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This is cold
desert country, characterized by large daily and seasonal
temperature fluctuations. The
average annual temperature is 5.6oC, and the frost-free
period is about 90 days. Mean
air temperatures during the 2 to 3 months of winter are below
freezing (Figure 2), and topsoils usually remain frozen from mid
to late November through February or early March.
Snow cover typically persists for at least 2 to 3 months.
During summer, low humidity and clear skies result in
relatively high maximum temperatures (30 - 35oC) and
high evaporative demand during the day;
at night, radiative cooling often drops temperatures to
below 10oC. Thus, diurnal temperature fluctuations often exceed 20 oC.
By the time air
masses moving inland from the Pacific Ocean reach the eastern
Snake River Plain, they have lost much of their moisture to the
many intervening mountain ranges of Oregon and central Idaho.
Consequently, mean annual precipitation at the INEEL is
only about 220 mm. Approximately
36% of that falls during April, May, and June (Climate
Diagram; note strong precipitation peak in the spring).
On average, precipitation exceeds potential
evapotranspiration from October through May, and potential
evaportranspiration exceeds precipitation through the summer
months (Climate
Diagram). Melting
snow and spring rains typically account for most of the annual
recharge of soil moisture (Anderson et al. 1987).
June,
1995, was the wettest month of record at the INEEL, when 118 mm of
rainfall were recorded at the Central Facilities Area.
The previous record month was 112 mm in May of 1957.
The record for a single day is 42 mm, which fell on June
10, 1969. The second
highest 24-hour amount (39 mm) fell on June 5, 1995. The 1995 “water year” (October - September) was also the
wettest on record, with 360 mm of precipitation.
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