Creepin’ Cirrus; pedantry on display

“Storms” at 30,000 feet, single ice crystals falling from various varieties and species of Cirrus clouds.  That’s about all we got for “weather” in the next few days as Cirrus creeps up from the tropics into Arizona.  But those Cirrus produce great sunrises and sunsets, so have camera ready.  And while not much is happening, you should practice logging what you see up there.

Cirrus clouds are the first clouds we see when something is up with the weather, even when it stays up high, but even in these “storms at 30,000 feet”, the moist level tends to decline over time, meaning there might be a chance for mid-level clouds to appear….such as, you guessed it,  say, Altocumulus clouds, clouds mainly comprised of droplets, in the near future.  That would be pretty exciting; mo’ better sunsets!

Maybe if it was a “cold one”, an Altocumulus cloud with virga hanging out of it, would give you a great opportunity to talk with your neighbors about the Wegner-Bergeron-Findeisen1 precipitation mechanism (be sure to use all three names to attain the greatest personal stature with them).

——–Warning!  Beginning pedantic unit———-

What’s “WBF”, you say?

Hell, you see it all the time!   Well, actually only once in awhile here in Arizona.  Below, in a pictogram,  is a representation of “WBF in action” from a few days into our cold spell just passed so you’ll know when you see ice virga hanging from a droplet cloud you’ll know what the HELL has happened up there. (Dry spells, such as we are in now, make me want to cuss that bit more.)

The background.

Your car has been parked outside all night, and the air was moist.  You finally wake up and go outside and you see that dew has formed on your car windows, well, all over.  But even though its a bit below freezing, not too much because we’re in Arizona, you also see that in a couple of spots,  ice has formed;  “horror frost” crystals as we call them here in Arizona because we don’t like frost and cold air of any sort.  (The real name is “hoar frost”, and watch out how you use that in a sentence.)  The remainder of the drops you see on the car are still in the liquid phase, or have JUST frozen.

But here’s the exciting, magical thing that happens around those “horror frost” ice crystals, demonstrated with a photo through a car window.  I’ve added stuff on this jpeg to help explain the magic show going on when the two, liquid and ice, mingle.

A recent example of the WBF in action.
A recent example of the WBF in action.


And what you see that has happened here is the same thing that happens in clouds when ice and droplets mingle, are co-located so-to-speak.  When an ice crystal forms in a droplet cloud, it becomes a vapor hog, water molecule hoarder, because at water saturation, the condition that results in the drops forming in the first place, its SUPERSATURATED with respect to an ice crystal!  Amazing, and CRITICAL for life as we know it on this planet because most of the precipitation that falls in mid-latitudes is related to this process.  We would have virtually no precip here in Catalina ever without this process.

What does that mean?  When the two phases are in proximity as here, the droplets nearest the crystal evaporate, and the ice crystal grows and falls out, usually, as precipitation.  Most rain on this planet is due to that process! There are two others that are also important, all ice, all liquid, but today I’m only talkin’ WBF, the “mixed phase” process.

In a cloud, especially a flat one like Altocumulus, this “mixed phase” condition results in ice crystals that grow too heavy to stay in it–its kind of like a “Thanksgiving-for-ice-crystals” inside a mixed phase cloud, and they fall out in those fine strands because they are so fat.

In another way, the ice crystals in a mixed phase cloud are like a low pressure centers, the droplets high pressure centers and the molecules move from high to low pressures.

Mixed phase clouds would go away completely, of course, UNLESS there was some upward motion to keep new droplets forming.  But, as in “Ghosts of the Perlucidus” blabbed about here a couple of days ago, sometimes there isn’t enough upward motion to keep the droplets “alive” and only a ghostly remains of the droplet cloud can be seen in a thin patch of ice.

Further detective work re the above jpeg.

Those ice crystals has to have formed when the drops around them were still liquid, probably just as the window reached a freezing temperature or a hair below.  If all the dew drops had frozen at once as clear ice, you would not have seen this crystal growth happen because everybody is in the solid phase, no “high or low pressures.”  So, while dew drops were forming and growing while the temperature dropped below freezing, there was something quite unique about a particle on the window that caused ice to form when most other places were quite happy to be liquid.  We call those special particles that might have triggered an ice crystal, “ice nuclei”, though, too,  there may have been a window surface imperfection that did it.

Anyway, ice nuclei are always much rarer in clouds than “cloud condensation nuclei”, particles that the cloud droplets form on.  A demonstration of that is in that photo above.

Cirrus clouds, almost never having water, “don’t need no water” because its often  supersatured with respect to ice above 30,000 feet without having a droplet cloud.  So, even without the water phase, an ice crystal can get fat and fall out in many Cirrus clouds, such as the revered, Cirrus uncinus with its pretty trails.  Veil clouds like Cirrostratus?  Not so much growth.

———-End of pedantic unit———

Yesterday’s clouds

You may have spotted those creepin’  Cirrus at sunset yesterday.  If not here they are, a classic example, ones that kind of drift up to the north out of the tropics into Arizona that from weak circulations down there, even in drought times here, periodically passing overhead, keeping our skies from being boring, particularly for those many of you who out there who are cloud-centric, head-on-a-swivel when outdoors:

5:48 PM.  Disant Cirrus, loaded with a few contrails, creeps toward Catalina.  Outta be here by now but its dark and I think I can make out something so this is not really a forecast because I kind of cheated by looking at the sky just now.
5:48 PM. Distant Cirrus, loaded with a few contrails because there’s an airway down there, creeps toward Catalina. Outta be here by now.   Its dark and I think I can make out something so this is not really a forecast because I kind of cheated by looking at the sky just now but I WOULD have said that without looking…  And if those clouds did get here, there might be nice sunrise for you.


What ahead?

Of course, there’s some rain on the model “horizon” (IPS MeteoStar rendering of WRF-Goofus model), but like that “puddle” on a desert highway on a hot day that you never get to, and I’ve tried, because it  moves away as you speed down the road, the “puddle” staying the same distance away, our model rains seem to do the same thing.   I’ve used this metaphor before, but I can’t think of a better one.   I think its pretty good, too; damn good, really, to cuss a bit more.  :} Here are a couple of examples of rain in southern Arizona from last night’s global model run to get your hopes up, most likely to be dashed:

Valid for 5 PM AST, January 28th, Monday.  Green areas denote the rain the model thinks has fallen in the prior 12 h.  Yeah, right!
Valid for 5 PM AST, January 28th, Monday. Green areas denote the rain the model thinks has fallen in the prior 12 h. Yeah, right.


Valid for 5 PM AST, February 1st, Friday.  I would gladly eat my words with whipped cream on them if this happens.
Valid for 5 PM AST, February 1st, Friday. I would gladly eat my words with whipped cream on them if this happens.




1They’re not “mostly dead” now, but “all dead”, to crib a line from “The Princess Bride.”

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.