196 ANNUAL JIEPORT SMITHSONIAN INSTITUTION, 1937 



Figure 9.— Monthly isokeraunic maps based upon the total number of thunderstorm days occtirring 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.) 



