Snowy day (overhead) – Cloud-maven

Cloud tops in those deeper Cu reached -30 C (-22 F) yesterday, plenty cold enough for lots of ice, with a few scattered very light showers reaching the ground, even a few drops here in Sutherland Heights a little before 5 PM, qualifying for a day with a trace of rain. Imagine! Rain! What izzat?

Here is that day below (if you want the short version, go to the U of AZ time lapse film department, online here).

Day Summary: Sunny with Altocumulus castellanus/floccus virgae in the early morning, then in the late morning, Cumulus humilis and mediocris, the latter with virga; Cu grew into shallow, but very cold Cumulonimbus capillatus (lots of “hair”-ice), with virga and RWU (rain showers of unknown intensity) in a couple of spots. Sunny again in mid-afternoon, but Cumulus re-developed in mid-late afternoon with more virga and some Cu reaching the shallow Cb stage with sprinkles here and there.

That’s it, in kind of a jumbled form. Hope you logged all these clouds and changes yesterday.

9:55 AM. Altocumulus castellanus/floccus virgae

11:06 AM. Cu developed rapidly over and downwind of the Catalinas. Lots of ice at far left, indicating how cold those clouds were, likely around -20 C (-4 F) at cloud top by this time. Bases, too, below freezing by this time.

11:10 AM. A classic, gorgeous example of Ac castellanus and floccus (the latter has no base, just a turret). No ice visible here.

2:07 PM. Cold Cumulus and Stratocumulus filled in as trough apex was about to pass overhead (wind shift line, one you can see in the movie linked to above).

2:08 PM. Rain showers reach the ground toward Marana whose city limit is pretty much everything you see in this photo I think…

4:00 PM. Clearing developed for awhile in the mid-afternoon, but then Cu were quickly reforming. Lots of ice again in this one at left, the oldest portion of the cloud where droplets are evaporating, but the ice becomes visible because it doesn’t evaporate as fast as the droplets do. BTW, air flows THROUGH the cloud, youngest portions on the upwind side, oldest portions downwind. You want to know that if you’re flying around with a research aircraft because if you’re only targeting the young portions, you’re not going to find the “correct” amount of ice that developed in that cloud. Some researchers apparently did this and reported in journals anomalously low ice particle concentrations for the cloud top temperatures that they sampled. For the sake of courtesy, I will not mention their names. But this is why at Washington, we always found a LOT of ice because C-M /we knew where to go!¹

4:31 PM. This was probably the deepest cloud of the day, and there is some suggestion of soft hail (“graupel”) falling out as would be an indicator of some higher liquid water contents before it converted completely to ice. Graupel comes from ice crystals or snowflakes that have bumped into a lot of supercooled cloud droplets that then freeze instantly on the crystal, helping it fall faster and collide with more droplets on the way down, a process called “”riming.” Pilots know full well about riming.

6:24 PM. Nice sunset color in clouds and on the mountains.

Don’t see any weather ahead that I like, and so not talkin’ about that today.

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¹One of the many pioneering innovations here at “cloud-maven” is the novella-sized figure caption.

By 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.

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