170 ANNUAL REPORT SMITHSONIAN INSTITUTION,, 19 42 



result of certain progressive wavelike movements of certain disturbed 

 areas, orginating in different parts of the world. With each cycle of 

 change in solar activity, the centers of high barometric pressure move 

 from high latitudes to low latitudes and back again. The amplitude 

 of their oscillations and the speed with which these waves progress 

 appears to be inversely proportional to the length of the period of 

 oscillation. 



In years of unusually high sunspot maxima, as was the case in 1937, 

 areas of high pressure appear to be pushed farther northward. The 

 return of these highs to low latitudes with accompanying colder and 

 clearer weather may, he believes, be so retarded under such instances 

 as to invert the phase of a cycle that may have persisted for some 

 time while the amplitudes of the oscillations were of less magnitude. 

 Thus there will occur several years when the differences in barometric 

 pressure between the equatorial region and North Temperate Zone 

 become greater than normal, to be followed by several years when the 

 pressure differences become less than normal. The shifting of these 

 centers of action, Clayton believes, is definitely associated with sun- 

 spots. 



Various attempts have been made to attribute climatic cycles to 

 changes in solar activity. Perhaps the most outstanding scientific 

 contribution in this direction has come from Prof. A. E. Douglass, of 

 the University of Arizona, who has spent a lifetime measuring varia- 

 tions in tree growth, especially in the forests of the Southwest and 

 in California. Douglass noted that sequences in periods of rapid 

 growth of trees, as measured by the widths of their rings, follow very 

 closely the sequences in the sunspot cycle. Since variations in tree 

 growth suggest variations in precipitation, he has accumulated a vast 

 amount of evidence for alternations of wet and dry periods variable 

 with the sunspot cycle, carrying records backward for some 3,000 

 years. His studies appear to indicate that at least for selected regions, 

 trees have shown most growth when sunspots were most numerous. It 

 does not appear improbable, however, that the growth of trees inte- 

 grates all favorable conditions and that temperature, the quality of 

 sunlight, and the amount of ultraviolet radiation all enter into the 

 growth rate of trees as well as does rainfall. 



Sunspot periods have also been traced with minor discrepancies in 

 the flow of rivers and the level of lakes, some regions responding much 

 more clearly than others to the sunspot cycle. 



Altogether we see there are many indications that the earth re- 

 sponds to the changing state of the sun over an interval of a little 

 over 11 years and often by double this period or approximately 23 

 years. Whether all the effects produced in the earth and its atmos- 

 phere that are noticed at sunspot maxima are the result of the sun- 

 spots themselves or whether the state of the sun and its whole sur- 



