Thought I would run down to Golden Goose Plaza here in Catalina to catch some sunset shots after I thought of this headline; “gold on gold” :{ Here are three shots from the GGP for your viewing pleasure:
Lesson time… The clouds in the first shot aren’t producing virga. Why? Not cold enough you would guess, AND, the liquid cloud drops in them very tiny. The smaller the drop, the more freezing is resisted in clouds, the larger the drops, and the lower the temperature, the more likely it is that ice will form.
When ice does form in a droplet cloud, the drops around the ice crystal evaporate and the vapor appears on the ice crystal, depositing on it as new molecules of ice. Under a microscope on a glass slide, the crystal magically gets larger while a liquid drop next to it gets smaller and disappears. This process occurs in clouds that are comprised of both liquid droplets and ice crystals and the crystals eventually fall out as precipitation. The folks who described this “mixed phase” process, and at one time thought to be the only one that produced rain at the ground, was Alfred Wegner1, Tor Bergeron2, and Walter Findeisen. We won’t mention “riming” an additive to that process today, which is the collection of instant freezing of drops by the falling ice crystals making them heavier…
This “mixed phase” (liquid and solid together) process is ALWAYS described in weather text books from elementary to graduate ones. It requires the presence of droplet clouds and the introduction of ice to get the ball rolling. Altocumulus clouds are always mostly comprised of droplets, and so virga coming out of them is ALWAYS due to the “mixed phase” precipitation process. Tell your friends.
Here’s some surprising facts about yesterday’s Altocumulus clouds, even to me.
How high were those clouds? Well, according to the TUS balloon sounding they were no less than about 27,000 feet above sea level, or about 24,000 feet above us here in Catalina, and along with that, they were extremely cold, with tops indicated to be about -30 C (-22 F). The thinner ones on the right side of these photos would have been only slightly warmer, and as you can plainly see, had no virga, no ice in them even at those low temperatures! Pretty remarkable. In thin clouds like those not producing ice, you can bet the droplets were very, very small, likely smaller than 10 microns in diameter, and that small size of droplets is associated with a resistance to freezing. Such clouds also tell you that there is a lack of what we call, “ice nuclei” up there, substances around which ice can form, usually soil particles.
In the clouds with virga off on the horizon (3rd photo), they are clearly deeper, meaning the drops were larger near the top of the clouds, AND likely a bit colder as well, both factors leading to prolific ice formation and heavy virga trails. Hope this makes some sense. Pretty skies, anyway.
The TUS sounding for last evening at 5 PM AST:
Today’s clouds and weather?
Some mid-level moisture is still around, and so more Altocumulus clouds, along with some Cirrus should move in during the day and evening hours with virga possible, since they’ll likely be cold again. The U of A mod also sees lower, but very shallow Cumulus clouds likely, ones too warm to rain via the “mixed phase” process that you now know about. So, rain chances are pretty dismal later today and tomorrow. Nothing in the longer term, two week view either. Man, this is a LONG dry stretch!
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1Remember Wegner? The meteorologist that came up with the idea of plate tectonics around the turn of the century? But the geographers/geologists laughed at him for about 50 years until they saw that he was right. Why did they laugh at him? He was a weatherman, not a paleogeographer/geologist ( i. e., not a member of the club).
2Here’s a photo of Tor Bergeron from 1968 in case you wanted to see what he looked like. I don’t know who that is standing next to him…looks like someone who might have been influenced by Buddy Holly.
