NATIVE WOODY PLANTS OF THE UNITED STATES 
25 
Average annual number of daj^s with snow cover from 1 to 10 in 
the extreme south to 30 to 60 in the extreme north. Average annual 
frost-free season from 150 days northward to 220 days southward, 
KEGIOW 2 0, SOUTHERN BLACK SOILS 
Subhumid and mesothermai throughout ; precipitation deficient at 
all seasons in the northwestern portion, adequate at all seasons in 
the eastern fringe and southeastern portion. Intermediate in mois- 
ture conditions between regions 16, 25, and 29. 
Average annual number of days with snow cover, 1 to 10 in the 
north to less than 1 in the south. Average frost-free season, 220 to 
more than 260 days, longest southward. 
REGION 21, NORTHERN PRAIRIES 
Subhumid, microthermal ; precipitation deficient at all seasons in 
the north, changing to adequate at all seasons southward; drying* 
winds and frequent droughts of more than 30 days occur in tho 
western portion. 
Average annual number of days with snow cover from 60 to 90 in 
the south to more than 120 in the north. Average annual frost-free 
season from 100 days in the north to 160 in the south. 
REGION 22, CENTRAL PRAIRIES 
Northwestern portion and western fringe subhumid, remaining 
portions humid; northern portion microthermal, southern mesother- 
mai; precipitation adequate at all seasons throughout. 
Average annual number of days with snow cover 30 to 60 except 
in the southern part of the western portion, where it decreases south- 
ward, ranging from 1 to 10. Average growing season 150 to 160 
days in the northern portion to 180 in the southeastern portion and 
230 in the extreme southwestern portion. 
REGION 23, WESTERN GREAT LAKES 
Western portion subhumid, eastern portion humid; microthermal; 
rainfall adequate at all seasons. 
Average annual number of days with snow cover, 60 to 90 in thei 
southern and southwestern portions to more than 120 in the northern 
and northeastern portions. Average annual growing season 100 days 
in the north to 150 days in the south, or in the southwest, 170 days. 
Upper Peninsula of Michigan, 110 to 140 days, 
REGION 24, CENTRAL GREAT LAKES 
Humid, microthermal, with adequate precipitation at all seasons. 
Winter temperatures more moderate than in region 23. 
Average annual number of days with snow cover from 60 to 90 in 
the southern half, from 90 to 120 in the northern half. Frost-free^ 
season averaging 110 days in the north to over 150 in the south 
longest close to the lakes. 
