Passages: an upper low one on the 18th disappoints; today is the 20th

I got behind….

Lot of great scenes on the 18th, but, ultimately with hopes raised for appreciable measurable rain in Catalina, it was a disappointing day. Nice temperatures, though, for May if you’re a temperature person.  Only a sprinkle fell (4:15 PM), and if you weren’t outside walking the dogs you would NEVER have noticed it.

Here is your full cloud day1, as presented by the University of Arizona Weather Department.  Its pretty dramatic; lot of crossing winds, as you will see, and an almost volcanic eruption in the first  Cumulonimbus cloud that developed near the Catalina Mountains. 

That blow up was indicative of an remarkable amount of instability over us yesterday morning, one that allowed really thin and narrow clouds to climb thousands of feet upward without evaporating.  Usually the air is dry enough above and around skinny clouds that even when its pretty moist, they can’t go very far without the drier air getting in and wrecking them (a process called, “entrainment”).  Here are a few scenes from your cloud day yesterday.

5:45 AM. Gorgeous grouping of Altocumulus castellanus and floccus. They’re coming at you. (If you thinking of soft orchestral music here, you may be remembering well-known orchestra leader, Andre Castellanus.
7:37 AM. Here a castellanus turret rises five to six thousand feet above its base. Had never seen one this skinny and THAT tall before. Was really pumped about the mid-level instability at this time. It wouldn’t last. The great height is indicated by the luminosity of the top,
Also at 7:37 AM, another amazinging tall turret rises up from quiescent bases, ones not connected to ground currents. The bouoyancy in these clouds is due to the heat released when moist air condenses (latent heat of condensation). When the temperature drops rapidly with increasing height, that bit of heat released is enough to allow weak updrafts to rise great distances, sometimes becoming Cumulonimbus clouds and thunderstorms. These clouds, due to their size, would no longer be considered just Altocumulus andre castellanus, but rather Cumulus congestus. Here’s where our cloud naming system falters some. Later, a couple of these grouping did become small Cumulonimbus clouds with RW- (light rain showers).
7:11 AM. The great height of these tops was also indicated by the formation of ice, that faint veil around the edges. Stood outside for a few minutes, thinking I might experience some drops, but didn’t.
7:38 AM. The top of this Cumulus congestus has just reached the level where ice will form in the top.
10:22 AM. Cumulus congestus clouds began their transitions to Cumulonimbus clouds early and often over and downwind from the Catalinas. Can you spot the glaciating turret in the middle, background? Pretty good skill level if you can.
10:23 AM. Here’s a close up of that turret in rapid transition to ice. It was this kind of phenomenon that led Hobbs and Rangno and Rangno and Hobbs to reject the Hallett-Mossop theory of riming-splintering as THE major factor in ice production in Cumulus to Cumulonimbus transitions like these. The high concentrations of ice particles happened faster than could be explained by riming and splintering, or so it was thought. Still think that, but am in the minority, though there have been reports of inexplicable, fast ice development like that Stith et al paper (with Heysmfield!) in 2004 that for a time appeared to put the “icing on the career cake.” Incredible ice concentrations were found in updrafts of tropical Cu for which there was no explanation! That finding hasn’t been replicated by others, casting doubt on the whole damn paper! “Dammitall”, to cuss that bit.
11:04 AM. Nice Cumulonimbus capillatus incus (has anvil) pounds up toward Oracle way. Tops are not that high, maybe less than 25 kft.
3:41 PM. The air aloft began to warm and an inversion capped most of the convection causing the tops of Cumulus clouds to spread out and create a cloudy mid to late afternoon. Nice, if you’re working outside in mid-May. Since the tops were colder than -10 °C (14 °F) the ice-forming levels, some slight amounts of ice virga and sprinkles came out of these splotches of Stratocumulus clouds. One passed through the Sutherland Heights, but if you weren’t outside you would never have known it!
4:38 PM. Isolated rain shafts indicate some top bulges are reached well beyond the ice-forming level. Note grass fire in the distance.
7:22 PM. Pretty nice sunset due to multi-level clouds, some Stratocumulus, Altocumulus, and a distant Cumulonimbus anvil.

More troughiness and winds ahead during the next week as has been foretold in our models, and reinforced by weather “spaghetti” plots, after our brief warm up today.  No rain here, though.   Seems now like rain can only occur at the very end of the month where weaker upper troughs coming out of the Pac appear to be able to reach down and fetch some tropical air. 

The End


1Its gone now because I couldn’t finish yesterday.  Went off to Benson for horse training with Zeus.

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.


  1. Some fantastic cloud scenes you have there, Art. The tall skinny Acc picture especially. Well, anyway, you’re not the only one disappointed: We were supposed to get some much needed showers the last few days, and even the next 2 or 3, but every time I check the forecast, the % chance goes down. You’re right; upper lows from the Pacific, ain’t what they used to be.

  2. Yes, I do follow Cliff Mass. The good news this morning is, we finally got some good showers last night (over half an inch in the gauge) and more is coming tomorrow. Looks like the heat is returning next week, but at least we have some water in the ground for all the trees and plants around here.

    1. Gee, I almost forgot to reply, Roland.

      Cliff does some good stuff, that’s for sure. Very erudite.

      Wish we could get some of that rain! Heat, well, we have enough of that!


      1. No problem, Art- I decided to wait before saying anything more. We actually got a LOT of rain finally on the 28th (I measured 1.73 inches). That’s not quite enough to make it a normal amount for the month, but no one’s complaining now. As for the heat, it’s not coming here as much as I previously thought- but that’s okay. We don’t want it as much as some places do.

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