198 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



maps were prepared from United States Weather Bureau records of 

 thunderstorm occurrence at stations scattered over the United States. 

 The records covered a 20-ycar period. One must, of course, know the 

 United States Weather Bureau's definition of a thunderstorm in 

 order to interpret the charts. Their observers were instructed to 

 report a thunderstorm if they could hear thunder. This probably 

 limits the range of each station to a roughly circular area frequently 

 not over 10 miles and probably never over 20 miles in radius, since 

 it is doubtful whether thunder can be heard farther than that. This 

 range would vary greatly from station to station. Stations in noisy 

 locations in large cities would probably report fewer than the actual 

 number of storms on account of the masking of the thunder by other 

 noises. Wliile the data are subject to these limitations, still they are 

 the best available on thunderstorm occurrence in the United States. 

 Alexander has reviewed liis monthly thunderstorm maps as follows: 



During the winter months — December, January, and February — the center 

 of thunderstorm activity for the United States is in the vicinity of Vickshurg 

 (Miss.). In February, however, the general thunderstorm area tends to drift 

 southeastward, with a marked secondary over Pensacola (Fla.). In March the 

 center of activity is still over the lower Mississippi Valley, with the general storm 

 area spreading rapidly northeast over the Tennessee and Ohio Valleys. In Apiil 

 the center appears to be in the vicinity of Shrevepoit (La.), with the general area 

 spreading northeast over a large part of the Eastern States, but also north and west. 



The interesting thing about the May chart is the definite appearance of the 

 primary center over Tampa (Fla.), and a strong secondary over the lower plains 

 States. Great thunderstorm activity now prevails over the entire eastern half 

 of the country, except in the Canadian border States, including the whole of 

 New England. There is also an increased activity in western Montana. 



During June the thunderstorm area continues to spread northward and covers 

 the entire country east of the Rocky Mountains except possibly the extreme 

 northeast. The center of greatest activity is in the vicinity of Tampa. One of 

 the most surprising things revealed by the July chart is the increased activity 

 over the Rocky Mountain States, with a secondary over Santa Fe (N. Mex.), 

 almost as strong as the primary over Tampa. Marked activity also continues 

 in southwestern Montana and in the vicinity of Yellowstone Park. The distribu- 

 tion in August is very much the same as in July, but with a notable decrease in 

 intensity along the Canadian border and a marked weakening of the center over 

 Santa Fc. The two centers, at Tampa and Santa Fe, persist, though weakening 

 through September. In October the southeastern (Tampa) center seems to 

 have dropiKjd a little south and is now over Key West, while the Santa Fe center 

 has disappeared or shifted to eastern Texas and the southern plains States, and 

 the general storm area is rapidly diminishing. In November, as during the 

 winter months, the active area is over the lower Mississippi and Ohio Valleys. 



Chart 13 (reproduced as figure 11 in this article), which shows the average 

 annual number of days vrith thunderstorms during the 20-ycar period at a large 

 number of stations in the United States and Canada, has a number of interesting 

 features and is worthy of considerable study. Note that no part of the country 

 is entirely free from thunderstorms, although they are comparatively rare along 

 the Pacific coast; and that there are two centers of maximum activity, one over 

 Tampa, with an annual average of 94 days with thunderstorms in the 20-year 

 period, and the other over Santa Fe, with an average of 73 during the same period. 



