July 15, 2014

Was the Best Sunset in Years the Result of Wildfire Smoke?

Sunday's sunset was spectacular--many people feel it was the best in years.    But why?

There has been some  analysis of the sunset in the Seattle Times and elsewhere, but I think they might have missed some of the key details.   Let's go further here.

But first, here are two pictures sent to me by Dave and Jill Going.  Unbelievable.

Scott Sistek's blog at KOMO TV has more pictures (Scott does a wonderful job gathering weather photos).

But why was this sunset so amazing?

We start with the right season in which sunset and the transition into twilight is longer than other times of the year.  Longer is better!

Then we get the perfect geometry.   You want a visible setting sun, whose red rays need sunset illuminate a solid, middle-level cloud deck.

Why the red light at sunset?  Because the atmosphere preferentially scatters shorter wavelengths (like blue, green, and to some degree yellow), leaving red.  At sunset, the light has to go through more atmosphere, thus more scattering and thus more red.

Sunday night, a mid-level cloud deck moved northward over central Puget Sound associated with an upper level trough that initiated mid-level instability, and there was clearing right behind and to the side of the clouds. Here is a visible satellite picture at 8:40 PM.  Clouds over Seattle, but clearing just to the west and southwest. Perfect.

Normally, we have large scale cloud systems that cover the sun or we have clear skies.  But to get this geometry just right only happens a few times of the year.   But what was this time so unusually beautiful?

There was something else.

There was a large amount of virga Sunday night...rain falling out of the clouds, with most of it not reaching the surface.   The virga (which is obvious in the pictures above) caught the red light from the setting sun, setting up shimmering curtains of red.  This was a very important component of the unique show and is obvious in the above pictures.  Or in the one below I borrowed from the KOMO website:

So we start with the ideal season, we get a fairly unusual cloud/sun geometry, and  have the very unusual situation of having extensive virga.   That alone was enough to give us an amazing and usual show.

But as the late night advertisements go, but wait!  There is more!

Particles from wild fire smoke are well known for improving sunsets...you get more particles scattering the light, providing deeper reds.

There are several wildfires burning in eastern Washington, including the massive Mills Canyon fire.  Lots of smoke east of the Cascades.

Where do you think simulated air trajectories starting from the fire on Sunday morning ended up  6 hr later?  Puget Sound. (see the WRF trajectories below)

The visibility camera  and particle sensor at the Puget Sound Clean Air Agency saw degradation on Sunday (shown at noon here)

And particles sensors at a number of locations near Seattle showed elevated amounts of small particles (smaller than 2.5 microns)....see the proof below.  You see the peak between 16 (4 PM) and midnight on Sunday?  Sunset time.  I also put the Ellensburg values so you can see where the stuff was coming from....it peaked a few hours earlier.

Although I won't show it here, one could follow smoke across the Cascades on visible satellite photos.
A number of people told me about a haziness in the sky on Sunday.    The modest amount of smoke coming across the Cascades would have enhanced the sunset.  The final contributing factor.

And one more thing, many of you also saw double rainbow to the east as the sun interacted with the falling rain. (picture courtesy of David Bruce)

And to make this totally surreal, there was lightning too!

Folks...this sunset was liking winning the Trifecta at a horse race.  An amazing syncing of various factors enhanced the show.  If you saw it, you were very fortunate.  If you didn't enjoy the pictures.

Global Warming, the Media, and Coal Trains

I will be giving a talk in Friday Harbor and Eastsound, sponsored by the San Juan Island and Orcas Is. libraries.

I will be discussing the serious threat of global warming, how the media is generally doing a poor job in educating about this issue, and how mankind is really not taking it seriously (e.g., the coal trains). 

Friday Harbor: July 22nd, 6:30 PM, The Mullis Community Center, 589 Nash St.

Orcas Island:  July 23rd, 5:30 PM, Orcas Center


  1. To take things over the top, there was a double rainbow over Lake Washington with that sunset. I saw it as I was biking home on the Burke Gilman...hopefully someone has a picture.

  2. Thank you so much for this post! One of the more amazing sunsets I think I've ever seen- great to know the reason! Do you have an email list?

  3. Buried in the middle of Shoreline, our lot is not exactly a great sunset location, so it wasn't something I was expecting. Fortunately, towards the end of the hour-long evolving event, my wife noticed. Usually the set is off below the west horizon with nothing noteworthy to capture, but not this one. "That's unusual" I thought, with reds and pinks high above the trees. I stepped inside to grab the Nikon and did manage one good shot, even though the extent of the color had diminished in that time. Fur silhouettes over deep pink and royal blue. Not bad for our drive way. https://www.facebook.com/media/set/?set=a.10152633658696995.1073741851.564261994&type=1&l=a3a50b5d1f


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