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 dry-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- 
