To a large extent, the climate of the Great Plains is determined by its geographic position within North America. The region is affected by several different air mass types that possess very different temperature and moisture properties. Air masses that move south over the region, from the dry, often snow-covered interior sections of central Canada, bring cold, dry air across the Plains. Air masses that form over the Gulf of Mexico and the Caribbean often move northward into the Plains with warm, very moist air. Occasionally, very warm, dry air will enter the Plains with air masses that originate in the desert areas of the southwestern United States and northern Mexico. Finally, air that originates over the Pacific Ocean will often move east, crossing the mountainous region of the western third of the United States into the Great Plains. The transport of this air up the windward side of a mountain barrier and then down the leeward side results in significant warming and drying. Depending on the time of year, one or a combination of these air masses typically dominate much of the Great Plains region.
Another major factor in the climate of the Great Plains is its location near the center of the continent, a great distance from any large body of water. For a given input or output of energy, the temperature of water changes much more slowly than that of soil surfaces. Thus the temperature in an area in close proximity to a large body of water typically experiences a more consistent climate because of the moderating effects of the water (such an area is cooler in the summer and warmer in the winter). As a result of soil surfaces heating and cooling rapidly, regions remote from large water bodies experience large variations in temperature throughout the course of a year and over a twenty-four-hour period.
The Great Plains, therefore, has a large range in both annual and daily temperatures. During the midwinter months (January and February), when cold, dry air from central Canada dominates, temperatures are very cold, with mean temperatures varying from 40ºF across the Southern Plains to as low as 10ºF across the Canadian Prairies. During midsummer (July and August), when the Plains are dominated by either warm, moist air from the Gulf of Mexico or warm, dry air from the Southwest, mean temperatures increase to approximately 80ºF through the Southern Plains and approximately 66ºF across the Canadian Prairies. This gives the region a much larger range in annual temperature than is found elsewhere in North America. For example, the range in mean monthly temperature between January and July in Omaha, Nebraska, is approximately 56ºF, while in Philadelphia, Pennsylvania, and San Francisco, California (each at a similar latitude), the ranges are 46ºF and 14ºF respectively. The daily temperature range across the Plains generally increases to the north and west, away from the moderating influence of the Gulf of Mexico. During July, the mean daily temperature range across the Plains is approximately 30\F, while in the eastern parts of North America the daily range is 20ºF and along the Pacific coast only 10ºF.
Precipitation across the Great Plains decreases dramatically from southeast to northwest. The main source of atmospheric moisture is the Gulf of Mexico. Areas near the Gulf, in the southeastern portion of the Plains, receive more than forty inches of precipitation annually. This total decreases to less than fourteen inches in eastern Montana and parts of the Canadian Prairies. The annual cycle of precipitation across the Plains is dominated by a summer maximum. During the summer months warm, moist air masses move north from the Gulf of Mexico. These air masses normally produce showers and thunderstorms because of their unstable characteristics. During the spring cold, dry air masses from central Canada often interact with the air from the Gulf to set the stage for the outbreak of severe thunderstorms that can bring heavy rainfall, high winds, hail, and tornadoes to local areas. During the winter snow normally covers a substantial portion of the Plains. Annual snowfall averages from less than one inch across the southern portion of the region to more than forty inches across the north. Because of the cold winter temperatures and the relatively heavy annual snowfall, snow cover blankets the Northern Plains throughout much of the winter season.
The year-to-year variability in temperature and precipitation across the Great Plains is very large. This variability is especially evident in the recurrent problem of drought. The very warm and often dry summer weather that is characteristic of the Plains leads to high evaporation and transpiration (water loss from plants) rates. Soils are often depleted of their moisture, leading to stressed natural and cultivated vegetation. A measure of the lack of available soil moisture for plants, the soil moisture deficit, has been calculated for the entire Great Plains region for the period 1895 through 1994. From this it is clear that the Plains as a whole has undergone recurrent periods of drought over the last century, especially during the 1930s (the Dust Bowl years) and the 1950s. The large annual (within one year) and interannual (year-to-year) variability of Great Plains climate makes the region a natural laboratory for studying the effects of climate variability on a host of problems associated with the interaction of humans with their environment.
Daniel J. Leathers University of Delaware
Borchert, John R. "The Climate of the Central North American Grassland." Annals of the Association of American Geographers 40 (1950): 1–39.
Lawson, Merlin P., and Charles W. Stockton. "Desert Myth and Climatic Reality." Annals of the Association of the American Geographers 71 (1981): 527–35.
Rosenberg, Norman J. "Climate of the Great Plains Region of the United States." Great Plains Quarterly 6 (1986): 22–32.