November 04, 2013

Freezing Fog Hits the Northwest Lowlands

Of all the meteorological hazards of our area, which is most likely to injure or harm you?

Winds?  No.
Floods?  No.
Tornadoes?  No way.

The answer?  Roadway icing associate with freezing fog.  

And last night freezing fog made its first visit of the autumn to several locations in western Washington.

Freezing fog icing occurs when fog passes over a surface that is at or below freezing.  Fog in our area is made up of small water droplets.  Most of the time the fog is above freezing, but sometimes the fog droplets are supercooled, a condition in which the droplets are liquid even though their temperature is at or below freezing.  Amazing but true.

Cooler air moved in behind the storm on Saturday, and as the winds calmed and the skies cleared later on Sunday that allowed good radiational cooling at the surface that allowed temperatures to plummet and areas of fog to develop.  The visible satellite image this morning at 8:30 AM shows some of the fog around Puget Sound.

Freezing fog was noted at several locations, including McChord Air Force Base in the south Sound.  Here are the observations last night at that location.  Time in UTC (same as GMT, 8 hr later than PST).  They got down to 1/8 mile visibility (.12) and air temperatures of 32F.  And fog!

Remember that all surface air temperatures are reported at 2m above the surface--the temperature at the surface could well have been colder.

When fog passes over a roadway that is at or below freezing, the droplets in the fog rapidly freeze on to the road producing sudden icing. 

This can be very dangerous.  When I first started at the UW, I received many calls from lawyers looking for meteorological assistance.  The number one type of case they needed help with?  Roadway icing.  As I investigated a number of these events, I found that many of them were associated with freezing fog.  In fact, I was so concerned about this threat I created a web site just on roadway icing (found here).  On the website I also talk about other causes of roadway icing (frost, refreeze of melting snow).  But freezing fog is the big Kahuna.

Now where is the biggest risk for freezing fog?  Over bridges in relatively moist rural areas.  Rural areas are generally cooler and moisture helps the fog. 

 Why bridges?  Because there is air under excellent insulator.  Roadway surfaces in contact with the soil are warmed a bit by heat being conducted up through the soil...particularly in the fall when the soils are still relatively warm.  But there is no soil under bridges and they can radiate heat to space and cool rapidly.   So this time of the year, bridges are a particular focus of trouble. One thing I learned from doing legal cases:  probably half of the serious icing  accidents occurred on bridges.

So drive carefully during the night or morning when air temperatures drop down to 35-36F (or your car thermometer drops to say 35F); you should be thinking of icing.  If you also  see any fog around, slow down immediately and be ready for trouble...the threat is very real.

Announcement:  Ivar's Mukilteo Landing restaurant (my favorite weather-themed eating establishment) will have a special dinner and a presentation (by me) to commemorate the tenth anniversary of the on storm that destroyed the restaurant in 2003.  It will take place next Thursday, November 7.  More information here.  Limited to 40 people.
Seattle Residents:   Please consider voting for Sue Peters for School Board. Check out my previous blog for more information.  Corporate ed interests, pushing charter schools, excessive testing, strict control of teachers, and heavy use of computerized instruction are pushing their own candidate, backing by hundreds of thousands of dollars from wealthy donors and Pacs.  Sue Peters will be a strong, experienced, and articulate voice on the school board....and she supports good math education.  If you vote for only one race, please vote on this one.


  1. Sue Peters has won. And typing for myself, and several friends, this is largely because of your recommendations in this regard, Dr Mass. congratulations to us all.

  2. This is good news, but there is another important issue with Seattle schools that directly affects the Wedgwood community. On Friday evening November 1 at 7pm, Seattle Public schools released a new Growth Boundary Proposal (version 3) that significantly impacts Wedgwood. We need community support to help get fight this. More information is available here:


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