196 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



per unit area occur directly under the storm center. The number 

 of hits per unit area decreases rapidly until there are no hits at 

 about 25 per cent of the cloud height away from the center. AVith 

 a rod O.G per cent of the cloud height, 10 per cent of the strokes 

 strike the rod (fig. 37). There is a protected area where no ground 

 hits occur (fig. 38). The maximum ground hits per unit area, 0.7 

 per cent, occur at about 8 per cent of the cloud height from the 

 center. No ground hits occur at 25 per cent of cloud height from 

 the center. 



CHANCE OF BEING STRUCK 



The above data offer a m«ans of estimating the relative chance 

 of objects of different heights being struck during a thunderstorm 



^^ 



V .-v. 



Area Protected by 

 Lightning Rods. 



Hits per Unit Area 

 per Hundred Strokes. 

 Rod Directly Under 

 Storm Center 



Curves may be read 

 either directly in 

 the inch dimensions 

 of the model or in 

 per cent of Cloud 

 Hei^t. 



1 



2 3 4 5 6 7 8 9 W II Inches 

 5 10 15 20 25loCloud Height 



Distance of Hits from Rod 



Fig. 38 



when the cloud is overhead and of sufficient voltage to discharge 

 to earth. 



Assume a cloud 1,000 feet high. A 6-foot man on a plane directly 

 under the storm center (from fig. 37) would be hit 15 times out of 

 everj-^ 100 strokes, while a 25-foot building would be hit every time. 

 A man flat on the ground (from fig. 38) would be struck about once 

 for every hundred strokes. An 18.5 foot building directly under 

 the storm center would be struck 84 times out of 100 hits. How- 

 ever, with the storm center moved only 206 feet this building would 

 not be struck. 



