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<br /> <br />CLIMATE <br />Weather in the basin is typical of the interior of a large <br />land mass in the temperate zone--variable. Rapid weather changes <br />are caused by invasions of large masses of warm, moist air from <br />the Gulf of Mexico, hot, dry air from the southwest, cool, dry <br />air from the Pacific Ocean, and cold, dry air from Canada. <br /> <br />Precipitation <br />The period 1941-1970 is presently considered by the U.S. <br />Weather Service to represent a "normal" precipitation period. <br />Figure 2-6 shows how the average annual precipitation for that <br />period varied. It decreased fairly uniformly from about 32 inches <br />in the most easterly part of the basin to less than 14 inches in <br />the western part. The amount of rainfall at any location generally <br />varies considerably from year to year. The figure also shows the <br />annual variation at several stations across the basin. <br />Intense thunderstorms are common although they are generally <br />localized in extent, seldom covering large areas of the basin. <br />Droughts have severely affected most of the basin at one <br />time or another. The periods of 1887-1896, 1924-1927, 1934-1941, <br />and 1952-1956 were particularly severe. However, droughts are <br />not restricted to years with low annual rainfall. They have oc- <br />curred in years with average total annual precipitation but where <br />the monthly distribution of rainfall throughout the year did not <br />follow the normal pattern. Figure 2-7 shows the normal monthly <br />distribution of precipitation at Scottsbluff, North Platte, and <br />Lincoln. <br /> <br />2-8 <br />