THE WATER VAPOR OF THE AIR 



223 



If, then, the temperature at 4 p.m. were 65 and the rate 

 of fall 3 per hour, notice the difference in the fall of tempera- 

 ture that would occur with different dew-point conditions 

 (assuming that the dew-point does not change during the 

 night). 



(1) If the dew-point were 29 (this would seldom occur 

 with a temperature of 65 except in dry regions), the tem- 

 perature would fall steadily through the night, reaching 29 

 at 4 a.m. and, probably, 26 or 27 at 6 o'clock. (2) If the 

 dew-point were 38, the temperature would reach it about 

 1 a.m., and the slow fall thereafter would reach 30 or 32 by 

 morning. (3) If the dew-point were 44, the falling tem- 

 perature would reach it before midnight, but would not go 

 lower than 35 by morning. 



Exercise 54. To Construct Typical Daily Temperature and 

 Dew-point Curves 



Figure 152 illustrates the rate of fall for somewhat different con- 

 ditions. The student will prepare another similar diagram, using 



FIG. 152. This figure shows the general or usual relation of temperature 

 and the dew-point throughout the day. The dotted line shows about how 

 the temperature would fall were it not for the heat of condensation set free 

 when dew forms. Note the rather sharp bend in the temperature curve 

 when dew begins to form. 



the same dew-point curve, but drawing a temperature curve showing a 

 maximum of 70 at 2.30 p.m., 65 at 4 p.m., and after that hour a fall 

 of 3 per hour until the dew-point is reached. 



