214 THE WEATHER 



PROBLEMS 



1. An acre of clover loses probably 500 to 750 tons of water per 

 season, largely given off through the plants themselves. What 

 depth of rainfall is required to grow this crop? (Water weighs 62.5 

 Ib. per cu. ft.) Ans. From 4.4 to 6.6. in. 



2. A single corn stalk may lose 300 Ib. of water during the season. 

 At 3 stalks to the hill and the hills 3>^ ft. apart each way, what depth 

 of rainfall is required to furnish this amount of water? Ans. 14.1 in. 



3. An apple tree, 30 years old, may give off 250 Ib. of water per 

 day or 18 tons during the growing season. If there are 40 such 

 trees per acre, how much water would they require per season? 

 What depth of rainfall is needed to replace this water? Ans. 6.034 in. 



242. Evaporation, How Controlled. The amount of vapor 

 given off is controlled by the temperature of the evaporating 

 surface and the space above it, and by the amount of vapor 

 already present over the evaporating surface (see Chap. I, 

 Sec. III). An accumulation of vapor over the evaporating 

 surface checks evaporation; calm air retards evaporation by 

 leaving the accumulated vapor over the evaporating surface. 

 Wind carries away the accumulated vapor and aids evapora- 

 tion. High temperature increases evaporation. 



243. Evaporation, Measurement of. A rough idea of the 

 rate of evaporation may be obtained by placing shallow 

 trays filled with water out of doors and measuring the re- 

 maining depths at intervals of a few days in summer and less 

 frequently in winter. The depth of rain falling into the pans 

 must, of course, be considered. Place one tray in the sun, 

 another in the shade, another where the wind blows freely, 

 and another in calm air, and compare results. Fig. 149 shows 

 the estimated yearly amount of evaporation from a water 

 surface in the United States. Explain the large amount in 

 the southwest. 



244. Evaporation Effects; Cooling. To evaporate a quart 

 of water having a temperature of 32 (i.e., to change it into 

 vapor having the same temperature) requires about 1,075 

 times as much heat as would be needed to raise the tempera- 



