October 01, 2012

Visibility: Its Back!

Both Sunday and today, something very precious returned to us.

It has been gone for weeks and months.


Mount Rainier was back in all its majesty, Mt. Baker jutted out to the north, the Cascades seemed close and well defined, and the Olympics again towered over the Sound.

Compare images from Puget Sound Clean Air Agency's visibility cam today and only a few days ago (Thursday). 

Why so good?

We have had a fairly major change in the trajectories of the air over us, with the flow in the lower atmosphere now from the north and northwest, thus pushing the smoke from the eastern Washington fires towards the east and south.  For example, here are the winds aloft over Seattle from 11 AM on Friday to 11 AM on Saturday (heights are in meters and time increases to the left).  Substantial wind change.

During the summer we often had bad visibility when air came off the Pacific as well, due to the enormous amounts of smoke coming from the Siberian fires.  But those fires are mainly out now.

In addition to clear skies, we had other visual treats....wonderful displays of jet contrails, and some of them were clearly apparent from space, particularly late Friday and Saturday.  First, a few contrail shots from my department.

Here is a MODIS visible satellite image for Friday afternoon over western Oregon...the white lines are contrails.  Lots of north-south tracks towards California.

Sometimes the contrails show interesting substructures.  I saw this one yesterday.

 Looking closely one can see two lines, one from each engine.  As the contrail aged, wavelike undulation appeared in the lines and then they appeared to revolve around each other.  I understand these are called contrail instability waves and represent some kind of shear-flow instability.  Pretty neat.


  1. hope that northerly air flow sticks around. i like the cleaner air.

  2. Also an amazing sunset last night...wish I'd had my camera...mandarin-orange behind purple mountains, clouds visible between peaks such that the mountains could clearly be seen one behind the other instead of appearing to be one solid flat monolith, and a massive dark blue-gray cloud shoving in from the south or southwest devouring it all. Absolutely brilliant.

  3. I couldn't believe how bright the sun was yesterday afternoon. Also, beautiful lenticular clouds last night. I would be happy to share photos with you Cliff but don't know where to send them or how to attach them.

  4. That contrail pattern is amazing. I remember first seeing it with a group of students in my cloud physics class. After class, I found out that it was called the Crow Instability, named after the guy who first described it. Crow's paper in 1970 gives a nice description about how it happens.

  5. Some of the contrail turbulence is due to wing tip turbulence, which follows behind the plane. I assume when I see contrail patterns like this that the winds aloft are very stable and allow the vortices to settle and disrupt the pattern in the sky.

    Watch this demostration:


  6. So much for the visibility. Today there was an east wind along the I-90 corridor. Thick haze was visible near Exit 38 and the air smelled like a campfire. Our old friend, the forest fire smoke, is back.

  7. Wow, Shelton hit 26F this morning: http://www.srh.weather.gov/data/SEW/RTPWA


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