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. 



BEGION 10, SOUTH WESTERiN 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 



