112 



FORREST SHREVE 



due to cold air flow than would take place in the relatively small 

 valleys and canons of the mountains. The Arizona Experiment 

 Station is located on the long, gentle mesa slope 85 feet (26 m.) 

 above the flood plain and about 2700 yards (2470 m.) from its 

 edge. The position of the Experiment Station is intermediate 

 between the positions of the two laboratories, but nearer that of 

 the Acclimatisation Laboratory. 



A comparison of the monthly mean minima for the two labora- 

 tories and the Experiment Station for April to October, 1911, is 

 shown in table 1 . The differences between the means for the two 



Average monthly minimum temper- 

 ature, Desert Laboratory 55.7 



Average monthly minimum temper 



ature, Acclimatisation Laboratory 41 . 9 



Difference 13.8 



Average monthly minimum temper 



ature, Experiment Station j 47.1 



Total monthly rainfall, Desert Lab-! 



oratory . 28 



Number of days with 0.01 inch or 

 more of rain, Desert Laboratory J 1 



TABLE 1 



APRIL MAY 



64.0 



46.2 



17.8 



JUNE JULY AUGUST 



74.3 74.1 



59.9 66.4 



14.4 ; 7.7 



75.6 



66.8 



8.8 



53.7 | 63.1 71.3 70. I 

 0.00 0.01 1.96 4.44 



SEP- 

 TEMBER 







I 



8 12 



72.5 



63.6 

 8.9 



OCTO- 

 BER 



58.8 



48.0 

 10.8 



68.1 



2.58* 0.94 



laboratories are greatest in April, May, and June, during the driest 

 and most cloudless months of the year, and fall to about one-half 

 these differences during the rainy months of mid-summer. This 

 indicates that the wetness of the soil, which affects its specific 

 heat, and the frequent cloudiness of the nights in that season, 

 which affects the rate of radiation, cause a greater difference 

 between the different months of the year than do the general 

 temperature differences of summer and winter. The tempera- 

 tures of the Experiment Station lie consistently between those of 

 the two laboratories and are nearer those of the Acclimatisation 

 Laboratory, in harmony with its elevation. 



The greatest difference between any of the daily readings for 

 the two laboratories is 24°F. (13°C.) on May 21, and the least 



