120 



THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



the dotted line representing San Francisco, the dot and dash line San Diego, and the 

 sohd hue the mean of these two. These have been averaged in 11-year periods, just as 

 have the measures of tree growth. Although this coast is 500 miles distant from the 

 .\i-izona trees, and lies beyond the mountains, yet the crests and troughs of the tree growth 

 in Arizona correspond closely to those of the rainfall in Cahfornia. This is not surprising, 

 for while the summer rains of northern Arizona have no relation to the coast of California, 

 the winter precipitation in the two regions varies in harmony. Below the rainfall curve 

 is placed another, showing the average temperature at San Diego during the 11-year 

 periods of the last .50 years. Shorter curves of temperature of other towns of the California 

 coast show the same ^characteristics as that of San Diego. Here we find in the first half 

 a marked similarity to the rainfall curve, especially to that of San Diego. In the second 

 half, however, the "temperature curve finds the minimum satisfactorily, but partially fails 

 to rise to the maximum. Thus in that coast region we find exemplified in the 11-year 

 period a change from a two-crested cycle of rain to a one-crested cycle of temperature. 

 This is not new, for it happens every year in Arizona. That State has a double ramy 

 season, winter and summer, giving, therefore, a yearly rainfall curve with two crests. 

 But its temperature curve, of course, has a high summer crest only. One maximum of rain 

 corresponds to a maximum of temperature, but the other maximum of rain corresponds to 

 a minimum of temperature. Dr. Lockyer has worked out a similar transition froni precipi- 

 tation to pressure in AustraUa, and doubtless it exists in other subtropical regions, and 

 probably in other cycles. The present case shows that it extends outside annual variation. 



Fig. 25.— Sun-spots and the Growth of Trees at Eberswalde, Germany. 



