Washington State has a reputation for rain, which is accurate for the western half of the state, particularly the coastal areas and the Puget Sound region. The climate here is defined by an abundant supply of moisture meeting a specific geographic barrier that forces it from the atmosphere. This localized phenomenon creates a climatic division, explaining why one side of the state is lush and green while the other is semi-arid. The scientific explanation for this pronounced rainfall is a three-part process involving global wind patterns, ocean influence, and high mountain ranges.
Prevailing Winds and Pacific Moisture
The foundation of Washington’s wet weather begins in the Pacific Ocean, the state’s primary source of atmospheric moisture. Prevailing southwesterly and westerly winds, known as the Westerlies, consistently push vast air masses toward the North American continent, acquiring water vapor as they travel across the mild Pacific. During the wet season, the Aleutian Low pressure system intensifies and shifts southward toward the Gulf of Alaska. The counter-clockwise circulation directs a steady, mild, and moist onshore flow directly into the Pacific Northwest, ensuring the air arrives at the landmass fully saturated.
The Mechanism of Orographic Lift
The sheer quantity of precipitation on Washington’s windward side is a direct consequence of a geological process called orographic lift. The state’s massive mountain ranges, specifically the Olympic Mountains and the Cascade Range, act as physical walls directly in the path of the incoming moist air. When the saturated air encounters these steep slopes, it is forced upward, expanding due to the drop in atmospheric pressure at higher altitudes. This expansion causes the air to cool adiabatically; as the temperature falls, the water vapor reaches its dew point, leading to condensation. This process forms dense clouds and ultimately releases the moisture as rain or snow on the western slopes, exemplified by the Hoh Rainforest, which receives upwards of 140 inches of rain annually.
The Resulting Rain Shadow
The final stage of this atmospheric process is the creation of a rain shadow on the eastern side of the mountains. Once the air crests the peaks of the Olympic and Cascade Mountains, it has already shed the majority of its water content on the western slopes. This now-dry air begins to descend the leeward (eastern) side, where it is compressed by increasing atmospheric pressure, causing its temperature to warm significantly. This phenomenon, known as adiabatic warming, drastically lowers the relative humidity, making the descending air extremely dry and clear. Cities like Sequim, located in the Olympic rain shadow, receive only about 16 inches of annual rainfall, while Eastern Washington cities experience a semi-arid climate with totals often below 8 inches.