10 



FIELD OPERATIONS OF THE BUREAU OF SOILS, 1017. 



The precipitation depends somewhat ii})on the h)cati()n with refer- 

 ence to mountains and canyons. The summer rains or showers are 

 often very local, and in the form of heavy downi)ours accomi)anied 

 by lightning and thunder, while the winter rains are more general 

 and fall more gently over a longer period of time. The summer 

 rains are of little benefit to agriculture directly and they hinder har- 

 vesting and fruit-drying operations, but they are important from 

 the standpoint of irrigation. It will be noticed from the tables that 

 to some extent wet cycles or years seem to be followed by extremely 

 dry ones. During the wet cycles there is- more underground water 

 present and the lands are farmed for the time being by irrigation 

 with pumped water, wdiich during dry periods is scanty. 



The following table shows the mean annual and the absolute maxi- 

 mum and minimum temperatures for several stations in different 

 parts of the area : 



Average annual temperature and extremes of temperature at Casa Grande 

 Ruins,^ Florence, and Sacaton. 



' Now Casa Grande National Monument. 



The mean annual temperature ns recorded at Phoenix, for an 8- 

 year period from 1896 to 1903, inclusive, was 70°. The absolute 

 maximum for Maricopa is 126°, and this is the highest temperature 

 on record in this area or vicinity. Temperatures ranging between 

 110° and 120° are recorded yearly, and the temperature frequently 

 remains above 100° for periods of several days or several Aveeks dur- 

 ing the summer. During the early summer the nights are cool, but 

 the high relative humidity accompanying the extremes in tempera- 

 ture during the rainy season of the middle and late summer makes 

 this part of the year uncomfortable. The dry part of the year is 

 one of ^remarkably low relative humidity, xllthough the tempera- 

 tures during the greater part of the year are moderate, during the 

 winter and spring low extremes of temperature for this part of the 

 United States are recorded. For instance, absolute mininuun re- 



