How sad.  A few contours to the south of us, that jet to the south of us, that is,  and we’d have got our tenth or more inch of rain.  But no.  That trough over southern California and northern Baja had to zip out and “up” (that is, to the northeast) like a jet taking off from an aircraft carrier.  Gone now.  All we’ll have is leftover wind and a threatening looking, but too shallow a deck of…….Stratocumulus.   Too warm on the top of this layer this morning for ice formation, and that, as you know here in AZ, means that nothing comes out the bottom because the droplets in the cloud are too small to fall out.

But yesterday afternoon, an ice bonanza!

Alas, those Cumulus and Stratocumulus complexes were too high-based for the considerable ice crystals and snow forming in them to reach the ground as melted drops,  except for “sprinkles-its-not-drizzle” here and there1.  Heck, the drops that made it down around 5 PM AST weren’t even that big.

I wonder if you saw the rapid transition to ice-producing clouds yesterday.   Not much going on up to 1:30 PM.  Then, all of a sudden, it seemed, there was ice almost everywhere in those little clouds.  It was fascinating since they did not appear to be deepening upward to lower temperatures.

Let’s review yesterday with a long cloud harangue, starting from that wonderful sunrise with an Altocumulus lenticularis undulatus (has something like ocean waves or rolls in it to produce this where the air is rising and falling to produce cloud, then clearing), here is yesterday.  Hmmm.  I wonder if you remember where THIS sentence started?

Next, that promising scruff of cloud (I would call it, Stratocumulus) topping Mt. Sara Lemmon.  It was promising because with cloud bases lower than the top of Mt. Sara, there’s a better chance of rain reaching the ground.  But, up they they went as it got warmer, a usual thing.  Has to be a flood of water vapor coming in to overcome the rise of cloud bases with daytime warming.   As boffo as that trough looked over southern California, it couldn’t really “bring the bacon” if bacon was moisture that is.

We did have a lenticular cloud, too, for awhile.  Let’s see that, too.  It will be good for you.  Notice how it is near the same spot as the “undulatus” cloud?  That’s what lenticulars do; they have favorite haunts.  When the flow is from the southwest, this is where they are going to be, over and over again, downwind from Mt. Sara L.

 By mid-day and early afternoon, Cumlus cloud bases were well above Mt. Lemmon, a couple of thousand feet at least.  Here is a mid-day shot of those non-ice producing, Cumulus fractus and humilis clouds next.



The first Cumulus photo was taken at 1:50 PM, and if you were a real sharpie, you would have seen some tell tale vales, but probably only Mr. Cloud Maven person did, they were that faint.  But here in this second shot of Cumulus humilis and such, you can PLAINLY see that in the center, one of those little guys has converted COMPLETELY to ice.  It was pretty amazing to see that in such small clouds.  Soon the whole sky was filled with clouds “icing out”, becoming nothing but ice crystals and snow flakes.  Here are some more photos of that stage, including a short rainbow demarcating where the snow was melting into drops.






So what caused all the ice to appear in clouds that didn’t appear to be growing in height?  Well, first of all, by the end of the afternoon, they were certainly colder at cloud top, so that would explain the late afternoon ice everywhere.  Also contributing, was that is was getting colder over us as the day wore on as that trough approached.  So, even if the cloud tops stayed the same height, they would have gotten colder.  Finally, dust has been known to have a role in causing clouds to glaciate at higher temperatures than if there was no dust getting into them.  This is something that we saw happen in Durango, Colorado during a randomized cloud seeding experiment when dust storms hit and “ice nuclei” measurements shot up.  So, dust, too, may have had a role.

The afternoon TUS balloon sounding suggested that the tops were only about -15 C (5 F), maybe -17 C in one that momentarily bulged above the main cloud top level-Cumlus clouds do that.

However, the amount of ice is not commensurate with a temperature that “high” and so I reject the sounding temperature.

I think, with bases around -10 C yesterday afternoon, that for clouds to produce as much ice as we saw, they would have to be -20 C (-4 F).  I think maybe the strong temperature drop to the northwest from the balloon launch site might have played a role, that the temperature of the balloon instrument was correct, but it was a few degrees colder over Calalina and to the northwest of us.  That “surmision”, a deduction,  you get from, say, the 500 mb map where it was far colder at Flagstaff than here.  Of course, you might think I am lying, and just made that last part up because I am really clueless about what happened.  Due to your doubt, I will now post the 500 millibar map from my home university so that you can see I did not make this up.

As you can see, while TUS is only at -18 C, Flagstaff is -23 C, and San Diego is -28 C!  So going to the NW (a heading of 310 degrees) from the balloon launch site their at Davis-Monthan meant it was a LOT colder in that direction, mile by mile even maybe.   Also, you can see by Flagstaff’s wind, that the jet core at this level had not passed over us, a key to wintertime rain here.  Never did.  Hence, a “whiff” on this storm, to use an old word right before a new word from baseball, as in, “he whiffed on that slider” (struck out). I can’t believe how I am educating you today!

The End.


Sorry, have to carry on this theme about what is drizzle and what’s not.  You should find another TEEVEE weather presenter if he or she calls what happened yesterday for a few minutes, “drizzle.”   Rain and snow mixed is NOT “sleet”, by the way, either, another looming corruption of our weather terminology.