You’re probably smiling now remember singing this little ditty as a kid, maybe singing it with your friends on the bus, whenever you saw “Altocumulus floccus virgae” clouds such as are pictured in the first photo. Wasn’t it great when you saw these kinds of clouds while on a vacation trip and mom and dad had to stop somewhere to get you some pie after you sang that song? Well, I nostalgiate here.
To the right of the dead yucca stalk, Altocumulus tufts are shedding snow. The opacity of the virga is a give away that its snow, and not rain. In some of these little tufts, the water droplet cloud that preceeded the formation of ice has disappeared, and all that is left is falling snow. How much snow is it? Just a flurry, if you were up there, even though it looks pretty thick. Once in awhile in our research on ice in clouds at the University of Washington, we got to sample these from top to bottom. Because the ice crystal concentrations are usually pretty low in clouds like the ones shown, a few per liter and often less than 1 per liter, those delicate ice crystals don’t bump into each other much and break up, and you find gorgeous images of star-like crystals in these fall out streaks, the kind you see on Christmas cards (examples here). How do I know what from ten miles away and 16,000 feet or so below them. Its a funny thing, but ice crystals are differently shaped depending mostly on temperature. To get the temperature of these clouds you can get a pilot report (unlikely) or examine the humidity profile of the Tucson sounding for “00 Z” (5 PM LST yesterday afternoon and make an educated guess. The highest relative humidity on that sounding was at 525 mb ) about 15-16, 000 feet above the ground) with a temperature of -16 C (about 3 F), namely, darn cold. Continuing, we in this field have a well known chart by Magono and Lee (1966) that shows the temperature at which certain forms of crystals grow. At the temperature I am guessing those clouds were at, those crystals would have grown as stellars and dendrites, which grow between about -12 C and -18 C. Sometime I will show you some of these crystals, but for brevity will quit here on this topic.
The second and third photos show what one of these tufts looks like before the crystals have grown and fallen out. Top center, the largest raggedy tuft (Altocumulus floccus) show no fallout of ice. In the last photo, twenty-nine minutes later, there is a fine veil of ice crystals below it (upper right hand corner). Only now, with virga, are they “Altocumulus floccus virgae”! I’m singing right now! And, if you look really carefully you’ll see that most of those little guys have a little ice fall underneath them. Certainly, in those clouds you would find PERFECT crystal specimens!
I’ll end here on an exciting note. The Enviro Can model CONTINUES to show a very strong system moving into our area on Saturday afternoon, likely accompanied by winds as strong or stronger than we saw last Saturday, before the rain and cold air hits on Sunday. Snow levels are going to be really low and we might see some ice in the rain on Sunday here in Cat (alina) Land. Amounts are looking substantial at this point. Man, do we need it!
In the meantime, an upper trough off Baja passes over tomorrow. It has enough moisture with it to provide more “clouds for pies” (Altocumulus floccus virga, and, of course, Altocumulus castellanus virgae, which also qualifies as well for a pie). And, some cirrus will be around, too. However, I am going to stick my neck out and say there will be sprinkles tomorrow. Mods really don’t have a thing, so you’ll have to keep that in mind.
Be sure to keep you’re camera ready for sunsets like last night (see below)!