SEATTLE, Wash. — Clear autumn nights bring on more mornings with dense fog in Western Washington, often creating a hazard for drivers.
The process for fog formation begins after sunset, when the ground starts losing the heat from the day. If we have clear or clearing skies overhead, that heat escapes easily out through the atmosphere. As the ground cools, the air right above it cools too. If the temperature drops enough, the moisture in the air will cool enough to become a cloud. That’s fog!
Light winds are also a key ingredient. Wind less than about 5-7 miles per hour often won’t keep fog from forming, but stronger winds will stir up the atmosphere too much. On the calmest nights, a very thin layer of fog can form and stay put into the morning. At first light, the ground can be enveloped in fog while the sky overhead remains completely clear.
Some of Western Washington’s foggiest spots are in the South Sound and south interior. Areas around Olympia, Shelton, Chehalis, and parts of Pierce and Thurston counties often cool more quickly at night than locations closer to larger bodies of water. This “extra” cooling is helpful for fog formation.
Valleys near the Cascades are another favorite location for fog. As the ground cools overnight, cold air naturally drains downhill and collects in low-lying areas. The valleys become pockets of colder air while surrounding hillsides remain a bit warmer. Places such as the Snoqualmie Valley, the Enumclaw area, and other river valleys can wake up to dense fog while nearby higher terrain has little or none at all.
In October, we usually can get the fog to “burn off” rather quickly. Once the sun rises, the rays warm the ground again, the air temperature climbs, and fog “disappears.” That’s why some of Western Washington’s clearest autumn afternoons can follow some of the foggiest mornings.
Through the weekend, we’ll have patchy fog, so if you wake up to gray and gloom, it’s likely to clear up in just a few hours!