Want to keep the excitement level up…after all, it is raining somewhere near by… Besides, if you’re excited, it will sound like more happened than really did. (Another ploy is to report large amounts of rain somewhere else: “An Avra Valley location got 0.39 inches late yesterday and last night.” Really did.)
Well, at least this rambling low “below” us in Mexico is not a total “gutter ball.” It was looking at little grim for rain last evening with all the stars out. Here is a loop of that low’s drift, with obs and satellite imagery on a 500 millibar map from my former “home” Department at the University of Washington:
You will see here that a blob of deep clouds is sneaking around in “back” of us and will likely deliver an odd fall of rain from the northeast as the low scoots off to Texas. The models, in all their wisdom, think the rain in those sneaky clouds can get over the Cat Mountains but to the east of us here in Catalina. Don’t expect a lot, then, maybe just a hundredth or two if we are lucky this morning before those clouds starting thinning and moving off. Still, it would be something. Rain and snow threats are still in the model outputs (last night’s 5 PM AST one) in the days ahead for us, but have diminished in magnitude; become more marginal in later runs, dammitall. Hence, this font size to reflect that.
Yesterday was one of the great days of clouds we have here so often, so pretty with all that snow coming down out of such little wispy clouds. Here are some scenes, beginning with the momentary underlighting of Altocumulus clouds shedding long virga trails. In the second shot, I will no doubt have to admonish people again that to see what I saw, you must hold your computer monitor over your head. This was a long fiber of virga (snow), like those shown in the first photo, but was overhead. So nice!
Cloud tops of these little clouds, those flat flakes at the top of the virga (sometimes called, “generating cells”, source regions for all that ice, by my reading of yesterday’s Tucson sounding, were about -25 C (-13 F) at about 15,000 feet above the ground here. These are low temperatures for us at that level, BTW.
The rest of the day was generally a joy, too, with terrific examples of glaciating (turning completely to ice) Altocumulus castellanus clouds (next shot). Those whitish turrets in the foreground above the bush have glaciated, while the others are still mostly liquid water clouds, on their way to becoming icy, and cirrus-like. This glaciation has just happened and you can infer that because the snow has not yet fallen from the bottoms of those clouds above the bush. If you really look hard you can see the telltale filaments, fibers, stranding in those clouds, too.
A really nice example of “Ac cas”, turreted middle-level clouds, are those whitish clouds in the distance toward Kit Peak. That next shot is of an Altocumulus turret long after it has glaciated with only a ghost of its former height shown protruding above a long, fibrous, icy trail. There may have been other smaller turrets as well.
What was happening here is that the air just below these Altocumulus clouds, was only moist for a short distance, and then became extremely dry. So, initiatlly, the ice that fell out of these clouds may have grown in size, but then when they starting falling into the dry layer, began evaporating rapidly. As they did, the fallspeeds of the crystals went down to practically nothing and so, being something like diamond dust snow, seemed to float horizontally in the air. And as they do this at these small crystals sizes, they lose the stranding become something akin to a fog of ice crystals that gradually disperse from the parent clouds. Sometimes layers of Cirrostratus clouds, or large patches of thick Cirrus (spissatus variety) form in this way. An example of ice clouds having formed like that yesterday in shown in the next shot.
Finally, just some shots of some pretty clouds, the next (supercooled) Altocumulus perlucidus (tending toward castellanus) and NOT showing any ice, and then just more shots of those little snowstorms in the sky.
The End, finally!