September 8, 1916] 



SCIENCE 



355 



ning and under-running winds. This leads to 

 the winter and spring thunderstorms; particu- 

 larly in the southern Mississippi Valley where 

 the lower air is warmest and dampest. The 

 cooling of the upper air while the lower re- 

 mains relatively warm is characteristic of a 

 marine location. With the aid of cyclones, 

 thunderstorms produced in this way are to be 

 expected in winter and at night. The Pacific 

 coast region thus tends to have its thunder- 

 storms, few at most, in winter. 



For illustration, the accompanying table 

 shows the monthly percentage frequencies of 

 days with thunderstorms at seven stations in 

 the United States. 4 A thunderstorm day is 

 now defined as one on which thunder is heard 

 whether or not rain falls at the observing sta- 

 tion. 



■agS 

 "13 



San Francisco, 

 Cal 



Fresno, Cal.. 



Boston, Mass... 



New York, 

 N. Y 



Chicago, 111.. 



Santa F6, 

 N. Mex 



Tampa, Fla.... 



37 



180 



284 

 400 



732 

 944 



At San Francisco, atmospheric instability 

 does not often occur in summer. Fresno has 

 its maximum early probably because the air is 

 too dry in mid-summer. The other stations 

 have the greatest number in summer. Boston, 

 New York and Chicago all have an abundance 

 of moisture. The greater number of thunder- 

 storms in Chicago for the year, and particu- 

 larly in spring, as compared with New York 

 and Boston, is due to its continental position 

 and exposure to rapid temperature changes. 

 The interior location favors more rapid warm- 

 ing in spring than is the case in the east. 

 Even New York appears markedly more con- 

 tinental than Boston. It is noteworthy that 

 there are more thunderstorms in May than in 

 September: May is moister; and the upper 

 air is colder. The great thunderstorm activity 

 at Santa Fe is favored by the mountain loca- 



tion (altitude 7,013 feet) east of the Kio 

 Grande. In June, July and August there is, 

 on the average, a thunderstorm every other 

 day. Thunderstorms are less than half as fre- 

 quent at the drier, lower places such as El 

 Paso. Tampa has more thunderstorms than 

 any other weather bureau station in the United 

 States. In the three summer months, thun- 

 derstorms occur on about two days out of 

 three. The summer on-shore winds supply 

 abundant moisture and the intense sunlight at 

 this low latitude effectively overheats the lower 

 air. Thus the joint distribution of atmos- 

 pheric instability and moisture dominate 

 thunderstorm frequency. 



Parts of Professor "Ward's abstract are 

 quoted here: 



" As essential characteristics of American 

 climate, thunderstorms have a broad human 

 interest. From the viewpoint of climatology, 

 the distribution of thunderstorms is of more 

 interest than their mechanism. The part 

 played by their rains in watering our crops 

 is of greater importance than the size of the 

 raindrops. The damage done by their light- 

 ning 5 and hail 6 concerns us more than the 

 cause of the lightning flash or than the origin 

 of the hailstorms. The thunderstorms of the 

 eastern United States are among the most 

 characteristic of American climatic phenom- 

 ena. In size, intensity and frequency of oc- 

 currence they are unique. 



" In relation to man's activities, it is of sig- 

 nificance that most thunderstorms occur at a 

 time of year and at the hours when outdoor 

 activities are at their height. 



" Thunderstorms bring us much that is of 

 benefit. To them we owe much, in parts of 

 our country even most, of our spring and 

 summer rainfall. Without these beneficent 

 thunderstorms our great staple crops east of 

 the Bocky Mountains would never reach ma- 

 turity. One good thunderstorm over a con- 

 siderable area at a critical crop stage is worth 

 hundreds of thousands of dollars to American 

 farmers. Our stock markets time and again 



5 See note on ' ' Thunder and Lightning 

 ence, N. S., Vol. XLII., 1915, p. 252. 

 e See note on ' ' Hail, ' ' below. 



Scr- 



