18 SNAKE RIVEE PLAINS OF IDAHO. [bull. 199. 
The mean annual precipitation is about 13 inches, but many local 
variations occur. Nearly all the water that reaches the thirsty lands 
comes in winter and spring. During the growing season the soil is 
invariably parched, and successful agriculture without irrigation is 
seemingly impossible. 
The summers are decidedly hot. Not infrequently for many con- 
secutive days and even for weeks at a time the temperature during 
the hours of sunshine is over 100° F. and frequently reaches 105° F., 
and reports by local observers of 110° to 115° in the shade are not 
rare. Owing to the dryness of the atmosphere, however, the heat is 
seldom oppressive, and sunstroke is said to be unknown. Under the 
prevailingly clear skies in summer radiation is rapid and the nights 
are nearly always cool. Exceptionally warm nights occur when the sky 
is clouded, and radiation from the heated soil and rocks is checked. 
In winter the atmosphere is even clearer than in summer, as the 
rain and snow remove the dust which is ever present when the soil is 
dry. The air is cold and the temperature has a greater daily range in 
winter than in summer, frequently falling far below zero, Fahrenheit. 
During the winter of 1898-99 the minimum temperature at Blackfoot 
was —30° F. and at Minidoka —28° F., while the year following, 
at Lost River, situated on the western border of the plains, in the 
entrance to a tributary valley, a temperature of —11° was recorded. 
During the }^ear 1898 the last "killing frost" in spring at Blackfoot 
was on June 5, and the first frost during the succeeding fall was 
on September 29; at Minidoka the corresponding conditions occurred 
on May 14 and September 1. The snowfall, while aggregating during 
certain winters from 30 to 15 inches or more, varies greatly from 
year to }^ear and from place to place, but seldom remains long on the 
ground. The sudden snowstorms are usually succeeded quickly by 
thaws during the hours of sunshine, and sleighing is seldom prac- 
ticable. Warm winds, termed "Chinook winds," frequently occur in 
winter, during the prevalence of which snow, if present, disappears as 
if by magic, leaving the plains and the lower slopes of the bordering 
mountains bare. Large numbers of horses, cattle, and sheep are 
pastured on the plains throughout the winter, and require to be fed 
only during the more severe storms. In winter the skies, although 
frequently clouded, are usually clear, especially at night, and the stars 
are of phenomenal brilliancy^. 
The prevailing winds, often heavily dust-laden, are from the west, 
and at many times, especially in the fall, blow with such strength and 
constancy as to become tiying to a person's nerves. In summer and 1 
fall strong afternoon winds frequently occur, which die away at sun- 1 
set; these are almost invariably from the west. These strong breezes 
are nearly as regular in their periods as the sea breezes on land^Hi 
bordering warm seas, but instead of bringing a refreshing coolnesjf I 
to 
I 
of 
jiid. 
