Frosty the Cumulus cloud

No NWS sounding from the U of AZ Weather Department yesterday afternoon, so’s we can’t really tell with solid data what the temperatures of yesterday’s frosty clouds were.

However, with a max here in the Heights of Sutherland of 71°F, and with a dry adiabatic lapse rate to the bottoms of the clouds (as is always the case on sunny afternoons with Cu), if we estimate how high the bottoms were with any accuracy we can get that bottom temperature.

You already know as a well-developed cloud maven person that they were WELL below freezing which could see by noticing how far the snow virga extended below the bases of the Cumulus, at least 3,000 feet.  and more from the larger clouds later on.   So we have something,

Let’s say bases were at 14,000 feet above the ground over Catalinaland–they were way above Ms. Mt. Lemmon at 9,000 feet which you could probably tell.  That would make the bases at about 16,000 to 17,000 feet above sea level in the free air, pretty darn high above us.

From a ground level of 3,000 feet, and with the dry adiabatic lapse rate of 5.4°F per 1000 feet, that would make the cloud bottoms a cold, cold, -2°F, or about -17° to -18°C!  COLD!  Then, tops, of  clouds only 3,000 feet thick (about 1 km), would be -28° to -30°C (assuming a mix of the dry adiabatic rate with the “moist adiabatic” rate, given yesterday’s conditions, or about 4° per 1000 feet, “plus or minus.”

Addendum–corrections, hope nobody see’s ’em:

Later analysis and the next morning’s NWS sounding from the U of AZ suggests that bases were closer to -10°C because they were not as high as CMP estimated.  Rather they were closer to 12,000 feet ASL.  Tops would not be quite as cold, too, more like -25° C and colder in the deeper clouds, plenty cold enough for ice in even the small clouds, and for the long snow virga trails.

Below, some samples of Frosty the Cumulus (Cumuli, plural):

1:22 PM. Small Cumulus (humilis and fractus) begin to form.


3:40 PM. By mid-afternoon, the slightly fatter Cumulus clouds (mediocris) start showing ice crystals coming out the downwind side, which is what that bit of haze is above the cloud outline.
3:41 PM. An ice haze is seen here, too, in the dissipating remnant of a humilis-sized Cu on the left edge, center.
4:10 PM.  Ice is appearing just about everywhere now as the air aloft cools a little more, and the Cumulus deepen upward some.  The clouds are likely no deeper than about 3,000 feet or about 1 km.  Tops, using the in-cloud lapse rate, typically a little less than the dry adiabatic rate, would be a frigid -27° C (-17°F) or so, which readily explains the ice-behaving nature of yesterday’s shallow Cumulus.  Even deeper clouds formed later in the afternoon and evening hours.  This image not left “blank” but has explanatory writing on it.
5:23 PM. Raindrops from heavy virga were beginning to reach the ground in several areas as evening approached. Overnight, a few fell on Catalina! These Cumulus complexes were likely more than 2 km thick, 6600 feet) topping out at temperatures well below -30°C (-22° F)


Not much ahead now.  Maybe a few more frosty Cu will form today… before things dry out and heat up.

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. Hi Art; Happy Easter to you! Not many cumulus clouds here- more of the stratiform cirro-stratus or alto-stratus types today. At least we’re NOT seeing the nimbo-stratus: That one was in abundance yesterday morning and all day on Monday.

  2. Hi, again Roland,

    Thanks. I was updating it since I had made a goof, and forgot about it until your e-mail. Nice you have something different these days rather than the usual Ns!


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