TART V — SEVERE STORMS 



Tornadoes — Their Forecasting and Potential Modification 



A tornado, also called cyclone or 

 twister, is defined as a violently rotat- 

 ing column of air, pendant from a 

 cumulonimbus cloud, and nearly al- 

 ways observable as a funnel. The 

 shape of a funnel varies from a cone 

 to a rope; its lower end does not 

 always touch the ground. A con- 

 firmed small tornado could be char- 

 acterized by a damage area of 10,000 

 square feet, while the swath of a 

 giant tornado covers more than 30 

 square miles. Thus, a giant tornado 

 could be 50,000 times larger than a 

 tiny one in terms of potential damage 

 area. 



The annual tornado frequency 

 changed from a minimum of 64 in 

 1919 to a maximum of 912 in 1967, 

 which represents a ratio of 1:14. This 

 does not mean that tornado frequency 

 increased by at least one order of 

 magnitude. Instead, reporting effi- 

 ciency — related to the reporting sys- 

 tem, urban development, population 

 density, and such — probably in- 

 creased the apparent tornado fre- 

 quency. It is preferable, therefore, to 

 evaluate the potential danger of tor- 

 nadoes according to damage areas 

 rather than their number of occur- 

 rences. 



Damaging Tornadoes 



When a tornado warning is issued, 

 the general public will be looking for 

 the nearest storm shelter for protec- 

 tion of life. Statistics show, however, 

 that 50 percent of the total tornado 

 damage area is produced by only 4 

 percent of the tornadoes. This means 

 that half of the potential damage 

 area can be warned efficiently if the 

 top 4 percent of tornadoes are pre- 

 dicted with great accuracy. If the top 

 10 percent of tornadoes can be pre- 

 dicted, their damage area would cover 

 75 percent of the total damage area. 

 Although these statistics do not sug- 

 gest that only large tornadoes should 



be predicted to the neglect of others, 

 accurate prediction of large tornadoes 

 would be of great value to local 

 residents. 



Small Tornadoes — The origin of 

 large, long-lasting tornadoes seems to 

 be quite different from that of the 

 tornadoes at the small end of the size 

 spectrum. Small tornadoes and water- 

 spouts are so similar in dimension and 

 appearance that the former can be 

 regarded as waterspouts traveling 

 over land. These small storms, al- 

 though they make up a large number 

 of all storms, are very difficult to 

 predict. They may form within a local 

 shear line associated with growing 

 cumulus clouds that may or may not 

 become thunderstorms. Small torna- 

 does last only a few minutes, leaving 

 a damage swath of only a few miles. 



Hook-Echo Tornadoes — Large tor- 

 nadoes frequently last 30 to 60 min- 

 utes. Furthermore, in many cases 

 several tornadoes of similar size and 

 intensity appear one after another, 

 thus forming a family of large torna- 

 does. When radar pictures of proper 

 gain and of low elevation angles are 

 examined, almost all tornadoes in 

 such a family are related to a thunder- 

 storm echo with rotational character- 

 istics — i.e., a rotating thunderstorm is 

 a spawning place for one to several 

 large tornadoes. 



When the view is unobstructed, a 

 rotating thunderstorm can be photo- 

 graphed at large distances as a bell- 

 shaped cloud with an over-all diame- 

 ter of 5 to 25 miles. The same cloud 

 would appear in a plan-position- 

 indicator (PPI) radarscope as a "hook 

 echo," with an eye at the rotation 

 center and several echo bands spiral- 

 ing around the eye-wall circulation. 

 Despite the fact that a family of 

 tornadoes comes from a rotating 

 thunderstorm, not every rotating 

 thunderstorm or hook echo spawns a 

 tornado during its lifetime. It is likely 



that only a maximum of 50 percent 

 of hook echoes spawn tornadoes — 

 usually large ones. Hook-echo torna- 

 does are responsible for more than 

 half of the damage areas caused by 

 all tornadoes. 



Detecting Large Tornadoes — The 

 above evidence leads to the conclu- 

 sion that large tornadoes spawn from 

 mesoscale vortex fields identified as 

 rotating thunderstorms, hook echoes, 

 or tornado cyclones. The outermost 

 diameter of such a vortex ranges be- 

 tween 5 to 25 miles. The eye, sur- 

 rounded partially or totally by a hook- 

 shaped echo, rotates at the rate of 

 20 to 40 miles per hour at its outside 

 edge and is 1 to 3 miles in diameter. 

 The central pressure of a tornado- 

 bearing mesoscale vortex or tornado 

 cyclone is only 2 or 4 millibars lower 

 than its far environment. An imprac- 

 tically large and expensive network 

 of barograph stations would be re- 

 quired for detecting tornado cyclones. 

 Unless a doppler radar network be- 

 comes available in the future, PPI- 

 scope pictures in iso-echo pres- 

 entation with better than one-mile 

 resolution will provide the only means 

 of detecting tornado cyclones within 

 some 10 minutes after their formation. 



Early detection of tornado cyclones 

 is the key to a warning within a 

 narrow zone in which there is a 

 chance of tornado formation. Such 

 an alley is only 5 miles wide and 50 

 miles long on the average, while a 

 tornado watch area extends 50 x 100 

 miles, some 20 times larger than one 

 alley area. 



Maximum Tornado Windspeed 



Windspeed is an important pa- 

 rameter, necessary for the design of 

 tornado protective structures. When 

 settlers first experienced the impact 

 of tornadoes in the Midwest, they 

 estimated maximum windspeed to 



144 



