THE OCCURRENCE OF FROST 



355 



objects to a very low temperature. An equal loss of heat from the same sub- 

 stances when they are loaded with moisture results in only a small lowering of the 

 temperature not only because the water must now be cooled in addition to the 

 ground and vegetation but, as we know, water requires the removal of consider- 

 able heat to cool it sUghtly. The radiation losses from the saturated surfaces 

 may also be less than from the dry surfaces." 



Evidently looseness in application of terms "wet" and "dry" has 

 led to some apparently conflicting results. Petit^*^ records observations 

 that at first seem contradictory to those of Cox, since they indicate 

 higher temperatures over the wetter soil (c/. Tables, 63 and 64). Petit 



Table 63. — Temperatures in Moist and Dry Soils 



{After Petit'^'>) 

 (Degrees Centigrade) 



Date and time 



Saturated soil 



Apr. 23, 4.00 p.m 

 Apr. 23, 7.15 p.m 

 Apr. 24, 5.20 p.m 



21.6 

 16.1 

 6.5 



states that the chief cooling influence in wet soil, evaporation, is inactive 

 at night, that the moist soil conducts heat more rapidly than the dry 

 and therefore can receive more heat from below; he evidently considers 

 that these factors offset the greater radiation he ascribes to wet soil and 

 the lower heat storage during the day. Curiously enough he finds 

 that dew forms earlier and is more abundant on the moist soil. It is 

 possible, however, that Cox and Petit worked with soils of different 

 texture and moisture content and that their results are not necessarily 

 conflicting. 



Table 64. — Surface Temperatures Over Wet and Over Dry Soils 



(After Petit^'^) 

 (Degrees Centigrade) 



Date 



Watered 



Sept. 23 

 Sept. 23 

 Sept. 23 

 Sept. 24 

 Sept. 28 

 Sept. 28 

 Sept. 29 



15.6 

 12.4 

 7.3 

 3.2 

 10.6 

 6.9 

 5.2 



Efifect of Cultivation. — In a series of- observations on the minimum 

 temperatures over cultivated and uncultivated soils at Peoria, 111., it 



