The Great Plains, stretching across the central United States and Canada, is often associated with severe dryness, leading many to wonder if it is truly a desert. This geographical area, characterized by its vast, treeless expanse, is often mistakenly grouped with the world’s arid regions. The definitive scientific answer is that the Great Plains is not a desert. While the region is undeniably dry, its climate and ecosystem fail to meet the rigorous metrics scientists use to classify a true desert environment.
How Deserts Are Classified
A desert is scientifically defined by a severe moisture deficit, where the potential for water evaporation greatly exceeds the amount of precipitation received. This chronic shortage of available moisture for plant life is quantified using an aridity index. This index compares the water supply (precipitation) to the maximum water loss that could occur (potential evapotranspiration).
Climatologists utilize the Köppen climate classification system, which designates true deserts with the letters BW, standing for arid climate. These regions typically receive less than 250 millimeters (about 10 inches) of annual rainfall. Precipitation in a BW climate must also be less than half of the potential water loss through evapotranspiration.
This classification is further broken down into hot deserts (BWh) and cold deserts (BWk), distinguished by their average annual temperatures. The determining factor is the extreme imbalance between water coming in and water leaving the system. True deserts exhibit very low relative humidity and irregular, unreliable rainfall patterns.
The Great Plains Climate and Hydrology
The climate of the Great Plains is best described as continental and semi-arid, scientifically known as steppe. In the Köppen system, this region is classified as BSk, representing a mid-latitude semi-arid climate. This designation signifies a transitional zone between arid deserts and more humid climates.
The primary reason for the region’s relative dryness is the rain shadow effect created by the Rocky Mountains to the west. As moist air masses from the Pacific Ocean rise over the mountains, they cool and drop their moisture, leaving drier air to descend onto the Plains. This phenomenon contributes to the high winds and low humidity experienced across the plains.
Despite this influence, the Great Plains receives significantly more moisture than a desert, with average annual precipitation ranging from 25 to 75 centimeters (10 to 30 inches). This places its water supply in a different category than a true desert. While evapotranspiration still exceeds precipitation, the deficit is not severe enough to meet the arid BW classification threshold.
Why the Great Plains Is Not a Desert
The most comprehensive refutation of the desert label lies in the Great Plains’ distinct ecology and soil composition. The region is fundamentally a grassland biome, possessing characteristics incompatible with a desert environment. The soil in the Plains is a deep, fertile layer of topsoil.
This rich soil, known as a Mollisol, is a product of the extensive root systems of perennial grasses that dominate the landscape. When these grasses die, their roots decay, contributing organic matter deep into the soil and creating a nutrient-rich environment. Desert soils, by contrast, are typically thin, nutrient-poor, and often have high mineral or salt content.
The vegetation itself is the strongest indicator that the area is not a desert. The Plains are covered by dense, deep-rooted grasses, such as buffalo grass and blue grama, which are adapted to survive drought and fire. These plants form a continuous cover that stabilizes the soil and supports a high biodiversity of grazing animals. True deserts support only sparse, specialized plants like cacti and succulents that conserve water using shallow, wide-spreading roots.
The Origin of the “Great American Desert” Misnomer
The misleading term “Great American Desert” originated with early American explorers unfamiliar with semi-arid grasslands. Major Stephen H. Long, who led a scientific expedition through the region in 1820, is credited with popularizing the phrase. His party’s report described the area as “unfit for cultivation,” reflecting their East Coast perspective where a lack of trees signaled poor land.
Long’s map from 1823 officially labeled the territory as the “Great American Desert,” cementing the idea in the public consciousness and on early geographical charts. Explorers like Zebulon Pike had earlier compared the treeless expanse to the “sandy deserts of Africa,” leading to a perception of the land as an uninhabitable wasteland.
This historical misnomer persisted for decades, influencing westward expansion by causing settlers to bypass the region in search of more familiar, forested areas. The term was a reflection of cultural bias and a lack of understanding of grassland ecology, rather than an accurate scientific assessment of the climate.