94. MISC. PUBLICATION 303, U. S. DEPT. OF AGRICULTURE 
in the western section 210 to less than 120 with altitude; in the east- 
ern section from 180 in the south to less than 90 in the higher parts 
of the extreme northern portion. 
REGION 15, NORTHERN GREAT PLAINS 
Except for the subhumid eastern portion and the Black Hills, 
semiarid throughout; microthermal and with a deficiency of rainfall 
at all seasons. In the Black Hills, with altitude a humid micro- 
thermal climate occurs, with adequate precipitation at all seasons. 
Average annual number of days with snow cover 30 to 60 in the 
south, but 60 to more than 120 over most of the central and northern 
portions, increasing northward. Average growing season 110 to 126 
days in the extreme north, 120 to 150 in the south and southwest, 
and 90 to 120 in Wyoming, northwestern South Dakota, southwest- 
ern North Dakota, and southeastern Montana. Extremely cold in 
winter, usually —80° to —40° F, 
REGION 16, CENTRAL GREAT PLAINS 
Climate warmer than that of region 15 and with greater evapora- 
tion. Generally semiarid except for the subhumid northeastern and 
southeastern portions; microthermal in the northern fringe and in 
the northeastern and southeastern portions, elsewhere mesothermal ; 
precipitation generally deficient at all seasons except in the northeast 
and southeast where there is a summer deficiency only. 
Average annual number of days with snow cover from less than 
1 in the extreme southern portion to 30 to 60 in the extreme north. 
Average growing season 150 days in the north to 210 days in the 
south. 
REGION 17, SOUTHERN PLAINS. 
Semiarid, except for a small arid area west of the mouth of the 
Pecos River; mesothermal, with rainfall deficient at all seasons. 
Average annual growing season 210 to more than 260 days, the longer 
season southward. Evaporation high throughout. 
REGION 18, NORTHERN BLACK SOILS 
Subhumid, except for the extreme semiarid northwestern portion; 
microthermal, rainfall deficient at all seasons. Intermediate in mois- 
ture between regions 15 and 21. 
Average annual number of days with snow cover from 30 to 60 in 
the south to over 120 in the north. Average annual growing season 
110 to 150 days, or slightly more in the extreme southern portion. 
REGION 19, CENTRAL BLACK SOILS 
Subhumid throughout; microthermal in the western portions, 
mesothermal in the eastern; precipitation deficient at all seasons in 
the northern portion, in summer in the southwestern and west-central 
portions, but adequate at all seasons in the eastern portion. Sudden 
variations common in winter; hot winds often blowing in summer; 
moisture conditions intermediate between those of regions 16 and 22. 
