6. 



The high tornado death rate in 

 relation to property loss is attribut- 

 able partly to our inability to warn 

 effectively against impending torna- 

 does. A tornado is a very destructive 

 phenomenon, but it usually exists for 

 only a short time and affects only 

 the thousandth part of a region cov- 

 ered by tornado-spawning thunder- 

 storms. Extreme variability is an 

 essential characteristic. Most tornado 

 losses are associated with just a few 



storms that utterly destroy the struc- 

 tures in significant portions of urban 

 areas or in whole small communities. 

 These events, sudden and never fore- 

 shadowed more than a few hours in 

 advance, leave the survivors stunned 

 amid desolation; they call for a sud- 

 den focused response, of a magnitude 

 akin to that demanded in war, by 

 the affected community and by state 

 and national governments. 



Tornado Prediction 



We have noted that the typical 

 tornado accompanies an otherwise 

 severe thunderstorm. Severe thunder- 

 storms are themselves hazards and 

 demand public forecasts, and the 

 possibility of tornadoes is usually 

 indicated when severe thunderstorms 

 are predicted. 



Our forecasts, which must start 

 from a description of the present state 

 of the atmosphere, are less specific 

 than we would like. This lack of 

 specificity is associated in part with a 



lack of knowledge, but also with 

 observations that are too sparse to de- 

 scribe atmospheric variability on the 

 scale of tornado or thunderstorm phe- 

 nomena. Thus, the extent of a severe 

 thunderstorm is 10 to 20 miles and 

 the lifetime of a storm system is 

 generally about six hours. But the 

 distance between first-line surface 

 weather stations is about 100 miles, 

 and between upper air stations about 

 150 miles. Observations are made 

 hourly at the surface stations (more 

 often under special conditions) but 

 usually at only 12-hour intervals at 

 the upper air stations. Therefore, 

 even if our knowledge were otherwise 

 adequate to the task, the observing 

 system would limit us to indicating 

 the probability of thunderstorms in 

 regions much larger than the storms 

 themselves. 



At present, tornadoes are fore- 

 shadowed from one to six hours in 

 advance, for periods of about six 

 hours and in regions of about 25,000 

 square miles. About 50 percent of 

 such predictions are correct, with the 



Figure V-6 — COMPARATIVE LOSSES DUE TO SEVERE STORMS 

 AND HURRICANES 



Loss of life is almost four times greater from severe storms than from hurricanes, 

 while property damage is less than one-half as great. 



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