IDAHO CLIMATE.
Edward L. Wells, Section Director, U. S. Weather Bureau.

Idaho extends through seven degrees of latitude, or as far as from Indiana to Florida. In altitude it ranges from about 700 feet to more than 12,000 feet. Its northern end lies within the path of the rain areas which pass eastward from the north Pacific ocean, while the southern portion lies well out of that path. As a result of these complex factors the climate is so diverse as to render description difficult and accurate graphic representation impracticable.

The entire state comes under the modifying influence of the equable climate of the north Pacific ocean, and is protected to a large extent from the cold waves that prevail east of the great continental divide, while the Chinook winds play an important part in determining the temperature, so that the entire state possesses a milder climate than might be supposed from the latitude and elevation. The normal annual temperature ranges from about 36 degrees in the mountainous interior to about 55 degrees along the middle reaches of the Snake river. This is a range greater than that found in traveling from Bismarck, North Dakota, to St. Louis, Mo.

The coldest part of the state includes that part of the main range of the Rocky Mountains which forms a portion of the eastern border, together with the sparsely settled elevated regions in the interior and the higher parts of the more important mountain ranges. Here the climate is characterized by cold, snowy winters and short, cool summers. It is sometimes quite warm for a few hours at mid-day in summer, but even in summer the nights are almost cool enough to make a fire enjoyable. In these mountains, above the agricultural sections, practically all the winter precipitation is in the form of snow and occasionally some drifts remain throughout the year. This region thus forms the storehouse of moisture for the many streams of the state, the waters of which are used for irrigation and power, and it also produces valuable timber and furnishes excellent summer grazing. In this section the temperature is about the same as that found in North Dakota.

The plateaus and higher valleys, which make up a large part of the central and extreme eastern portions of the state, are somewhat warmer than the mountainous regions already referred to. The winters are cold and the summers are short, but are long enough to enable the staple grains, grasses and vegetables to come to maturity. The normal annual temperature in this section is about the same as that found in parts of Wisconsin.

The great Snake river plain, together with the lower valleys of the streams that join the Snake during its course across this plain, and a part of the region draining into Great Salt Lake, possess temperature conditions particularly suited to the needs of agriculture. The eastern portion is characterized by moderately cold winters and moderately warm summers, and has a normal annual temperature agreeing closely with that found in central and southern Michigan. The western portion is characterized by mild winters, and by summers in which there are some days of relatively high temperature but in which the nights are usually cool. In this section the normal annual temperature is about the same as that found in central Illinois and northern Missouri.

Some damage is occasionally done by spring frosts, but the period of Iow temperature is short and conditions are such as to render protective measures highly effective. Uniformly cool nights, together with other conditions the operation of which is not well understood, render Idaho-grown products somewhat immune from the effects of low temperature, so that even the tenderfruit bloom will frequently withstand a temperature considerably below the freezing point. The Weather Bureau is planning a frost-warning service for the fruit belt of Idaho which, with proper co-operation on the part of the growers, will render the loss from frost inconsequential Damage by frost in the fall is a rare occurrence except in the higher altitudes.

The diversity in precipitation is quite as pronounced as in the case of temperature. The normal annual precipitation ranges from about 8 inches in part .of the Snake River Plain to about 40 inches in the Bitter Root mountains. This is a range greater than that found in going from Albany, New York, to Phoenix, Arizona. The heaviest precipitation occurs in the mountains, much of it there in the form of snow. On the Snake River Plain and in the valleys adjacent the rainfall is generally inadequate for growing crops without irrigation, though there are local exceptions, and scientific dry-farming methods are reclaiming a larger acreage every year. Elsewhere there are large areas where irrigation is not necessary. Over much of the State the precipitation is heavier in November, with a secondary maximum in May. However, in parts of central and eastern Idaho the heaviest rainfall is in May or June, with the secondary maximum ranging from November to March. Practically everywhere the driest month is July or August. The distribution of precipitation is quite favorable for "dry-farming," while in the case of irrigation by the use of stored water the period of storage is comparatively short, for the climatic conditions are such that the maximum flow of the streams comes in the height of the growing season. The intensity of the rainfall is less than in many other western states, thus lessening the likelihood of damage from floods. It is seldom that more than one inch of rain falls in 24 hours. Another important feature is the regularity of the rainfall. If the normal annual precipitation for a certain place is 20 inches, and in a given year the actual amount is 15 inches, we say that the departure is 5 inches. At Boise the average annual departure for a period of 28 years has been 2.40 inches, or 18 per cent. of the normal amount, while at E1 Paso, Texas, the average annual departure for a period of 39 years was 3.30 inches, or 35 per cent. of the normal amount.

Thunderstorms are light and infrequent. Damage by hail is by no means common. Tornadoes are practically unknown. The average wind movement, where records have been kept, ranges from five and one-half to eight and one-half miles per hour. In Iowa and Nebraska the wind movement averages from eight to nine miles an hour, while the average wind movement at Chicago is about fourteen miles an hour. There is an abundance of sunshine in the growing and ripening season, averaging almost ten hours a day at Boise during July.

It is suggested that persons desiring more detailed information than can be given in this article may obtain it by calling at the nearest local office of the Weather Bureau or addressing the l~cal office of the Weather Bureau at Boise.



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