Grade. 

 Date. . 



(58) EXERCISE 22 



Effect of Evaporation on Soil Temperature 

 Purpose. — To show that soils are cooled when water evaporates from them. 



PLAN 



1. Take two dairy thermometers which agree in their reading. Tie a few 



inches of cloth around the bulb of one thermometer and suspend the 

 lower part of the cloth in a glass of water that is slightly warmer than 

 the room. 



2. From time to time note the difference in readings of the two thermometers, 



and decide how much difference is produced by evaporation of water 

 from the cloth. 



QUESTIONS 



1. Show the analogy between this exercise and the evaporation of water from 



soils. 



2. Would soils be kept cold by water evaporating from them in the spring 



of the year? 



3. How many units of heat are absorbed or dissipated by the evaporation 



of one unit of water (see any text-book on physics). 



4. Why should farmers wishing to warm their soils in early spring, prevent 



the evaporation of water from them? Would harrowing do this? 



5. If harrowing produces a dust mulch and checks evaporation, would the 



free water below the surface be held there or tend to find its way do\A-n- 

 ward? 



6. Would the loosening of the surface crust of the soil tend to check do^^^l- 



ward percolation of soil water or increase it? 



7. Would the loosening of the soil crust increase or retard the entrance of 



warm spring air? 



