CLIMATIC CONDITIONS OF THE UNITED STATES. 



311 



Summer march of evaporation at selected stations. — ^The variation in 

 the intensity of atmospheric evaporating power as the season advances 

 at any given station is surely of very great importance in determining 

 the kind and extent of the corresponding variations of developmental 

 changes occurring in plants. These variations are therefore of consid- 

 erable interest in other ways than because of the general novelty of 

 atmometric data. We therefore give, in the following paragraphs, a 

 short discussion of this feature for each of the 10 stations represented 

 by the graphs of figures 3 to 12. These graphs are of gradatory form 

 and each one is double. The upper graph of each pair represents the 

 weekly evaporation rates (in cubic centimeters) from the standard 

 cylindrical porous-cup atmometer, and the lower one represents the 

 weekly precipitation. The dates given below, in each case, denote 

 the endings of the consecutive weeks of the series, and the numbers 

 on the graphs themselves are the ordinate values, representing cubic 

 centimeters of evaporation and inches of rainfall. The features 

 brought out by these figures will now be considered. 



FiQ. 3. — Weekly precipitation and evaporation indices, summer of 1908, Seattle, Washington. 



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FiQ. 4. — Weekly precipitation and evaporation indices, summer of 1908, San Diego, California. 



Seattle, Washington (fig. 3) : In this graph the intensity of atmos- 

 pheric evaporating power attained an early maximum of 200 c.c. 

 (week ending July 13) and then fell generally to the end of our series. 

 The weeks ending October 12 and 19 showed an average rate of 15 

 c.c. per week, which is the minimum rate encountered. In the latter 



