Date of this Version
Great Plains Quarterly Vol. 1, No. 1, Winter 1981, pp. 5-15.
Changes in climate are major factors shaping the history of human occupance in the Great Plains region. Although Americans have often acted as though climates are fixed, the record indicates that in the past the climate of the Great Plains has changed drastically over relatively short periods of time. In order to acquire some understanding of what the Great Plains climate may become in the future and how human society may prepare for it, we must first comprehend what it was at various times in the past.
An important element in the climate of the American West is the presence and variability of "chinook" winds--streams of Pacific air that flow eastward over the mountains and plateaus and descend upon the Great Plains, where they create "chinook climates." These air movements arc best understood in terms of unique topographical features.
Americans have long understood that the special North American pattern of great mountain ranges (cordillera) athwart the midlatitude westerlies dominates the climate of most of the continent, including areas far to the east of the Rockies. Lorin Blodget first pointed out in 1857 that the West is primarily a high plateau with mountain ranges that modify the f10w of air across its surface.1
In general, the region is too high to allow most of the moist low-level air impinging on the West Coast to pass over to the plains. The kinetic energy of the west winds is usually insufficient for the air to rise from near sea level to the crest of the Sieras and Cascades. However, much more air leaves the cast side of the cordillera near the ground than arrives at the West Coast near the sea.2 The excess comes from atmospheric levels near the height of the plateau (about four to five thousand feet above sea level). Because of the location of the mountain ranges and the character of the air, there are three dominant routes by which this midlevel Pacific air crosses to the plains-the southern, middle, and northern routes.3 The three major railroads to the West were built along these routes to take advantage of the least difficult general passages through the cordillera (Fig. 1).
The southern route, which runs roughly along the Mexican border, carries the most air in winter when the westerlies are far south. This air, passing over the south western desert and south of the Rockies, flows across the Llano Estacado or northward along the Front Ranges roughly as far as Denver. It is seasonally hot and very dryas it descends from the cordillera down the slope of the high plains.
The middle route for the eastward passage of Pacific air is the broad, lower area of the Columbia River valley, the Snake River area of southern Idaho, and the break between the southern and northern Rockies represented by the basins of Wyoming. This route is at a latitude near that of maximum westerlies, and the mild, dry airstream that flows eastward roughly parallel to the Union Pacific route can often be traced eastward into Ohio and Pennsylvania. It is this dry airstream that coincides with the most easterly extension of the western grasslands-the "prairie peninsula."4