(2) Inland, from about the 70th meridian eastward to the Atlantic coast, the amount varies between 70 and 75 per cent.

(3) The dry region is in the Southwest, where the average annual value is not over 50 per cent.  In this region are included Arizona, New Mexico, western Colorado, and the greater portion of both Utah and Nevada.  The amount of annual relative humidity in the remaining portion of the elevated district, 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 Southwest 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 humid 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 he 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 rainfall decreases.  Over the larger part of the arid and semiarid sections the sun shines over 70 per cent of the time.

Winds

The winds of any locality, owing to their moisture-dissipating power play an important part in the success of dry-farming.  A persistent wind will offset much of the benefit of a heavy rainfall and careful cultivation.  While great general laws have been formulated regarding the movements of the atmosphere, they are of minor value in judging the effect of wind on any farming district.  Local observations, however, may enable the farmer to estimate the probable effect of the winds and thus to formulate proper cultural means of protection.  In general, those living in a district are able to describe it without special observations as windy or quiet.  In the dry-farm territory of the United States the one great region of relatively high and persistent winds is the Great Plains region east of the Rocky Mountains.  Dry-farmers in that section will of necessity be obliged to adopt cultural methods that will prevent the excessive evaporation naturally induced by the unhindered wind, and the possible blowing of well-tilled fallow land.

Summary