Forgetting about the fact that not one of the cloud types the so called “cloud maven” predicted showed up yesterday–perhaps he is not really a cloud maven after all–today we will look at cloud microstructure again which will help us to forget.
First, the rain: Baja low beginning to spin toward Catalina, nice rainband forming in western AZ, should get here this afternoon. Monitor it here. Also, current clouds overhead will be getting colder on their tops as they deepen upward, and so showers are likely to begin appearing on radar before that rainband gets here, especially on the Catalinas. Monitor those local shower formations here.
Today’s “lesson” will be that it was interesting that the TUS sounding for yesterday afternoon was almost exactly a replica of the day in which we had slightly snowing clouds and I wrote a kind of long, boring piece about how exciting it was for ME to see shallow Stratocumulus clouds here in Arizona snowing when the tops were only -10 C (14 F). See fine, hair-like virga emanating from Stratocumulus clouds at right on that day. They were the most interesting clouds.
Not supposed to happen, or is very rare here in AZ. I blamed it on clean air after the rain in that tome, and that extra clean air likely led to larger cloud drops (cloud drops are defined as those smaller than about 50 microns in diameter; they don’t have appreciable fallspeeds). “Larger” cloud drops, to us met men, would be ones bigger than about 25 microns. Between 30 and 40 microns in diameter, they can then start bumping into each other and coagulate-coalescence to form drizzle drops (defined as those bigger than 100-200 microns in diameter because they then have appreciable fall speeds, a coupla meters per second), or grow into even rain drops (defined by met men as bigger than 500 microns in diameter). These definitions are somewhat gray, not exactly black and white; not because the drops are in gray clouds, but because these are based on a continuum of fallspeeds. For example, some scientists such as the great Judy Curry, have referred to drizzle as a drop but 50 microns in diameter. BTW, to add some human interest to this dry piece, I know an awful lot of Judy’s these days and the first girl I had a crush on was named Judy.
So, with yesterday’s afternoon sounding we had these clouds below, tops according to the TUS sounding at -10 C, probably even a little colder over the Catalina Mountains, and nary one ice crystal was to be seen. These clouds resisted ice formation even though they were cold enough to require quite the bundle of clothes had you been up there. What happened?
I blame it on air again, but this time, “bad” air, air full of particles. Take a look at this photo from yesterday. You can just see that bit of haziness below the Stratocumulus clouds.
A real cloud maven might have started to ruminate that, “Its gonna be harder today for these clouds to form ice.” I had no such thought myself, but was still looking for all those “no show” clouds. Then at the end of the day, as the clouds filled in I took a couple of shots of “crepuscular rays”, those rays of sun between the clouds that illustrate the presence of dirty air. Here is one of those shots. By this time I was looking for virga, knowing full well that those clouds up there had to be “supercooled” (still composed of liquid drops though below freezing–the kind of thing that causes icing on aircraft.
Contrast the strong “rays” of yesterday with the fainter ones of those on the “clean day” shown next. Note, the air always has some particles in it and there will always be something in the way of “crepuscular rays.” I hope you can notice some differences here. What that dirty air meant was smaller drops in those clouds than we saw on the clean day, and the smaller the drops, the lower the temperature at which ice forms.
Cloud seeding potential yesterday because the clouds were below freezing and not shedding ice?
Yep. However, because of the realtive high bases of those Stratocumulus clouds yesterday, little rain would have reached the ground in Catalina or elsewhere in the lower areas. Mt. Lemmon? I think you could have produced a couple of inches. Also, the SE flow was creating the “best” thickest clouds over and upwind of Pusch Ridge rather than over Mt. Lemmon as seen here in the U of A time lapse movie to some degree. Gee, I did see some brief Altocumulus lenticularis here. So, one of the clouds mentioned did show up.