November 29, 2020

When Dense Fog is a Sign of A Sunny Day Ahead

 When I woke up this morning and looked out, very dense fog dominated the landscape (see below).

I smiled to myself:  a "clear" sign of a very good day ahead with plenty of sun.

Or take a look at the amazing image from the SpaceNeedle PanoCam at 7:10 AM this morning.  A dense fog layer is below, but the tops of high buildings extend above the murk and if you closely, Mount Rainier is in the distance.  Above, blue skies and some beautifully illuminated cirrus clouds.

Very dense fog is inevitably shallow and forced by high pressure aloft, with clear skies above that allow intense cooling at the surface, which effectively radiates infrared radiation to space when clouds are not in the way.  Such intense radiative cooling brings the moist layer near the surface down to the dew point, and condensation occurs--producing the dense fog.

The latest visible satellite image shows the story from space, with the fog evident at lower elevations, but clear skies above a few hundred feet.

The sun is weak this time of the year, but this shallow fog will dissipate this morning.

But there is more.  You want warm temperatures today?   Say upper 50s in bright sunshine?

You can have it.  And I plan to enjoy it, by the way.

Near the surface, pressure is higher over eastern Washington, with lower pressure west of the Cascade crest (see image for 4 PM)

That will produce easterly (from the east) winds over the Cascades, and particularly in lower gaps and passes.   The same will be true on our coastal mountains.   As the air sinks on the western slopes of these barriers it will warm by compression, as the air moves from lower pressure aloft to higher pressure near the surface.    Just like your bicycle pump.

And because of this compression warming, there will be a band of warmer temperatures along the mountain slopes.  This features is clearly seen in the  forecast surface air temperatures at 1 PM today (see below).

So you want warmth?  Perhaps enjoy 60F in bright sun?  You know where to go.  A hike in the Issaquah Alps--like Tiger Mountain--will give you the warmth.  As will any hike or park above 1000 ft  on the western slopes of the Cascades should do the trick.

Time to find my sunscreen.

And by the way.....after some rain tonight, we expect an extraordinarily dry week ahead.


  1. The last significant storm showed us very low pres. (mB 970's) moving into the No.Vancouver Island area, followed by a very RAPID rise to High Pres. from a system to the South or SW. I mean, it was hours, not days! I hope you will explain sometime why there was little to no associated wind here with that very rapid and dramatic pressure change. Most perplexing, I say. Thank you. J P

  2. Went for our usual Alki walk. Fog had lifted on the Sound, but lingered in Elliot Bay in the afternoon.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

The Weather Regimes of Summer

 Weather patterns tend to get "stuck" for extended periods and we have certainly seen such persistent conditions this summer.    W...