THE THUNDERSTORM— EVANS AND McEACHRON 



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FiQUEE 10.— Monthly isokeraunic maps based upon the total number of thunderstorm days occurring each 

 month over the 20-year period 1904-23. Divide the numbers on the maps by 20 to obtain the average 

 number of thunderstorm days to be expected per month. (After Alexander.) 



