February 09, 2010

Why dense morning fog is a good sign!

You wake up and look outside only to find amazingly dense fog. Like the example above. You can barely see across the street, if that. Time to get depressed about another cloudy day? Pull the covers over your head and think of Maui? No way! Super dense fog is ironically a very good sign...one suggesting a sunny day ahead. Get out your sunglasses and your favorite recreation gear...you will probably be able to use both.

How can this be true?

Super dense fog, like what we had two days ago, requires intense cooling near the surface. Cooling that brings the low-level air to saturation to produce a large number of fog droplets.

And the most effective means of delivering such intense cooling? Intense infrared radiational cooling to space from the surface. To get such cooling you need a lack of clouds and relatively dry air aloft. Why? Clouds greatly lessen surface radiational cooling because they intercept some of the radiation moving upwards and send their own radiation back down to the surface. Dry air is good, because water vapor in the atmosphere can act like clouds, but to a lesser extent. The above conditions...lack of clouds and dry air aloft...are often produced by high pressure aloft, with sinking motion. High pressure is also associated with low winds.. which is good for fog, since strong winds can mix dry air down and prevent fog formation.

So relatively clear skies aloft, high pressure, and low humidity allow strong radiational cooling from the surface, which cools the ground intensely. The ground in turn cools the air right next of it. Strong cooling can bring the air temperature down to the dewpoint and fog forms. Dense fog.

This kind of fog is generally quite shallow because it is dependent on the strong surface cooling, so eventually the radiation from the sun can burn it off.

Thus, dense fog implies high pressure, lack of clouds aloft, and a good chance for a private sunny day...like Tuesday.

Want to see the changes in temperatures as this mechanism occurs? Below are temperatures in Seattle from 4 PM to 10 PM on Monday night...see the cooling develop at the surface?

We can watch the fog burn off from space! Below are two high-resolution satellite pictures...one at 10 AM, before the fog burned off, and the other in mid-afternoon. Note how much of the Puget Sound lowlands became completely clear. Another beautiful winter afternoon in Seattle! By the way, as noted by one of the comments below, fog tends to burn off from the outside in...something we learned in the 70s after the advent of high frequency visible satellite pictures.

In the middle winter (December and January) the sun is sometimes so weak that the dense fog never burns off. But now that we are in February the increasing sun strength makes such a depressing situation less likely. But even during those super-foggy days of mid-winter another strategy is available to get your sun...go up! Get above the shallow, cold fog layer Even modest mountains (Tiger or Cougar Mts) are enough. On these days hiking in the foothills or the Cascades can be warm, sunny and glorious, while Seattle is in the murk.

I have a detailed explanation of the above in my book...with pictures of an ascent up Cougar Mt. during such a surface cooling event.

My next blog will be on the inside story of the local math "wars", including the Superior Court Decision telling Seattle to rethink its textbook selection, another court decision throwing out the Discovering Math publisher's lawsuit against the State of Washington, the cancellation of a Bellevue Math night to keep its citizens in the dark, and some very good books selected by Lake Washington School District. And the impact of the Obama administration's math education policies. And much, much more! This may take two blogs....


  1. For the first time I thought I didn't have anything to say, but i do.

    You posted that visible sat. pic and what's interesting about watching the fog burn off during the day is its uniform pattern. It most always burn off from the outside inward to a central point.

    But at night, w/out visible sat., we rarely see how fog is formed. I believe it forms in spots and morphs into one layer, but i dunno. I know we have infrared satellite, but Cliff you need to build a night vision visible satellite that uses light from the moon and the stars. You'd make millions

  2. On these super foggy days it's often beautiful and sunny right from sunrise where I live up at about 600 feet in the north end of Sammamish (near Redmond).

    Can the fog layer really be that thin?

  3. Cliff: Hope someone showed you the Colbert Report top story on weather. Great lampoon of local news and weather disasters.

  4. Cliff, your blog is habit-forming! I often check in from east of the Cascade crest (Cle Elum). It's amazing how different the weather can be over a distance of less than a hundred miles. Dense winter fog here is a bad sign. Once it rolls in, it might stay for a couple weeks. It still means high pressure, but the valleys are socked in under the temperature inversion. We have had a lot of that this winter.

  5. For a television meteorologist, would you say fog burning off is a valid way to describe fog dissipating ?

  6. Dense morning fog is a good sign for most people, but not for those of us who love to hit the water before sunrise...

  7. Thanks Cliff for making such an indepth weather blog. Im very interested in the weather and I have book marked your site and recommended it to many of my weather friends.

    Weather Station Reviews


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