50 e. a. McGregor 



in August but, on the other hand, July was the month of maxi- 

 mum irrigation. From August to November there is a consid- 

 erable downward trend both of water use and humidity, but fol- 

 lowing November another remarkable discrepancy occurs. 

 Whereas the December water distribution drops nearly to the 

 season's minimum, the humidity simultaneously rises practically 

 to the seasonal maximum. 



From this analysis of the progress of humidity throughout the 

 season the writer is forced to the opinion that the successive 

 changes in the atmospheric moisture at the different periods of 

 the year are of a geographically wide-reaching nature, and are 

 not induced by local causes. To test the validity of this assump- 

 tion it occurred to us to procure similar data for some point on 

 the Pacific Coast outside the barrier of the Laguna Mountains. 



00% 



80% - 



Fig. 2. Graph showing annual range of humidity at San Diego, Cal. Average 

 for 1916-1918. 



Through the courtesy of Meteorologist H. F. Alciatore, of the 

 San Diego Weather Bureau office, we secured the humidity data 

 for that station for the identical period of our studies in the 

 valley. 



San Diego is over 100 miles distant from El Centro and owing 

 to the intervening mile-high range, as well as to its situation on 

 the ocean, it might be expected to offer local conditions quite 

 uninfluenced by Imperial Valley factors. A study of figure 2, 

 which graphically presents the seasonal changes of humidity at 

 San Diego, corroborates the writer's previous deductions, namely, 

 that the year's humidity behavior in the Imperial Valley is regu- 

 lated by meteorological conditions of a rather wide geographical 

 influence. From January to May there is a somewhat general 

 drop in the humidity, followed by a rise to the seasonal crest 



