NFNG 



Figure V-15 — THE INITIATION OF A LIGHTNING STROKE 



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 (Illustration Redrawn with Permission. BEK Technical Publications, Inc. Carnegie. Pa) 



The drawing shows the initiation of a stepped-leader from a cloud base. The time 

 involved is about 50 millionths of a second. As the downward-moving leader gets 

 close to the ground, upward-moving discharges meet it. A return stroke then propa- 

 gates from the ground to the cloud. The time for the return stroke propagation is 

 about 100 millionths of a second. Propagation is continuous until the charges are 

 dissipated. 



lightning at any given time during 

 this period has been only about ten, 

 of which perhaps half have contrib- 

 uted to our understanding of light- 

 ning. As an example of the general 

 lack of scientific interest in lightning 

 phenomena, the first technical book 

 on lightning was not published until 

 1969. 



While we have available a number 

 of observational "facts" about light- 

 ning, we do not understand lightning 

 in detail. Areas of particular igno- 

 rance are: (a) the initiation of light- 

 ning in the cloud and (b) propaga- 

 tion of lightning from cloud to 

 ground. Unfortunately, these are just 

 the areas in which a detailed under- 

 standing is essential if lightning con- 

 trol is to be practiced. 



It is important to know what we 

 mean by a "detailed understanding." 

 A "detailed understanding" implies a 

 mathematical description or model of 

 the lightning behavior. The mathe- 

 matical model is adequate when it can 

 predict the observed properties of 

 lightning. The mathematical model 

 can then be used to determine the 

 effects on the lightning of altering 

 various parameters of the model. 

 For the case of lightning initiation, 

 these parameters might be the am- 

 bient temperature, ambient electric 

 field, number of water drops per 

 unit volume, etc. The predictions of 



the mathematical model must be 

 tested by experiments. The results 

 of these experiments can suggest 

 changes in the model or can verify 

 its validity. It follows that experi- 

 ment and theory must advance to- 

 gether to achieve a complete descrip- 

 tion of the lightning phenomenon. 



The physics of lightning initiation 

 and propagation is exceedingly com- 

 plex. Some idea of its complexity can 

 be gauged by noting that the proc- 

 esses involved in electrical breakdown 

 between a rod and a flat plate in the 

 laboratory (an electric spark) are at 

 present only vaguely understood. It 

 appears that, despite about thirty 

 years of experimental work, a real 

 understanding of the laboratory spark 

 will not be available until a mathe- 

 matical description of the spark is 

 forthcoming. Only recently have 

 digital computers become available in 

 a sufficient size that a mathematical 

 solution to the spark problem is in 

 principle possible. 



The Future 



Significant progress in our detailed 

 understanding of lightning could 

 probably be made in the next ten to 

 fifteen years, although given the pres- 

 ent level of scientific activity and 

 ability in the lightning area, it is 

 unlikely that this will be the case. 



Lightning research has been neither 

 glamorous enough nor quantitative 

 enough to attract the attention of 

 many good graduate students or 

 senior scientists. Several excellent ex- 

 perimentalists are presently working 

 in the lightning area, and their work 

 needs to be continued and enlarged. 

 More important to the goal of de- 

 tailed understanding of lightning, 

 however, is the need for mathemati- 

 cally oriented scientists to become 

 involved in the problems of lightning 

 initiation and propagation. The 

 mathematically oriented scientists and 

 the experimentalists should work 

 closely together in both the construc- 

 tion of suitable mathematical models 

 and in the planning and analysis of 

 experiments. 



In studying lightning, the time- 

 scale on which meaningful results 

 can be expected is relatively long. 

 From an experimental point of view, 

 the necessity of staying in a given 

 location for a long enough time to 

 observe enough lightning to be able 

 to compile statistically significant re- 

 sults determines the time-scale of any 

 particular lightning research pro- 

 gram — generally, several years. The 

 mathematical approach to lightning is 

 exceedingly complex and thus must 

 also take place on a time-scale of 

 several years. With a coordinated 

 work force of perhaps five senior 



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