22 MISC. PUBLICATION 303, U. S. DEPT. OF AGRICULTURE 



western side varies from wet to humid. Precipitation is adequate at 

 all seasons in the northern portion but deficient in summer from 

 central Oregon southward. The climate is microthermal in the 

 northern portion, varying to mesothermal in the more southern part. 

 Average annual number of days with snow cover ranges from 10 

 to 60, increasing with altitude ; more than 60 at very high elevations. 

 Average growing season is less than 90 to 150 days, somewhat shorter 

 northward and on the eastern side and decreasing with altitude. 



REGION 5, SOUTHERN CALIFORNIA 



Climate subhumid, mesothermal, with a summer deficiency of rain- 

 fall northward. Southward the climate becomes semiarid with a 

 rainfall deficiency at all seasons. 



Average growing season 210 to more than 240 days. Snow cover 

 is rare. 



REGION 6, COLtTMBIA RIVER VALLEY 



In the north subhumid ; in the south semiarid ; or arid in the south- 

 east corner. A microthermal climate with a deficiency of rainfall at 

 all seasons. 



Average annual number of days with snow cover 10 to 60, lowest 

 southward; highest, northward. Average annual growing season, 

 120 to 150 days. 



REGION 7, PALOUSE-BITTERROOT VAT.T.KY 



A microthermal climate, varying from semiarid on the extreme 

 west to subhumid in the central portion, humid in the central part 

 of the narrowest portion, and subhumid in the extreme eastern por- 

 tion. Rainfall deficient at all seasons except in the very center of 

 the area where it is deficient during the summer only. 



Average annual number of days with snow coyer 10 to 90, the 

 southern half with 10 to 30, but with the number increasing north- 

 ward. Average annual growing season 180 days in the south to 90 

 days in the north or at higher elevations. 



REGION 8, SNAKE RIVER PLAIN-UTAH VALLEY 



Northward generally subhumid, passing to semiarid and arid 

 southward; microthermal and with a deficiency of rainfall at all 

 seasons. 



Average annual number of days with snow cover 10 to 30 in the 

 northwestern portion, 30 to 60 most common, especially southward. 

 Average annual frost-free season 90 to more than 180 days, shorter 

 with altitude, longer near large bodies of water and in the northern 

 portion. 



REGION 9, GREAT BASIN-INTERMONTANE 



Generally semiarid, microthermal, with a deficiency of rainfall at 

 all seasons, but with local arid portions, especially in the south- 

 western part. 



Average annual number of days with snow cover, 30 to 60 in the 

 larger western and southern portions; 60 to 90 days in the northern 

 half of the eastern portion. Average annual frost-free season less 

 than 90 days in the northwestern portion ; between 90 and 120 in cen- 



