Advection Fog Over Lake Washington!

Folks west of the Cascade crest smashed record high temperatures today as a Pineapple Express continued to pump extremely warm and moist air originating from the Hawaiian Islands into the region. The fact that today was relatively calm and dry definitely helped, as we were able to get a little more solar insolation and didn’t have any winds aloft mixing cooler temperatures down to the surface.

The chart below shows today’s maximum high temperatures. Sea-Tac hit 68 degrees today, crushing it’s old record of 58 set way back in 1959. Auburn and Puyallup hit 72 degrees, and Tacoma hit 71. Only one place in Washington recorded a high temperature at freezing or below, and that was Camp Muir at 10,110 feet.

But in spite of our record highs, we had some pretty ‘cool’ weather features around the area. The coolest, by far, was the advection fog over Lake Washington.

My mom sent me the picture above earlier this morning from our back porch looking towards a fog-covered Lake Washington. This picture threw me off at first; I had never seen this before and wasn’t sure how you could see such thick fog over the lake. But after thinking about it, it quickly became obvious that this was advection fog.

Advection fog forms when warm, moist air flows over a cool surface, usually water. The surface cools the air to the dewpoint, and as a result, fog forms. Advection fog is common in mornings along the coast of the Pacific Ocean in the summer due to the cold ocean temperatures and the relatively moist, mild airmass above it. This fog tends to “burn off” over the course of the day as the high summer sun warms the air above the dew point. San Francisco is famous for its advection fog.

While it’s common for advection fog to form over the ocean in the summer due to the warm air temperatures and cool ocean temperatures, it’s much more difficult for it to form in the winter. For advection fog to form, you need the dewpoint of the air to be higher than the surface the air is advecting into, and you need to have a high enough relative humidity so that a slight drop in air temperature can lead to saturation.

There’s a buoy in Lake Washington (the red point above) that measures both weather and lake data. Taking a look at the relative humidity at the buoy, you’ll see that relative humidity never actually touches 100%, meaning that if these measurements are correct, fog didn’t form at this location. I found this surprising, as there were widespread reports of fog all over the lake. It would be interesting to have an aerial shot to see the distribution of fog over the lake and determine if this was a faulty RH reading or not.

The profile plot below shows that surface temperatures at the lake were approximately 10.3 degrees Celsius (50.5 Fahrenheit). Air temperatures were around 11.5 degrees C, so with a relative humidity of 94%, the dewpoint was around 10.6 degrees C.

As the plot below shows, the Lake Sammamish buoy had fog from 11 pm 11/21 to 1 pm 11/22. I didn’t hear of any fog reports over the sound, but the observations from the West Point buoy support the idea of fog early Wednesday morning.

This fog even made it into Renton as winds briefly switched northerly and pushed the fog onshore.

I don’t think we’ll see advection fog over the lakes Thanksgiving morning simply because we’ll have stronger winds and the air will not spent an adequate amount of time over the lake to cool to the dew point. But it sure was cool to see it this morning.

Finally, here are some other pics of the fog!

A view from our rooftop of Lake Washington. Very warm and humid air over the cool surface of the lake making fog this morning. Likely occurring across many area lakes this morning! #wawx #Seattle pic.twitter.com/nklZ9GIfPD

— NWS Seattle (@NWSSeattle) November 22, 2017

Yes indeed it is at Green Lake pic.twitter.com/1AqZmCGKtV

— Dustin Guy (@GreenLakeWx) November 22, 2017

Have a great Thanksgiving!
Charlie