Seattle is often linked to the frequent presence of low clouds and fog, earning it the nickname “Gray City.” This observation is meteorologically sound, as the region experiences a high number of foggy days, particularly during the cooler months. The phenomenon arises from a specific combination of atmospheric conditions and the unique physical layout of the Pacific Northwest landscape. Understanding Seattle’s fog involves recognizing three interconnected factors: the source of the moisture, the geographic container, and the atmospheric condition that causes the fog to linger.
The Essential Ingredient: Marine Layer and Cool Ocean Air
The foundation of Seattle’s fog is the vast, cool body of water to the west: the Pacific Ocean. Fog creation begins over the water as a process known as advection, where warm, moist air moves horizontally over a much cooler surface. Cold ocean currents influence the Pacific coastline, which keep the surface water temperatures relatively low compared to the air moving over it.
This interaction causes the air closest to the ocean surface to cool rapidly to its dew point, leading to the formation of fog or a low-lying cloud deck. This cool, dense, moisture-laden air mass is termed the marine layer. Prevailing westerly winds consistently push this blanket of moist air eastward toward the Washington coast.
The marine layer serves as the primary source of the moisture that eventually envelops the Puget Sound region. This cool, damp air is constantly replenished from the ocean, ensuring a steady supply of fog and low clouds for the area. The air mass is stable and heavy, making it prone to settling into lower elevations as it moves inland.
Geographic Confinement: The Puget Sound Trough Effect
The physical landscape of Western Washington plays a significant role in channeling and concentrating the marine air mass. Seattle sits within a long, low-elevation corridor known as the Puget Sound Trough. This trough is defined by two massive mountain ranges that act as atmospheric barriers.
To the west, the Olympic Mountains block the eastward flow of the marine layer, forcing the air to detour around their northern and southern edges. The air funnels through the Strait of Juan de Fuca to the north and the Chehalis Gap to the south, effectively concentrating the moisture into the Puget Sound basin. The Cascade Mountains to the east form a second, substantial wall, preventing the air mass from easily dissipating further inland.
This geographic configuration creates a semi-enclosed bowl that the cool, moist air readily fills. The marine air is guided and trapped within this north-south oriented valley. The funneling action ensures that the fog and low clouds are efficiently delivered and contained over the Seattle metropolitan area.
The Mechanism of Persistence: Wintertime Temperature Inversions
The reason Seattle’s fog can persist for days or even weeks, especially in late fall and winter, is due to a phenomenon called a temperature inversion. Normally, air temperature decreases as altitude increases, allowing for vertical atmospheric mixing. An inversion reverses this pattern, with a layer of warmer air settling above a layer of cooler air near the surface.
This layer of warm air acts like a lid, capping the air below it and preventing it from rising. The cool, moist air from the marine layer, or locally formed radiation fog, becomes trapped underneath this stable layer. Because the air cannot mix vertically, the moisture and any suspended particles remain concentrated near the ground, leading to dense, persistent fog and overcast conditions.
Wintertime inversions are particularly strong because the sun angle is low, providing insufficient energy to warm the surface and break the cap. High-pressure systems often settle over the region, which enhances the sinking and warming of air aloft, reinforcing the inversion structure. The winter inversion locks the moisture in place, resulting in the prolonged gray periods that define the season.