Special low cloud base day ahead; yesterday’s pretty cloud scenes

Today will be a special one in the desert.  Cumulus bases are going to be really LOW for summer,  maybe only 3-4 kft above the ground, and likely warmer than 15 C (50 F).  Maybe 50 F doesn’t sound special, but it is.   A base temperature of summer clouds that warm is rarely observed here.  And with that, and all the posts here about the temperatures that ice forms (around -10 C, 14 F) are out the window.  Ice will form at much higher temperatures than usual.

This is because on a day when the Cumulus bases are that warm, rain forms by collisions between droplets before clouds even reach the -5 C (23 F) level, the highest temperature at which Ma Nature can produce ice.  Rain mgiht even form in our clouds today even before the freezing level itself!

This is so exciting for an Arizonan who has studied ice-in-clouds development over the years because today ice will form in clouds around the -5 to -10 C level, and the mechanism of Mssrs Hallett and Mossop will be heavily involved as well as other lesser understood mechanisms to form ice in clouds today.  And, along with that high ice-forming temperature will be categories of ice crystals that are rarely seen here, needles and hollow sheaths, ones that form at temperatures in clouds warmer than -10 C!  You can see how excited Mr. Cloud Maven Person is. For comparison, it would be like a bird watcher seeing a _________,  something pretty rare go by.

Dewpoint temperatures are running in the upper 60s and was 70 F (!) at TUS earlier this morning!  Indicative of a really, really moist day from a cloud standpoint even now is that line of Stratus fractus cloud halfway down on Samaniego (Sam) Ridge.  And this is BEFORE rain has fallen.  Not too unusual to see something like that AFTER a good rain, but before, its pretty rare.

All in all, a very tropical day ahead, very “Floridian” I would call it, and that means more water in the clouds above us ready to fall out, and more “fuel” to send those warm plumes of Cumulus turrets spaceward.  That’s because heat is released to the air around cloud droplets as then form, and the more “condensate” the more heat.  The warmer the cloud bases, the more condensate that occurs.  Its quite a feedback loop.

The last time we had bases this warm and low, some “lucky” areas got “Floridian” dumps of rain, that is, 3 inches in an hour.  (Three inches in an hour is pretty common in Florida in the summer.)

However, need some heating and/or a good symoptic situation to gather up the clouds today if we are to get more than just high humidity from Norbert’s remains.  Last night’s model run from the U of AZ was not real supportive of a great day because while the humidity is here, and upper level situation is going in the wrong direction, is not going to help much.  A lot of what we needed was expended over night in huge storms that are raking central and northern AZ now, with some sites in PHX reporting up to 2 inches since midnight!  And as that upper air configuration responsible for their great rains moves away, what’s right behind it up there, will try to squash clouds.

So, while we have the ingredients down low for an exceptional rain day, its not in the bag.  What’s worse is that drier air is now foretold to roll in from the west by tomorrow, further diminishing (not eliminating, though)  the chances for a decent rain here in Catalina.  “Egad”, considering all the promise that “Norbert” once held for us!

So, in sum, a bit clueless here as to what exactly kind of day we’re going to have.  “Truth-in-packaging” portion of blog.  I see rain has formed just now (6:41 AM) on Samaniego Ridge, AND to the S-SW, very good sign!

—a note on air quality—as inferred from visibility in a humid situation——–

Another thing you will notice is how clean the air is.  We have tremendous humidity, and unlike smog-filled air back east, the sky will be blue, and the visibility good.  If you’ve ever been back East, you’ll know that in most areas the sky between the clouds on humid days is pretty white, and horizontal visibility is reduced in the moist air, say ahead of a cold-cool front in summer.  This is due to large haze particles that have become droplets before water saturation has been reached, a phenomenon called deliquescence.  Its horrible.  Really ruins the sky back there on humid days.

Enough semi-technical blather. We’re mostly about pretty cloud pictures here.

Yesterday’s clouds

There were some spectacular scenes yesterday, even though it was disappointing as a rain day, only a late afternoon trace here in Sutherland Heights.  Here are some of the best.

6:34 AM. Altocumulus lenticularis hovers over and a little downwind of the Catalinas.
10:03 AM. First Cumulus begin to form on the Catalinas, later than expected. I will using the words, “expected” and “unexpected” a lot today.
1:21 PM. First ice seen, lower left top of blue sky and cloud border. Can you see it?
1:24 PM. Soon after the first ice is seen, out pops the rain, that very faint haze in the center of the photo.
2:12 PM. Mt Lemmon receiving 0.79 inches of rain in about an hour from this little guy. Note that the peak is TOTALLY obscured by this rain shaft.
12:24 PM. Cumulus clouds kind of muddled around up there when yours truly was expecting a sudden eruption at any time. Really did not happen yesterday.
12:26 PM. Mostly just a pretty scene, the blue sky, the Altocumulus perlucidus, and the Cumulus congestus erectus.
4:17 PM. It was especially gratifying, after kind of a non event day, to have this unexpected late eruption of a Cumulonimbus NW of Catalina. Meant chances weren’t quite over for nearby developments.
5:03 PM. Cumulus cloud street trails off the Catalinas. Will it do anything?
5:21 PM. The ragged edge of the higher layer leads to a series of crepescular rays in the falling rain, while the Cu congestus turret sends a long shadow Catalinaward, A gap in the clouds allows the sun to shine on the rain falling in Oro Valley then. Can you imagine how great the rainbow was on the other side, say from the Tortolita Mountains? The rainbow isn’t seen in the forward scattering direction because its due to reflected light back toward the sun from within the raindrops.
5:22 PM. Nice lighting on Samaniego Ridge, rainbow imminent,
5:24 PM. Magnificent, the lighting, the rainbow. How lucky we are to be here!
6:56 PM. Just when you thought the day was finished, this surprise. Well, it was to me, that’s for sure!

DSC_0175 6:58 PM. Ghostly-like late blooming Cumulonimbus calvus and Cumulus congestus clouds rise up against the falling temperatures. Pretty neat sight.[/caption]

6:58 PM. More unexpected strong developments to the west after sunset.


 The End!

Heck, if I worked on this much longer, the answer to what’s going to happen today would be in!

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.