CORRELATION WITH RAINFALL. 73 



into meteorological districts. Yet, in spite of local differences, mountain 

 regions may be alike in major characteristics, for all the Prescott 

 groups, though differing among themselves, cross-identify excellently 

 with the Flagstaff trees 60 miles away. The sequoias also cross- 

 identify perfectly in mountain localities 50 miles apart, showing that 

 there is enough similarity in different parts of the high Sierras to cause 

 the trees to agree in many variations. 



Arizona and California. Fully 450 miles intervene between the 

 sequoias of California and the pines of Arizona, yet there are strong 

 points of identity between them in the last 300 years. The dates of 

 notably small rings are much alike in each. The details of this com- 

 parison have not yet been fully studied, but they support the idea long 

 since expressed (1909) that Arizona and California, especially its 

 southern half, form parts of a large district which has similarity in 

 certain variations. A long acquaintance with this region throws light 

 on the details of this similarity. The winter precipitation, which is 

 largely in the form of snow at the altitude of the trees studied, has the 

 major influence on tree-growth, for it is largely conserved near the 

 trees, whereas the summer rains are usually torrential and the water 

 quickly flows away. The winter storms moving in an easterly direction 

 reach the coast region first and after about 24 hours are felt in Arizona. 

 Thus, in spite of the coast range of mountains and the intervening 

 low-level deserts, each winter storm passes over both regions and 

 causes an evident similarity between them. In a large view they belong 

 to a single meteorological district. 



Meteorological districts and solar correlation. In searching for a 

 link of connection between solar variation and meteorological changes, 

 we must bear in mind the effect of possible reversals in neighboring 

 meteorological districts, such as noted above in Norway. It may be 

 the lack of such precaution which has caused many meteorologists to 

 condemn at once the suggested connection between the distant cause 

 and the nearby effect. We must remember that districts may be small 

 in area, and in combining many together we may neutralize the result 

 for which we are in search. Some illustration of correlation found in 

 small districts will be given in the final chapter. 



