A persistent area of fog has spread over the central and southern Sound, and some of the pictures are stunning.
The 6 PM view from the Space Needle cam is simply beautiful, with some of the taller buildings extending above the murk. The lighting of the fog from below....
From the hills east of Seattle, the fog layer spread to the Olympics, with a beautiful sunset glow above.
Picture taken by Kristi Benda.
Not to be outdone, the image from space captured by the NASA Aqua satellite this morning was extraordinary, with fog and low stratus clouds constrained by local terrain.
Why so foggy?
Blame it on high pressure and clear skies aloft, which produce a strong inversion, in which temperature warms with height in the lower atmosphere.
Clear skies allow good radiational cooling of the surface, and high pressure aloft creates the clear skies and produces sinking air aloft, which warms the air by compression.
Want proof of the inversion? Below is a plot of the temperature change with height at 6 PM around SeaTac Airport.
Temperatures warm from the mid-30s near the surface to about 50°F around 2000 ft. That is a strong inversion! Inversions are associated with great stability, which means they suppress mixing of the air in the vertical. Good for keeping the fog in place!
Unfortunately, inversions have a major downside; they also keep pollutants in place, suppressing vertical mixing, which improves air quality near the surface.
Below are the current pollution levels (something called PM2.5), based on the PurpleAir network.
Moderate air quality at lower elevations (yellow colors) but far better at higher elevations (green colors).
This situation should continue tomorrow, but will dissipate on Friday.
Happy New Year!
What is it about the area in question that influences the development inversion-induced fog? Up in Western Whatcom County we aren't seeing much fog despite saturated (or nearly so) air and the presence of the strong temperature inversion.
ReplyDeleteThe winter of 1976-1977 had a similar though much more persistent western ridge/deep eastern trough or vortex over North America. For many days at a time, and recurring week after week, the PNW had monotonous foggy weather while lake-effect snows buried many Great Lakes shorelines and exceptional cold covered eastern parts of the continent. Even southern Florida had snow one day.
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