24 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 120 

 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 -30 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. 



