Frost and Freezing Fog: Drive/Walk Carefully

 We a moist lower atmosphere, a cool air mass over the region, and relatively clear skies (which allows good cooling to space),  we had good conditions for frost and fog across the region.

So be careful this morning.   Fog and freezing temperatures are the most dangerous combination, capable of laying down a thick layer of ice on cold roadways.

But first:  the sunrise.  It is spectacular this AM, with a poor sample from a cam this morning.


Surface air temperatures dropped to below freezing over much of the state this AM (see below, click on image to enlarge), with near 0F numbers in really cold locations (like Mazama to Winthrop).

And never forget the differences around our region, with temperatures near the water above freezing, but well below freezing in the interior (e.g., Woodinville, Duvall)

For road safety, the temperature of the road surface is what counts and that temperature can be VERY different than the temperatures of the air, which is measured 2-meters above the surface.  In fact, air temperatures at 2-m can be 3-6F cooler than at the surface under strong inversion conditions.

Consider this morning.   Below is the latest (7AM) graphic of surface air temperatures and road temperatures (in boxes) from the City of Seattle SNOWWATCH web site.   A lot of below freezing air temperatures, with warmer temperatures near the water.  But there is a lot of variability in the road temperatures.

Road temperatures on bridges, like the University of Washington viaduct (30F) are below freezing-- a real icing threat if the city did not pretreat the surface.   In contrast, road sensors in contact with the ground (e.g. West Seattle, 40F) are "toasty."    Heat is transferred up to roadway surfaces when they are in contact with the earth.....and that warming is particularly significant if temperatures have been mild (and they have been).


Again, be particularly careful if temperatures are cold and fog is around, something that is occurring right now over the south Sound area.  Here are a few shots of WSDOT cams south of Olympia this morning.   THIS is exactly the kind of situation you should fear.  If you see such conditions, slow down and be particularly careful around turns.




Finally, a final sunrise picture from WSDOT:







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