Saturday, July 9, 2016

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.


4 comments:

Houseboat guy said...

They were lovely last night seen threw patchy low level clouds. PRO tip... Put on you polarized sunglasses when looking at the clouds. It clears the haze, darkens the sky and makes the clouds really pop.

Justin Bodeutsch said...

I was wondering about that cloud in Olympia, it was indeed very freakish. I took a panorama of it since it was across the whole sky and very straight.

https://photos.google.com/share/AF1QipMwp4pT7GfIapQue9yXYstCavVplCko9bQSQehDsxrCeH8S9QHB63Lcka4PNktutQ/photo/AF1QipMkL2MWQQ1SWlJ1tgR_-odd4b2NLpF9R7ECSbqh?key=ZlhKOHlCQmJpTzJQTjRBZnVuSG5uUGo5b3d4Um9R

Nanoo Visotor said...

Hyde's photo reminded me of something I saw over NCentralOR, am 05Jul. A long contrail was evenly punctuated along about half it's length (like a rapid-cycle p.d.e.).

Grace Oliver said...

Here's link to a fantastic video made by a storm chaser. Recommend turning off sound. http://www.dpreview.com/news/4861277900/vorticity-the-beauty-of-storm-chasing/1
It's about 6 minutes with a couple quick black outs for a few seconds.