Nov., li)21 THUNDERSTORMS 33 



greater the danger. No tree is immune but those trees having 

 an extensive root system or a deep tap-root are most apt to be 

 struck because they are the best grounded and therefore offer 

 the least electrical resistance. Then again if one is caught 

 out of doors and is exposed to a violent thunderstorm it is 

 best so far as danger from lightning is concerned, to let one's 

 clothes get soaking wet, because wet clothes are much better 

 conductors and dry clothes much poorer conductors, than the 

 human body. It might even be advisable to lie flat on the 

 wet ground, undignified as this may be. For any given local- 

 ity, the lower the cloud the greater the danger; hence, when 

 the humidity is high it is favorable for a dangerous storm, since 

 the cloud will form at a low level and the rain is apt to be very 

 abundant. For the same reason a winter storm is likely to be 

 more dangerous than a summer storm of equal intensity. 

 And now how do we account for the thunder — that par- 

 ticular feature that gives name to our storm? It has taken 

 quite a while to answ^er this question satisfactorily. Many 

 very silly theories still persist. The electrical discharge, the 

 "lightning," furnishes the key to the explanation. The sudden 

 and intense heating of the air along the path of the discharge 

 causes it to expand suddenly and violently, sending out from 

 every part of its path a steep compression wave, which, as we 

 understand it, is the real cause of the thunder. The "rumbling" 

 that sometimes follows is due, chiefly perhaps, to the inequality 

 in the distances from the observer to the various portions of the 

 lightning's path, to the crookedness of the path, to a succession 

 of discharges, and to some extent to reflection under favorable 

 conditions. The distance to which thunder may be heard 

 varies from 7 to 15 miles. 



9. Forecasting the Thunderstorm. 



The forecasting of conditions favorable for the formation 

 of thunderstorms one or two days in advance is comparatively 

 easy but to say, even a few hours in advance, that a thunder- 

 storm will occur at a given place, at or about a given time, is, 

 to say the least, a hazardous venture. It is only after the 

 storm has actually begun and its direction and rate of move- 

 ment have been determined, can one speak with even a small 

 degree of assurance. As every one knows, a storm may occur, 

 in fact several of them, in sight of the observer and yet not at 



