NATIVE WOODY PLANTS OF THE UNITED STATES 23 
tral and western Nevada, and between 120 and 150 on the east side 
of the Sierras; in the southernmost portion more than 240 days 
along the Colorado River, increasingly shorter farther from the river 
to less than 90 days, depending upon elevation. 
REGION 10, SOUTHWESTERN DESERT 
Arid, mesothermal, with a deficiency of rainfall at all seasons. 
Growing season averages more than 240 days annually, locally some- 
what less with high altitude. 
REGION 11, SOUTHERN PLATEAU 
Climate semiarid in the northern and southwestern portions to 
arid in the south-central and southeastern portions; microthermal 
in the northern portion, elsewhere mesothermal; rainfall deficient 
throughout. 
Annual number of days with snow cover averages less than 1 in 
the southern and western portions, but ranges from 10 to 30 in the 
northern part. Proximity to mountains increases the figure locally. 
Annual growing seasons range from 90 to 150 days in the north, or 
with altitude, to more than 240 days in the south. 
REGION 12, NORTHERN ROCKY MOUNTAINS 
Great variation because of elevation and exposure. Generally sub- 
humid except for the semiarid western extension and extreme east- 
ern and south-central portions; microthermal and with an all-season 
deficiency of precipitation except in the extreme northwest where 
rainfall is adequate at all seasons. 
Annual number of days with snow cover ranges from 30 to over 
120, the number increasing with altitude. Somewhat less in south- 
central Idaho and the Blue Mountains. Average frost-free season 
generally less than 90 days; or with decreasing altitude, longer. 
REGION 13, CENTRAL ROCKY MOUNTAINS 
Climate similar to that of region 12, but temperatures for the 
same elevation about 7° warmer. Generally arid to semiarid at 
lower elevations, approaching subhumid to humid climate at higher 
elevations; microthermal, with rainfall adequacy at all seasons 
reached with altitude. 
Annual number of days with snow cover, 30 to 120, increasing 
with altitude. Average annual growing season less than 90 days; or 
at much lower elevations, longer. 
REGION 14, SOUTHERN ROCKY MOUNTAINS 
Climate similar to that of region 13, but temperatures for the same 
elevation about 6° warmer. Arid or semiarid at low elevations to 
subhumid or humid at higher ones; microthermal except for the 
mesothermal southeastern portion. Precipitation deficient at all, 
seasons except at very high altitudes. 
_ Average annual number of days with snow cover, 10 to 60, increas- 
ing with altitude, but fewer southward. Average frost-free season 
