Seattle, Washington, holds a reputation for persistent, drizzly weather, often leading to the assumption that it is one of the wettest cities in the United States. This perception is not the full meteorological story. The unique climate of the Puget Sound region results from a specific geographic and atmospheric configuration. Seattle’s weather is determined by an interplay between the Pacific Ocean, global air currents, and the towering mountain ranges that flank the city, driving frequent precipitation, especially during the cooler months.
The Mechanism of Moist Air Delivery
The moisture that reaches Seattle begins thousands of miles away over the North Pacific Ocean. Prevailing global wind patterns, known as the Westerlies, consistently draw air masses eastward across the ocean basin toward the North American continent. These air masses travel over temperate waters, allowing them to become saturated with water vapor through evaporation.
This moisture-rich air is then transported rapidly by the Polar Jet Stream, a fast-moving, high-altitude river of air. In the winter months, the Jet Stream often dips southward, steering low-pressure storm systems directly toward the Washington coast. These low-pressure centers gather and concentrate the Pacific moisture, directing a steady influx of damp air inland over the Puget Sound region.
The Role of the Olympic and Cascade Mountains
The presence of two mountain ranges—the Olympic Mountains to the west and the Cascade Mountains to the east—converts the incoming moist air into precipitation. As the moisture-laden air from the Pacific encounters the Olympic Mountains, it is forced to rise. This process is known as orographic lift.
As the air mass is pushed upward along the windward slopes, the pressure decreases, causing it to cool adiabatically. This cooling reduces the air’s capacity to hold water vapor, leading to condensation into clouds and subsequent rain or snow. Much of the moisture is released on the western slopes of the Olympic Peninsula, explaining why areas like the Hoh Rainforest receive extreme annual rainfall totals.
After passing over the Olympics, the drier air descends before rising again to cross the taller Cascade Range. The Cascades repeat the process of orographic lift, forcing any remaining moisture to fall on their western slopes. Seattle lies between these two ranges, receiving residual moisture that passes the Olympics but falls before the air hits the Cascades. The air that descends on the eastern side of the Cascades, having lost its water, warms up, creating a rain shadow effect that makes Eastern Washington arid.
Frequency Versus Total Volume
Seattle’s reputation for rain is rooted in the frequency of precipitation events rather than the total annual volume of water that falls. The city averages between 150 and 165 days each year with measurable precipitation, a significantly higher number of rainy days than most other large American cities. This high frequency contributes to the persistent feeling of wet weather.
The total amount of rain is modest compared to other parts of the country. Seattle receives approximately 37 to 39 inches of rain annually, which is less than cities like New York, Houston, or Miami. Typical Seattle precipitation is characterized by light rain, drizzle, or mist, which can last for hours or days without developing into a heavy downpour. This pattern of light, lingering precipitation creates the city’s signature gray and damp climate.