Why a giant fog cloud is parked over Seattle area, and when it will clear

Would you rather have dense fog or constantly on-and-off rain? That’s the question many Seattle-area residents might be asking themselves this week.

Low visibility fog is projected through Thursday due to an area of high pressure above Seattle, according to the National Weather Service.

Visibility through Tuesday will be less than a quarter-mile, and Seattle will likely be back to its typical rain showers by Friday evening when a weather system moves in, according to NWS meteorologist Mary Butwin.

Some of the fog that is lingering around the lowlands right now is “radiational fog,” she said. Thanks to clear skies, surface temperatures dropped to the dew point, allowing moist air to condense into a fog. The absence of wind is allowing the low-level cloud to linger around with no air to mix it out, Butwin said. While visibility may improve by Thursday, the fog is expected to clear by Friday afternoon, she said.

A stagnant air advisory is also in effect through noon Wednesday for Western Washington. People with respiratory illness should follow their doctor’s advice for dealing with high levels of air pollution during periods of stagnant air, NWS said in the advisory.

The Washington Department of Ecology is urging people avoid unnecessary burning because the air stagnation throughout Washington could allow pollution to build up.