The landscape of Eastern Washington often presents a visually striking image of dryness, leading many to question if the region is, in fact, a desert. Vast expanses of arid scrubland certainly convey the appearance of a desert environment, contrasting sharply with the lush, green western side of the state. While the climate is extremely dry, climatologists classify the region just outside the definition of a true desert. The environment is more accurately described as semi-arid, a distinct classification that explains the unique conditions and ecology of this part of the Pacific Northwest.
What Defines a Desert
The scientific classification of a desert relies primarily on the average amount of annual precipitation a region receives. An area is classified as an arid desert if it receives less than 250 millimeters (approximately 10 inches) of rainfall per year. This standard is based on the limited moisture available to support plant life.
Regions that receive slightly more moisture, between 250 mm and 500 mm (10 to 20 inches), are categorized as semi-arid. These semi-arid areas, often referred to as steppes, are intermediate zones between true deserts and more humid climates. The higher moisture content supports more continuous vegetation, typically grasses and small shrubs, unlike the sparse, specialized plant life of a true desert.
The Cascade Mountain Rain Shadow
The aridity of Eastern Washington is a direct consequence of the rain shadow effect. The Cascade Mountains, which run north-to-south through the state, act as a massive barrier to moisture-laden air moving inland from the Pacific Ocean. This process begins when moist air masses are forced upward as they encounter the steep western slopes, a mechanism called orographic lifting.
As the air rises, it cools rapidly, causing the water vapor within it to condense into clouds and precipitate heavily as rain or snow on the western side of the Cascades. Cities west of the mountains average nearly 40 inches of rain annually. By the time the air crosses the mountain crest, it has lost the majority of its moisture content.
The now-dry air then begins its descent down the eastern, or leeward, slopes. As air sinks, it is compressed by increasing atmospheric pressure, causing it to warm significantly—a process known as adiabatic heating. This warming effect further reduces the relative humidity of the air, making condensation and precipitation extremely unlikely in the valleys below.
This dramatic shift creates an exceptionally dry environment just a short distance from the moist coastline. The resulting rain shadow is responsible for the arid conditions that extend across the entire Columbia Basin.
Eastern Washington’s Shrub-Steppe Environment
The climate resulting from the Cascade rain shadow is classified as cold semi-arid, which supports the unique Shrub-Steppe ecosystem across Eastern Washington. This biome is characterized by low-lying, drought-resistant vegetation, predominantly big sagebrush interspersed with various native bunchgrasses. The term “steppe” refers to a vast, treeless plain, reflecting the open, scrubby nature of the landscape.
The moisture levels in this region fall near the desert classification threshold. While some areas receive annual precipitation totals just under the 10-inch desert threshold, the overall regional average trends slightly higher, placing it firmly in the semi-arid category.
The Shrub-Steppe experiences significant seasonal temperature variations, typical for interior continental climates. Winters are moderately cold, often with snow, while the summers are notably hot and dry. The lack of moisture in the air contributes to large temperature swings between day and night, a characteristic shared with desert environments. This combination of low precipitation and specific vegetation defines the Shrub-Steppe as a semi-arid grassland, not a true desert.