Located in the heart of Washington State, Yakima is surrounded by a landscape that often appears more like the American Southwest than the lush Pacific Northwest. East of the towering Cascade Mountains, the region experiences hot, dry summers and cold, relatively dry winters, a climate far removed from the coast’s temperate rainforests. This stark contrast leads many to ask: Is Yakima a desert? Determining the true nature of Yakima’s climate requires examining the precise scientific definitions used by climatologists, moving beyond visual impressions.
The Scientific Definition of a Desert
Climatology defines a desert primarily by the relationship between water received and water lost, not just by sand dunes or high temperatures. The most comprehensive measure of aridity is the ratio of annual precipitation (P) to potential evapotranspiration (PET). PET represents the maximum water that could evaporate and transpire from plants if water were unlimited. A true arid environment is defined by a consistent deficit, where water loss greatly exceeds water gain.
Scientists classify degrees of dryness using this P/PET ratio. A semi-arid climate (steppe) has a ratio between 0.20 and 0.50. A true arid climate falls between 0.03 and 0.20, and a hyper-arid climate, such as the Sahara, has a ratio below 0.03. Traditionally, a less precise definition holds that a region is a desert if it receives less than 10 inches (250 millimeters) of annual precipitation.
Yakima’s Official Climate Classification
Based on internationally recognized systems, Yakima is classified as a cold semi-arid climate (\(BSk\)) under the Köppen-Geiger system. The “BS” indicates a dry steppe or semi-arid climate, and the “k” signifies cold winters. This classification places Yakima in a transitional zone between humid climates and true deserts.
The city’s average annual precipitation is remarkably low, typically receiving only about 8 inches (203 millimeters) per year. This figure is below the average for most of the United States, yet it places Yakima just above the 10-inch threshold for a true arid desert. The semi-arid designation is fitting because the area receives enough moisture to support a sparse covering of grasses and shrubs, rather than the barren conditions of an arid zone.
The Role of the Cascade Rain Shadow
The primary geographical feature responsible for Yakima’s semi-arid status is the towering Cascade Mountain Range, which creates a pronounced rain shadow effect. Prevailing winds carry moist air masses eastward from the Pacific Ocean toward the coast. As this air encounters the steep western slopes of the Cascades, it is forced upward in a process called orographic lift.
This rapid ascent causes the air to cool and condense, leading to heavy precipitation (rain and snow) on the western, windward side of the mountains. Once the air crosses the crest, it descends on the eastern, leeward slope where Yakima is situated. This descending air compresses and warms adiabatically, causing the relative humidity to drop dramatically.
This meteorological process effectively strips the air of its moisture before it reaches central Washington. While locations on the west side of the Cascades can receive over 60 inches of rain annually, Yakima’s location deep in the rain shadow explains its meager 8 inches. The result is a profoundly dry climate dictated by the state’s mountainous geography.
Ecology and Landscape of Arid Central Washington
The semi-arid climate directly shapes the ecosystem of central Washington, which is characterized as a shrub-steppe rather than a desert biome. The natural landscape is dominated by drought-resistant vegetation, such as various species of sagebrush and short bunchgrasses. These plants are highly adapted to the dry conditions, featuring small leaves, fuzzy surfaces, and deep root systems to conserve and seek out limited moisture.
The soils in the region are typically light-colored and dry, containing high mineral content due to the lack of intense rainfall. Because natural precipitation is insufficient to support large-scale tree growth, the visual landscape is vast and open. This steppe environment supports specialized wildlife, including the pygmy rabbit and greater sage grouse. The agricultural bounty of the Yakima Valley, famous for its orchards and vineyards, is only possible through extensive irrigation, highlighting the region’s natural aridity.