PART V — SEVERE STORMS 



strikes produce little if any damage, 

 the lightning being confined to the 

 plane's metal skin. Sometimes, how- 

 ever, potentially serious structural 

 damage, such as the melting of large 

 holes, does occur. There have been 

 two cases of the total destruction of 

 aircraft which the Federal Aviation 

 Administration has attributed to igni- 

 tion of the aircrafts' fuel by lightning. 

 The most recent case was that of a 

 Pan American Boeing 707, which 

 exploded over Elkton, Maryland, in 

 December 1963 after being hit several 

 times by lightning. 



In addition to the radio waves and 

 heating effect produced by lightning, 

 the direct electrical effects of light- 

 ning are often deleterious. They can, 

 for example, result in the disruption 

 of electrical power, as is often the 

 case when lightning strikes a power- 

 transmission line or a power station. 

 Direct electrical effects can also result 

 in malfunction or destruction of criti- 

 cal electronic equipment in aircraft 

 and missiles. A spectacular example 

 of the foregoing was the lightning- 

 induced malfunction of the primary 

 guidance system of the Apollo 12 

 moon vehicle. Further, individual 

 deaths from lightning, about 200 per 

 year in the United States, are pri- 

 marily due to electrocution. 



Control of Lightning 



What can we do to control light- 

 ning? Are there possible harmful 

 consequences of such control? Let us 

 look at the second question first and 

 attempt to answer it by two examples. 

 Suppose technology were advanced 

 enough that we could stop lightning 

 from occurring. What would the 

 result be to forests and the at- 

 mosphere? 



1. If there were no lightning, 

 would the incidence and de- 

 structiveness of forest fires de- 

 crease? In many cases, forest 

 fires would be less common, but 

 those that did occur would be 

 more destructive. Lightning- 



induced forest fires and the 

 forests have lived together in 

 some sort of equilibrium for a 

 a long time. (The oldest ar- 

 cheological evidence of light- 

 ning is dated at 500 million 

 years ago.) There is now some 

 evidence to indicate that fre- 

 quent forest fires will keep a 

 forest floor clean so that the 

 fires that do occur are small 

 and will not burn the trees. 

 Further, in some cases, rela- 

 tively clean forest floors may 

 be necessary for the germina- 

 tion of new trees. For example, 

 Sequoia seedlings can germi- 

 nate in ashes but are suppressed 

 under a thick layer of needles 

 such as would cover an un- 

 hurried forest floor. Thus, it 

 is not obvious that blind control 

 of forest fires is desirable. 



2. If the frequency of lightning 

 were diminished, would there 

 be an effect on the atmosphere? 

 Nobody knows. Lightning cur- 

 rents and other electrical cur- 

 rents flowing in the atmosphere 

 during thunderstorms deliver 

 an electrical charge to the earth. 

 An approximately equal charge 

 (a balancing charge) is thought 

 to be carried from the earth to 

 the ionosphere in areas of fair 

 weather by the ambient fair- 

 weather electric field between 

 the earth and the ionosphere. 

 Changing the lightning fre- 

 quency might upset this charge- 

 transfer balance with a result- 

 ant effect on the fair-weather 

 field. The change in the fair- 

 weather field might trigger 

 further reactions. 



The study of the effects of light- 

 ning on the environment is in its 

 infancy. The control of lightning is 

 not necessarily desirable unless the 

 full consequences of that control are 

 evaluated. 



Now, let us look at lightning con- 

 trol. When "control" is mentioned 

 it is reasonable to think either of (a) 



stopping lightning or (b) harnessing 

 its power. To harness appreciable 

 power from lightning would require 

 a worldwide network which could 

 tap energy from a reasonable fraction 

 of the world's total discharges. Even 

 if science were to devise an efficient 

 way to tap energy from a lightning 

 stroke (which it has not yet done), 

 the construction and maintenance of 

 some sort of worldwide network ap- 

 pears at present to be impractical. 

 On the other hand, stopping lightning 

 from a given storm, or at least de- 

 creasing its frequency, is certainly a 

 practical goal, and some initial steps 

 in this direction have been taken. 

 For example, it has been experi- 

 mentally demonstrated, although not 

 to the satisfaction of everyone con- 

 cerned, that cloud seeding can some- 

 times decrease the number of light- 

 nings produced by a thundercloud. 



Understanding of Lightning 



A number of photographic, elec- 

 trical, spectroscopic, and acoustic 

 measurements have been made on 

 lightning. From these we have a 

 reasonably good idea of the energies, 

 currents, and charges involved in 

 lightning, of the electromagnetic 

 fields (radio waves, light, and so on) 

 generated, of the velocities of propa- 

 gation of the various luminous 

 "streamer" processes by which the 

 lightning discharge forms, and of 

 the temperature, pressure, and types 

 of particles comprising the discharge 

 channel. In short, we have available 

 both an observational description of 

 how lightning works (e.g., the dis- 

 charge is begun by a luminous leader 

 which is first seen at the cloud base 

 and moves toward ground in steps, 

 as shown in Figure V-15) and most 

 of the data needed for routine engi- 

 neering applications (e.g., power-line 

 design and lightning protection). 



A good deal of what we know 

 about lightning has been determined 

 in the United States in the past fifteen 

 years. However, the total number of 

 U.S. researchers primarily studying 



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