

be in excess of 500 miles per hour. 

 Some even estimated a supersonic 

 speed. 



Damage investigation since then 

 has reduced general vvindspeed esti- 

 mates to between 300 and 500 miles 

 per hour. If these maximum-speed 

 estimates are accurate, they would, 

 where combined with the storm's 

 pressure reduction, make it impossi- 

 ble to construct tornado-proof struc- 

 tures at reasonable cost. 



Fujita's study of tornadoes during 

 the past ten years, however, has now 

 led to the conclusion that the maxi- 

 mum windspeed of tornadoes is much 

 less than previously thought. Maxi- 

 mum rotational windspeeds, as esti- 

 mated from scaling motion pictures 

 and characteristic ground marks, are 

 about 200 miles per hour. The trans- 

 lational motion of the storm must be 

 added to the right side and sub- 

 tracted from the left side of the rotat- 

 ing core. If a tornado travels at its 

 average speed of 40 miles per hour, 

 the maximum combined speed above 

 the frictional layer would be 240 

 miles per hour. Some tornadoes, such 

 as the ones on Palm Sunday, 1965, 

 traveled eastward at 62.5 miles per 

 hour. For these storms, the maximum 

 combined windspeed would be 260 

 miles per hour. Inside the boundary 

 layer, the gust speed must be added 

 to the mean flow speed, which de- 

 creases toward the ground. Under 

 the safe assumption that the peak 

 gust speed could overpass the de- 

 crease in the flowspeed toward the 

 ground, a maximum gust speed of 

 300 miles per hour seems to be quite 

 reasonable. Thus, one has: 



Maximum rotational speed. . . .200 mph 



Maximum traveling speed 70 mph 



Maximum gust speed 300 mph 



It should be noted that the higher 

 estimated speeds were obtained by 

 assuming the cycloidal ground marks 

 were produced by one rotating object. 

 Fujita's study has indicated that there 

 are 3 to 5 spots which produce cy- 



cloidal marks. Thus, the speed for 

 any one tornado of a family must be 

 reduced bv one-third to one-fifth. 



Minimum Pressure Inside 

 Tornadoes 



As in the case of tornado wind- 

 speed, in earlier days pressure reduc- 

 tion at the center of tornadoes had 

 been overestimated to be a near 

 vacuum or 2,000 pounds per square 

 foot. Since then, meteorologists have 

 tended to agree that the pressure re- 

 duction at the storm center is between 

 200 and 400 millibars. 



It should be noted that a building 

 will suffer also from differential pres- 

 sure from its form resistance. A 300 

 miles per hour wind will produce a 

 positive stagnation pressure of about 

 90 millibars at its windward side. 

 Over the roof, however, the pressure 

 may be negative, with the result that 

 the roof is lifted. (The lifting force 

 cannot be estimated unless the com- 

 plete shape of the building is given 

 and a wind-tunnel test is performed.) 



Potential Tornado Protection and 

 Modification 



As a result of more recent wind- 

 speed and pressure estimates, criteria 

 for designing tornado-resistant struc- 

 tures have now become feasible. Such 

 structures could be expensive, al- 

 though future designs and improved 

 material could reduce costs to a level 

 where at least public buildings in a 

 tornado alley could be built to with- 

 stand tornado wind and pressure. 



Tornadoes vary in both shape and 

 size. The most commonly observed 

 four shapes are: 



Cone shape: Large tornadoes drop 

 down in the shape of a cone; as 

 the storm develops, the tip of 

 the cone reaches the ground. 



Column shape: A tornado or a 

 large waterspout takes the shape 

 of a large trunk. 



Chopstick shape: Thi 



shape of weak tornadoes and 

 waterspouts with small diame- 

 ters. 



Rope shape: When tornadoes be- 

 come very weak, they change 

 into a rope which often extends 

 miles in a semi-horizontal direc- 

 tion. 



Although tornadoes have such 

 different shapes, all tornadoes and 

 waterspouts are characterized by a 

 core circulation surrounded by a cir- 

 cle of maximum wind. Outside this 

 circle, the tangential windspeed de- 

 creases in inverse proportion to the 

 distance from the circulation center. 



Chopstick- or rope-shaped tunnels 

 may be considered axially symmetric. 

 When the core diameter increases, as 

 in the case of the cone and trunk 

 shapes, there are several spots of 

 strong suction around the edge of the 

 core; thus, they are no longer axially 

 symmetric. These spots of strong 

 suction rotate around the funnel at 

 the speed of the funnel rotation. 



Three ways of modifying tornado 

 windspeed may be considered. They 

 are: (a) a reduction of the circulation 

 energy; (b) an increase in the core 

 diameter without changing the circu- 

 lation intensity; and (c) reduction of 

 the windspeed near the ground. 



Reducing the Circulation Energy 

 — This possibility depends on the 

 counteracting energy that can be cre- 

 ated artificially. The total kinetic 

 energy of a tornado is on the order 

 of 10 7 kilocalories, which is just about 

 1/1,000 of a small, 20-kiloton atomic 

 bomb. The energy of even the largest 

 of tornadoes is comparable only to 

 1/100 of the energy in a small atomic 

 bomb. Atomic bombs obviously can- 

 not be used to modify a tornado. 

 We might however, investigate such 

 power sources as an artificial jet in 

 order to learn more about how the 

 relatively small and concentrated en- 

 ergy of a tornado might somehow 

 be dispersed. 



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