Visibility: Its Back!

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

It has been gone for weeks and months.

Visibility.

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.