Stunning Cloud Photos and Falling Ice Crystals

During the past several days I have enjoyed a wonderful treat, as folks around western Washington have sent me some spectacular cloud photos.   And as I will talk about later, many have a common element: ice crystals falling out of upper-level clouds.  But first, lets take a look at a few.

Kirsten Owens of Bainbridge Island sent this one.

 Another taken by Mike Doherty near Green Lake

One from KPLU head, Joey Cohn, looking west from West Seattle.

And Chris Hyde of Olympia sent this one, which he described as a "freak cloud"

All of these pictures share wispy streaks of clouds that curve and twist.   In fact, these are ice crystals falling out of mid and upper level clouds.    Such features are often called fall streaks or mares tails, and get curved and distorted because wind varies in the vertical (wind shear).   Sometimes they are also called virga, particularly if they evaporate before reaching the ground.

Your perspective makes a big difference.  In the last picture, the fall streaks look horizontal because the view point was under the cloud.

Let's go back to the curved structure of many fall streaks, such as the one shown below.  The horizontal winds in the atmosphere vary with height.   Generally, winds are stronger aloft (often highest at jet stream level---25,000-35,000 ft).  So precipitation generated at higher tropospheric levels near the jet stream falls into slower air, tends to slow down, thus producing a curved structure,

But there is another subtlety.  As the ice crystals fall, they tend to evaporate and become progressively lighter.  The fall velocity of the ice particle is dependent on the weight of the particle and thus the descent of the ice particle progressively slows.   With less vertical motion, horizontal motion dominates, causing the ice particle to predominantly move horizontally.  The schematic below illustrates this:

But whatever the physical origins, the beauty of falling ice crystals, illuminated by the colored light of sunset or sunrise, can be spectacular, moving, and possessing an ethereal quality of subtle beauty.