132 CLIMATIC CYCLES AND TREE GROWTH 



heat to come through than before the increase. What is the balance of effect? 

 Again, half of the heat received from the sun falls in equatorial regions be- 

 tween 30° north latitude and 30° south. The cloudiness at the poles with 

 ice and snow reduces the effective heat at high latitudes. Atmospheric 

 circulation is induced with the motive force applied in equatorial regions. 

 The earth is rotating and the cold polar areas are small and north temperate 

 zones have continents and oceans. All these conditions make the distribu- 

 tion of solar energy with movements of air and water a most complex opera- 

 tion and many investigators ask why there should be any system anywhere. 



Newcomb, thirty years ago, took averages over the earth as a whole and 

 concluded that there was no evidence of connection between meteorological 

 changes and the sun ; Bigelow found that solar effects can be positive in some 

 regions and negative in others. That statement of Bigelow's opened the 

 way for tree-ring studies in Arizona and our effort to find the solar cycle in 

 the Arizona trees. 



The disturbing feature in all comparisons between solar and terrestrial 

 cycles has been the presence of other cycles on the earth of very different 

 lengths and only rarely one of 11 years. The cyclogram method of analysis 

 helps to solve this difficulty by the cycle complex and its correspondence 

 to actual cycles in the sun. We feel justified in assuming the hypothesis 

 that there is a physical relationship between our climatic conditions and 

 the sun. 



