1018 
part in determining local temperatures 
on frosty nights. The entire region may 
be classed as arid, having approximately 
13 inches of precipitation annually at 
Boise, and somewhat less at points away 
from the mountains. Water for irriga- 
tion is supplied by the Boise river. 
In fair weather there is quite a no- 
ticeable mountain and valley breeze 
blowing down the valley, or from the 
southeast from early morning to about 
10 a. m. and up the valley or from the 
northwest in the afternoon. Frosts oc- 
cur ordinarily upon the approach of a 
strong high-pressure area from the 
northwest. The outflow from this high, 
combined with the ascending currents 
already mentioned, makes a_ strong 
northwesterly wind in the afternoon, 
which has come to be considered as the 
surest indication of frost. At night, in 
addition to the ordinary nocturnal les- 
sening of the wind velocity, the descend- 
ing current opposes the outflow from the 
high, causing a stagnation of the air 
highly favorable for the occurrence of 
low temperatures near the ground. At 
such times there is a noticeable tendency 
for the colder air to settle into the shal- 
low depressions along the water courses 
already mentioned. When conditions for 
Gatan Sows 
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ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 
rapid radiation are particularly favor- 
able no two thermometers in the valley 
will indicate the same temperature. At 
other times the distribution of tempera. 
ture is fairly uniform. Whenever there 
is any considerable amount of wind at 
night frost does not occur. 
Sometimes when the crest of the high 
reaches or passes this region before 
morning an easterly wind will spring up. 
A brisk easterly wind, coming as it does 
off the mountain range, partakes of the 
nature of a chinook. Usually the effect 
of these chinooks is hardly noticeable 
except over Boise and the belt of land 
lying between the foothills and the river. 
Sometimes, however, the effect becomes 
noticeable on the bench lands, and on 
rare instances the chinook has been 
known to pass over the city and mate- 
rially affect the temperature on the 
bench. 
Lewiston-Clarkston District 
For the purpose of a study of the tem- 
perature conditions at different points in 
the valley a temperature station was es- 
tablished in a favorable location in the 
Clarkston (Wash.) section, one and a 
half miles southwest of the Weather 
Bureau station. 
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Lewiston-Clarkston Fruit District. 
—After Reed. 
