Yesterday, one of these went over (“the rest of the story”)

First, the background, precursor shot:

11:23 AM. Altocumulus perlucidus.
11:23 AM. Altocumulus perlucidus.

Then this, looking straight up when CMP first noticed it because he wasn’t paying attention:

11:24 AM.
11:24 AM.

A few minutes  later, as it moved away:

11:27 AM.
11:27 AM.

What happened?  How cold were these Altocumulus clouds?

(Answers printed upside down below).

There were more, off in the distance, too.

Here are a few more shots of this phenomenon:

11:29 AM. As it went over the horizon to the east.
11:29 AM. As it went over the horizon to the east.  You can really see how strange it made the sky look.  Note, too, the contrail from a much higher flying aircraft (at Cirrus levels).
4:02 PM. Those fine trails of virga, center of photo, were "probably" created by an aircraft. They don't look natural to me, though when this happens without producing a ice canal or a clearing, its much more difficult to be certain. Those trails look too flat, a result of likely very high concentrations of ice crystals, all of which remain small due to the competition for the vapor in that cloud, and would be too small to collide with cloud droplets. There would be no hole or canal because the rise rate of the layer is producing droplet cloud faster than the ice crystals could take it away. In the prior photos with the ice canal, which did not fill in, you can guess the rise rate of the layer that produced the Ac per was nil. Those clouds did not fill in as the ice settled downward and out of the layer. Whew, lotta typing just then.
4:02 PM. Those fine trails of virga (center of photo, trailing in strands to the right), were “probably” created by an aircraft. They don’t look natural to me, though when this happens without producing a ice canal or a clearing, its much more difficult to be certain. Those trails look too flat, a result of likely very high concentrations of ice crystals, all of which remain small due to the competition for the vapor in that cloud, and would be too small to collide with cloud droplets. There would be no hole or canal because the rise rate of the layer is producing droplet cloud faster than the ice crystals could take it away. In the prior photos with the ice canal, which did not fill in, you can guess the rise rate of the layer that produced the Ac per was nil. Those clouds did not fill in as the ice settled downward and out of the layer. Whew, lotta typing just then.
DSC_0878
5:23 PM. Numerous holes were being punched in that cold Altocumulus layer out to the southwest of us. If you are pretty observant, you know that there is an airway out there, often filled with contrails. These, though are likely produced by those aircraft below the normal Cirrus levels, but rather would be ones departing or landing, in descent or climb modes, maybe from TUS?
5:41 PM. Pretty, but not natural.
5:41 PM. Zooming; pretty virga, but not natural.

Now, we’re really quitting because I have other things to do, ones that have to be done, like discovering why our attic has so many rodents in it?  Well, one, every so often,  dammitall.  Why is life one problem after another?

2017010212Z_SKEWT_KTUS
The pre-dawn TUS sounding, PRESUMMED representative of that Ac per layer. It would have been where the two lines pinch together, and if somehow you can read the temperature, its between -25° and -30° C.
For the evening aircraft effects, there are two choices of layers. Probably was the lower, warmer one IMO.
For the evening aircraft effects, there are two choices of layers. Probably was the lower, warmer one IMO.

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Answers not printed upside down instead:

It was an ice canal created in a highly supercooled layer of Altocumulus perlucidus.  How cold?  Whenever you see one of these in  a middle cloud like Altocumulus, you can guess that its colder than -20° C.  They’re rarely seen in warmer clouds.  The TUS soundings suggested this layer was between -25° and -30° C.  It mostly was ice free, bur regions of some slight (natural virga) were seen,

It was probably created by a jet, though the rarer prop aircraft can’t be ruled out.  Seems to be associated with cooling around prop tips or some say over the wing cooling momentarily below around  -40° C, though visually I would offer that  its from the water-loaded exhaust, at least in jets, rather than air cooled as it goes over the wing.

The End

Author: Art Rangno

Retiree from a group specializing in airborne measurements of clouds and aerosols at the University of Washington (Cloud and Aerosol Research Group). The projects in which I participated were in many countries; from the Arctic to Brazil, from the Marshall Islands to South Africa.