Weather Blog

Seattle awakens to a Needle in a fog bank

Seattle awakens to a Needle in a fog bank

SEATTLE - Those of you who were lucky to be atop Seattle's Space Needle or other skyscrapers early Thursday morning were treated to something that much of the rest of downtown missed out on: sunshine.

A thin fog bank just barely made it in from Elliott Bay at dawn -- obviously only a few hundred feet thick and shorter than the 520 feet where the Needle's top observation deck sits.

As the time lapse video above taken from our camera atop Columbia Tower shows, this fog didn't just form and sit parked over the city, it blew in from the west; its origins from the coast.

There are two different types of fog, and the Northwest gets both varieties.

"Radiation fog" (sounds scary, but it's not) usually begins on clear nights. As the temperature drops, the air cools to where it becomes totally saturated. The air condenses into water droplets, which we see as fog.

"Advection fog" is more like an imported fog and is created from a warm air mass that moves over a colder area -- very common here in spring and summer -- and then gets blown into another area. In our case, it's created by the warm air of summer sunshine condensing into clouds when it interacts with the cool air over the chilly Pacific waters. That marine layer is out over the ocean on most days this time of year.

The typical wind flow this time of year is from west to east, known as "marine flow" or "onshore flow" -- air coming from the ocean. Those winds will blow in the fog from coast overnight -- known as a "marine push" -- but it then depends on the strength of the winds as to how far that fog gets pushed inland.

Our unique topography also decides who gets fog and for low long. A branch of fog/clouds go east down the Strait and into the North Sound, another push comes in through the Chehalis Gap in between the Olympics and Coastal Range and socks in the south Sound. The Seattle area is the toughest place to get the marine layer because it’s essentially the farthest point the clouds have to travel, so downtown only gets it when it’s a decent push.

And that's what happened Wednesday night, the push was strong enough for clouds in Everett and Tacoma, but just barely strong enough to make a thin layer of clouds into the Puget Sound region -- thus the clouds hugging the shores that burned off as they got over the warmer land mass away from Elliott Bay.

The clouds then all burned away as the air warmed further in the morning and evaporated the fog bank... and voila! A sunny day.

This pattern is going to stick around through the weekend -- again, we'll dance with the morning clouds and fog but plenty of afternoon sunshine. (There is a risk of isolated thunderstorms in the afternoon and evening through Sunday. Keep tabs on those on our weather forecast page.) But if you do need some morning sunshine and the fog layer is thin enough, just head to higher ground.