Dry-Farming | Page 9

the west slope of the Sierra Nevadas to the
east slope of the Rockies. The degree of snowfall on the agricultural
lands is very variable and dependent upon local conditions. Snow falls
upon all the high mountain ranges.
Temperature
With the exceptions of portions of California, Arizona, and Texas the
average annual surface temperature of the dry-farm territory of the
United States ranges from 40 deg to 55 deg F. The average is not far
from 45 deg F. This places most of the dry-farm territory in the class of
cold regions, though a small area on the extreme east border may be
classed as temperate, and parts of California and Arizona as warm. The
range in temperature from the highest in summer to the lowest in winter
is considerable, but not widely different from other similar parts of the
United States. The range is greatest in the interior mountainous districts,
and lowest along the seacoast. The daily range of the highest and
lowest temperatures for any one day is generally higher over dry-farm
sections than over humid districts. In the Plateau regions of the
semiarid country the average daily variation is from 30 to 35 deg F.,
while east of the Mississippi it is only about 20 deg F. This greater
daily range is chiefly due to the clear skies and scant vegetation which
facilitate excessive warming by day and cooling by night.
The important temperature question for the dry-farmer is whether the
growing season is sufficiently warm and long to permit the maturing of
crops. There are few places, even at high altitudes in the region
considered, where the summer temperature is so low as to retard the
growth of plants. Likewise, the first and last killing frosts are ordinarily
so far apart as to allow an ample growing season. It must be
remembered that frosts are governed very largely by local topographic
features, and must be known from a local point of view. It is a general

law that frosts are more likely to occur in valleys 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.
(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