22 MISC. PUBLICATION 303, U. S. DEPT. OF AGRICULTURE 
western side varies from wet to humid. Precipitation is adequate at 
all seasons in tlie 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. 
EEGION 5, SOUTHESIT CALIFOSNIA 
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, 
EEGIOK 6, COLUMBIA EIVER VAULEY 
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. 
EEGION 7, PALOUSE-BITTEBEOOT VALLEY 
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. Bainfall 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 cover 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. 
EEGION 8, SNAKE BIVEE 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 connnon, 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, 
EEGION 9, GEEAT BASIN-INTEBMONTANE 
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- 
