504 



Journal of Agricultural Research voi. xvui, no. io 



summer time than in winter because of the greater difference in tempera- 

 ture between earth and upper atmosphere in the summer. The daily 

 temperature change curve, or thermograph record, is very much flatter 

 in winter than in summer. Figure 3 clearly shows this fact. Because 

 this curve for the same place is continually changing as the season pro- 

 gresses, no direct attempt was made to determine its equation. 



Figure 4 shows thermograph records from four widely separated cities 

 of the United States. It is to be observed that the hottest time of day 

 is about 4 o'clock — it varies with the season — instead of noon, and the 



ao 



TO 



6^ 



^O 



«2K? 



JO 



^c? 



/o 



/o /^ ^ ^ 6 & /o /£> ie ^ s ,9 Ap /^ ^ ^ 



Fig. 3. — Average Utah thermograph records for various seasons. 



coldest time is just before sunrise, instead of at midnight, as is sometimes 

 thought. Heat is being received most rapidly at noon, but the temper- 

 ature will continue to rise until the heat received and the heat lost per 

 second are equal. As soon as the radiation rate exceeds the absorption 

 rate the temperature will fall. Just after noon, even though the rate of 

 absorption of heat is somewhat less than it was, yet it is still in excess 

 of the radiation rate, and hence the net result is a rise in temperature. 

 Inasmuch as this phenomenon is common to all localities, the thermograph 

 records for clear days are very much alike in shape the country over. 

 The convexity of the curve on the rising part or during the morning 



