The Seattle area has been under incredibly dense fog for the last 4-5 days and it’s not going to end for a couple more days. The fog is caused by a temperature inversion. As you gain elevation, the temperature INCREASES. The temperature is right around freezing near the ground but if you go up 2500 feet, you’ll be at a balmy 60 degrees.
Inversions happen in the winter when we have a very stabile atmosphere. As cold air sinks and stays near the ground, a high pressure system can roll in on top of it and trap it. The hills and mountains provide the edges of a giant imaginary bowl. The higher pressure air up top is warmer because if you pressurize air it warms up. That warmer air sits like a cap on top of the cold air. If you head up about 1000 feet, you’re above that transition point and the skies are brilliantly clear, but down near sea level it’s thick pea soup. The air quality is incredibly poor because we aren’t getting any circulation. It’s also causing a lot of accidents as the thick fog freezes on the roadways overnight. Cliff Mass’s last few posts have been about this inversion if you want to read more.
Thankfully this should all blow out of the area on Wednesday the next system comes through.