46 DRY-FARMING 



between the 100th meridian on the east to the 

 Sierra Nevada and the Cascades on the west, 

 varies between 55 and 65 per cent. In July, August, 

 and September, the mean values in the South- 

 west sink as low as 20 to 30 per cent, while along 

 the Pacific coast districts they continue about 80 

 per cent the year round. In the Atlantic coast 

 districts, and generally east from the Mississippi 

 River, the variation from month to month is not 

 great. April is probably the driest month of the 

 year. 



The air of the dry-farm territory, therefore, on the 

 whole, contains considerably less than two thirds 

 the amount of moisture carried by the air of the hu- 

 mid states. This means that evaporation from 

 plant leaves and soil surfaces will go on more rapidly 

 in semiarid than in humid regions. Against this 

 danger, which cannot be controlled, the drj'-farmer 

 must take special precautions. 



Sunshine 



The amount of sunshine in a dry-farm section is 

 also of importance. Direct sunshine promotes plant 

 growth, but at the same time it accelerates the 

 evaporation of water from the soil. The whole 

 dry-farm territory receives more sunshine than do 

 the humid sections. In fact, the amount of sunshine 

 may roughly be said to increase as the annual rain- 



