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Cllimate

Differences in elevation are one of the main causes for the variable climate which characterizes the District (Figure CL). Precipitation ranges from 11 to 25 inches per year and snowfall ranges from 3 to 54 inches per year. Snowpack accumulations at higher elevations supply irrigation water for the City of Lewiston. Rainfall distribution varies with lower elevations having a more even distribution per month. Higher elevations tend to have high intensity short duration spring and summer rainfall events. Nez Perce County includes geographic areas with the lowest precipitation in the Clearwater River Basin (Figure CR).

Temperatures range from 115° F to -20° F. Lower elevations have summer highs of 115° F and winter lows of 26° F. The higher elevations are much cooler and have summer highs of 89° F and winter lows of 19° F. Hot, dry periods with as many as ten consecutive days of 100° F or more are common in the lower elevations.

Winds are light, usually prevailing from the east, with occasional stronger winds accompanying the well-developed frontal systems from the west.

Relative humidity averages about 70 percent during the winter months and gradually lowers to about 40 percent during July and August.

Figure CL. Average Annual Precipitation within the Nez Perce SWCD.

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Not to Scale Source: USDA Natural Resources Conservation Service

Figure AG. Agroclimate Zones in Idaho.

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Air Resources

Monitoring in and around the City of Lewiston has shown that PM-101 annual arithmetic mean values have been gradually decreasing in the time period from 1999 to 1986 (Table 1).2 Concentrations over 50 are considered to be hazardous.

Table 1. PM-10 Annual Arithmetic Mean Values 1986-1999 (μg/m3) for Lewiston, Idaho.

YearLevel
198638.0
1987*
198836.0
198939.0
199035.8
1991*
1992*
199333
199428
199526
199624
199727
199824.9
199924.5

* Missing data

The 2000 Conservation Needs Assessment survey identified air quality as one of the top ten resource concerns of those surveyed. Air quality is considered in all conservation plans within the District.

1PM-10 = particulate matter less than 10 μg/m3
2Idaho DEQ, 2000

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