One of the great humilis days of our time; began with virga above Ms. Mt. Lemmon

I was really happy for everyone out there when the skies were dotted with so many perfect examples of Cumulus humilis.   It was like a numismatist finding a perfect Indian head penny.  If you were like me, and I suspect you are, you were just going CRAZY taking pictures of those flat little pancake clouds.  Those clouds were pretty much limited to about 1,000 feet (300 m) thick at most

Not cold enough for ice in them, of course, since the temperatures at Cumulus cloud tops were only around -3 ° to -5 °C (28 ° to 23 ° F, respectively).  Around here, ice USUALLY does not appear in clouds until the temperature is lower than -10 °C at cloud top.

Yesterday began with some light snow falling on Mt Lemmon…well, it was falling downward TOWARD Ms Lemmon, actually.  Fell out of some thick Altocumulus clouds up there around where the cloud top temperature is… what?  OK, silly question for you, probably lower than -15 °C (5 ° F).

Let’s check the sounding to be sure, remembering that the launch site (University of AZ) was downwind of air flowing from the NW yesterday that went over the Catalinas, so a sounding at the U of AZ might suggest higher temperatures than this cloud was actually at since the air was probably descending before it got there.

Indeed, as just seen by me, the TUS sounding indicates that layer, up around 14 kft above sea level, 11 kft or so above Catalina, not a city, but rather a Census Designated Place or CDP, was “only”at  -10 ° C.

I reject that as the temperature of the virga-ing cloud over Ms. Lemmon!  Its a little too warm IMO.

8:41 AM. That white haze under the Altocumulus cloud is composed of ice crystals, concentrations probably a couple or less per liter of air. Likely stellar or plate crystals, ones that form at temperatures less than -10 ° C.
8:41 AM. That white haze under the Altocumulus clouds is composed of ice crystals, concentrations probably a couple or less per liter of air. Likely stellar or plate crystals, ones that form at temperatures less than -10 ° C.   Almost certainly no aggregates of crystals; concentrations too low to form “snowflakes” which are aggregates of single crystals.  Snowflakes form when higher concentrations of crystals collide and get locked together, as in stellars, and their cousins, dendrites, that grow in a similar temperature regime.  Dendritic crystals are usually seen in deeper clouds than these because those crystals have time to grow extensions in various directions, are not just “planar” ones.  If the cloud is thin like this one, not much growth can take place in the droplet cloud and simpler crystals like hexagonal plates and stellars (Christmas card crystals) fall out.  There is a lot of hand-waving here….
10:20 AM. A horse named “Zeus” looks to see if any Cumulus clouds are forming over the Mogollon Rim to the NE, or, maybe he’s fixated on the horses in that corral below…

By afternoon, the skies over Catalinaland were spotted and dotted with spectacular Cumulis humilis examples.  (The littlest shred clouds are Cumulus “fractus.”)

I’ve left the time of the photos off today.  After all, there was only one true time yesterday, “perfect humilis time!” or as we like to say, “PHT.”   Immerse yourself.

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The End

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.

1 comment

  1. Art: You should be receiving a piece of ‘snail mail” from me this week. I photocopied a couple of cloud pictures and a cacti for the 3rd picture on the sheet. Hope you like them- I’m just a bit disappointed with one of the cloud shots- it turned out too faded for me, but maybe you can still make out the details of the cloud structure.

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