PART V — SEVERE STORMS 



Increasing the Core Diameter — 

 This definitely reduces the maximum 

 tornado windspeed that occurs just 

 outside the core. Modification of hur- 

 ricanes through eye-wall seeding is 

 based on the similar principle in 

 which the release of latent heat 

 around the eye wall will literally ex- 

 pand the eye diameter, thus reducing 

 the extreme pressure gradient around 

 the eye. In the case of tornadoes, it 

 might be possible to cool the lowest 

 portion of the core circulation. If we 

 inject water droplets into the core at 

 a certain level between the ground 

 and the cloud base, they will evapo- 

 rate as they slowly centrifuge out, 

 thus cooling the core to increase the 

 descending motion inside the core. 

 The lower portion of the core will 

 then expand, reducing the maximum 

 windspeed. 



Contrary to older reports, a tor- 

 nado cannot suck up a body of water 

 beneath its core. Investigation of 



ground marks has revealed that the 

 suction power of a tornado is weaker 

 than a suction head of a household 

 vacuum cleaner placed closed to the 

 surface. It is, therefore, necessary to 

 deliver a large amount of water in 

 drop form into the core. 



Reducing Windspeed Near the 

 Ground — This could be achieved by 

 constructing a number of deflectors 

 to the west and southwest of an im- 

 portant structure such as an atomic 

 power plant. The deflectors should 

 be oriented in such manner that they 

 change the southeast winds on the 

 advancing side of a tornado to a 

 northeast wind or possibly to a north- 

 northeast wind, thus creating a flow 

 converging toward the tornado cen- 

 ter. The net effect of the convergence 

 will be to reduce the speed near the 

 surface. Design of deflectors should 

 be made through aerodynamic calcu- 

 lations and a wind-tunnel test. 



Other Activity 



Methods of estimating tornado 

 windspeed should be explored and 

 tested whenever feasible. Direct 

 measurement is desirable if "maxi- 

 mum wind indicators" are to be de- 

 signed to stand against tornado wind. 

 Measurement of object motion inside 

 the tornado does not always give the 

 air motion. Especially when an explo- 

 sion of a structure is involved, the 

 initial object velocity is likely to be 

 overestimated. The designing of a 

 low-priced "minimum-pressure indi- 

 cator" for placement over the area of 

 expected tornado paths is also recom- 

 mended. 



Basic research on tornado modifi- 

 cation also needs to be carried on 

 through various model experiments 

 and theoretical studies. Furthermore, 

 although the probability of tornadoes 

 is small, some important structures 

 must be protected against severe 

 destruction. 



Tornado Forecasting and Warning 



Tornado frequency within the 

 United States varies from 600 to 900 

 per year, with the major concentra- 

 tion through the Central Plains. 

 Ninety percent of all tornadoes have 

 a path-length between 0.5 and 50 

 miles and path-width between 40 and 

 800 yards. The median tornado has 

 a path-length of 5 miles with a path- 

 width of 200 yards. The median 

 destructive period is less than 30 

 minutes. Less is known about tor- 

 nado velocity profiles, but one can 

 estimate that 90 percent of the peak 

 speeds are between 100 and 225 miles 

 per hour, with a median peak velocity 

 of 150 miles per hour. Unfortu- 

 nately, the upper limit appears to be 

 around 300 miles per hour. 



Thus, the problem is to forecast 

 the occurrence of a rare meteorologi- 

 cal event which has median dimen- 

 sions of one square mile over a 

 30-minute period, and to forecast it 



sufficiently far in advance to allow 

 effective use of forecasts by all in- 

 terested parties. There should be 

 suitable differentiation for tornado 

 classes based on width, length, and 

 peak velocity. None of the above is 

 possible at this time for areas of 

 less than several thousand square 

 miles and for more than one hour in 

 advance. 



Matters Contributing to the 

 Forecast Problem 



Data Network — The average dis- 

 tance between full-time surface re- 

 porting stations is 100 miles. Reports 

 are made every hour, oftener when 

 special criteria are met. Unless the 

 special report is taken and trans- 

 mitted near a free time-period in the 

 teletype schedule, it is quite probable 

 that the report will be delayed 10 

 minutes in reaching the user. Thus, 



the spacing and frequency of reports 

 taken with the standard data network 

 is not adequate to fully describe the 

 severe weather events taking place 

 within the confines of the data net- 

 work. 



The average distance between 

 upper air stations is 150 miles — and 

 slightly more than that in the areas 

 of high tornado incidence. Rawin- 

 sonde releases are scheduled only 

 every 12, and on occasion every 6, 

 hours. But the 1200 Greenwich Mean 

 Time (GMT) release is made in the 

 Midwest at 6 a.m. Central Standard 

 Time (CST), a minimum thunder- 

 storm period, while the midnight 

 GMT release is made at 6 p.m. CST, 

 a maximum thunderstorm period. Ef- 

 fectively, this produces only one use- 

 ful report per day per station. These 

 reports are not adequate to fully de- 

 scribe the temperature, moisture, and 

 wind patterns within the tropo- 



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