Valley Climate

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Lewis Clark Valley Climate

Lewiston, Idaho and Clarkston, Washington are located at the confluence of the Snake and Clearwater Rivers at an elevation of 738 feet above mean sea level. The Weather Office is located on the bench at an elevation of 1,413 feet above sea level and about 2 miles south of Lewiston. Although Lewiston is at about the same latitude as Duluth, Minnesota, the climate, especially in the wintertime, is comparatively very mild. This mildness can be explained by its location with respect to the effects of Pacific air masses from the west and by the sheltering effects of the mountains that surround the valley in almost every direction.

Snowfall in the valley averages about 18 inches during the year, concentrated mostly in the three months of December, January, and February, but in the higher country surrounding the valley the snowfall is much heavier.

Precipitation normally amounts to about 13 inches annually, which is rather evenly distributed through the year except for the months of July and August, which are characterized by infrequent thunderstorms that usually drop only small amounts of rain. Records show that several times during these two months not more than a trace of rain has been recorded and at times not even a trace. The thunderstorms on the prairie are, at times, accompanied by heavy hail and windstorms.

Many winters have gone by without a temperature of zero being recorded in the valley, but the prairie sections usually experience lower temperatures. The summers experience hot and dry periods with as many as 10 consecutive days with afternoon temperatures reaching 100 degrees or more. Considerable cooling after sunset makes the nights very comfortable. Cold waves occur when arctic air, originating in the Yukon Territory, moves southward. Such cold waves are relatively infrequent when compared to the number of arctic outbreaks east of the continental divide in Montana only a short distance away.

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.

The growing season of approximately 220 days in this part of the country, makes conditions favorable for the growing of many types of fruits, vegetables, and berries.

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