than on hillsides, owing to the downward drainage of the cooled air.
Further, the danger of frost increases with the altitude. In
general, the last killing frost in spring over the dry-farm
territory varies from March 15 to May 29, and the first killing
frost in autumn from September 15 to November 15. These limits
permit of the maturing of all ordinary farm crops, especially the
grain crops.

Relative humidity

At a definite temperature, the atmosphere can hold only a certain
amount of water vapor. When the air can hold no more, it is said to
be saturated. When it is not saturated, the amount of water vapor
actually held by the air is expressed in percentages of the quantity
required for saturation. A relative humidity of 100 per cent means
that the air is saturated; of 50 per cent, that it is only one half
saturated. The drier the air is, the more rapidly does the water
evaporate into it. To the dry-farmer, therefore, the relative
humidity or degree of dryness of the air is of very great
importance. According to Professor Henry, the chief characteristics
of the geographic distribution of relative humidity in the United
States are as follows:--

(1) Along the coasts there is a belt of high humidity at all
seasons, the percentage of saturation ranging from 75 to 80 per
cent.

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