Category: Ice crystals

24 h of temperature infamy, Catalina, AZ

What an awful past 24 h here in “Catland”!  We’ve not only had low, perhaps, unprecedented low temperatures for a day with full sun, but also a noxious 15-30 mph north wind.  (Why didn’t we retire to Kauai where we would never be this cold????!!!  Just kidding, sort of.)

In some quarters(such as in a recent commentary by Al Gore),  it has been reported that we should expect colder winters as it gets warmer due to global warming (I am not kidding here-check the last paragraph here).

If this is the case, and our current godawful cold spell here in AZ has been enhanced by GW, then we had better reconsider where we grow citrus crops!  South Florida just had a record cold December.  Perhaps Florida is too far north for citrus crops as global warming proceeds and winters get colder, or at least cold spells more severe.  California, Arizona, are you listening?

Now, for a diatribe on clouds, about which Mr. Cloud-Maven person, me, is an expert (he sez).

If you saw the few small clouds we had yesterday, you saw something extraordinary for SE AZ.  Why?  Because of all the ice crystals that formed in such tiny clouds (ones called, cumulus fractus).

Below is a photo of cumulus fractus clouds forming lots of ice from yesterday afternoon.  The top most cloud is a pure cumulus fractus cloud, not yet showing ice.  But beyond that cloud, farther to the east, are similar clouds spewing forth a large plume of ice crystals, seen as the wispy, semi-transparent cloud downwind toward the right half of this photo.   Cu fra forming ice are common in the high elevation areas of the Rockies, but not here because our lower clouds that are small are almost never as cold as those, even in the wintertime.  Our clouds, according to the NWS sounding launched from Tucson around 5 PM LST yesterday were about -25 C, an extraordinarily low temperature for clouds not having any depth to speak of.   And, due to that low temperature, voila, ice forms!

So, a good eye with a little knowledge about ice formation could have guessed that these little clouds would have to be colder than about -20 C to have been so prolific in ice production, those veils seen downwind of them.  Ice forming in such shallow clouds are too small to fall out as precip as a rule, though some “virga” or flurries were observed here and there in SE AZ, such as at Stafford yesterday afternooon and west of Wilcox as well.  (Of course, I had hoped a couple of days ago that we here in Catalina would see a flurry.  Still, with flurries at Stafford, it was a damn close call, astronomically speaking, so I don’t feel that bad for getting a little too excited about the possibility of snow here a couple of days ago.  Besides, it was a “learning experience” as well….)

Amaze your friends with such trivia!!!

However, it will be a long time before you see such clouds, as low and small as these were, produce ice like this.  Well, we hope so anyway, or we are moving to Costa Rica where I will not have to experience even one second of being too cold!  If you want a really good look at what they were doing, go to the U of A’s timelapse for yesterday (here).  (Forgot to point this out until now…darn.)

Altostratus: a misunderstood cloud and for good reason

Yesterday afternoon the clouds thickened and dimmed the sun, and our high temperature struggled only into the mid-50s.  What cloud was that?  Here it is, with Twin Peaks on the horizon.

Our names for clouds, originating with English pharmacist, Luke Howard, are based on visual attributes from the ground.  Here, “Altostratus”  (As) does RESEMBLE its lower namesake cloud, Stratus, a low fog-like cloud with little definition often found in summer along the West Coast.  See a rare example of Stratus (St) hereabouts below.  Note that it is topping the Tortolita Mountains to the west, it is that low.

However, about the only thing that these clouds have in common is that they are both relatively smooth looking clouds.  Inside them, they are totally different. Also, St is a shallow cloud usually less than 1 km (3,000 feet) in depth, while As is normally 2-3 km  6,000 to 10,000 feet) in depth.  In Stratus, you just have cloud drops and maybe, as below, a few drizzle drops (mist-like)  falling out.  OK, once in awhile in cold locales you have a few ice crystals falling out, but drops rule!  On the other hand, in Altostratus, if you were flying in them with a 1998 version of the Stratton Park Engineering Company’s Cloud Particle Imager ($130,000 or so–I’ve added a link in case some of you want to go shopping now),  you would find nothing but ice crystals for the most part.  Water droplet clouds are sometimes found in them, and, oddly, if the top is not too cold (warmer than about -30 C), at cloud top, the coldest place!  So, it is not unusual to see, even in journals, a thin layer cloud consisting of drops called, As.  Makes sense really.   (A name change of As to “Altonimbostratus” would be helpful to emphasize its internal ice and falling snow particles.)

An example of the kinds of crystals found in a As clouds is shown below, collected over Barrow, AK, in a 1998 project called FIRE/ACE/SHEBA.

These typical crystals, having grown on the way down from simple plates or tiny columns, or sphere-like  “germs”, are called “bulett rosettes.”


Arizona: Colorado temperatures, Colorado clouds

It was a mind-boggling, hiking-challenging -30 F at Grand Canyon AP yesterday morning.  Overhead of Flagstaff,  at 5 AM MST yesterday it was -38 C (-36 F) and that temperature was the lowest temperature at 500 millibars in all of the US.   It is really, really rare to see -38 C over Arizona!   Temperatures in the Tucson and north area in the shallow cloud deck we saw creep over the sky from the west near dawn, were running around -15 to -17 C (5 to 1 F) at cloud top (around 11,000 to 12,000 feet above sea level) according to the Tucson sounding at 5 AM.   Bases were just above Mt. Sara Lemmon.  For those of you who think I might lie about how high the bottoms of those clouds, I present a photo of Ms. Mt. Lemmon at that time (slight hump beyond first range).

Who cares, you’re thinking?  Well, in these photos, there is a curiosity; the lack of snow coming out of the bottom of these clouds (called “virga”, and you’ll want to concentrate when you pronounce this word so it doesn’t sound like a popular drug for older males).  Normally, in the Arthur’s experience,  clouds this cold produce virga, that is,  there are natural “ice forming” aerosol particles in them that  result in snow crystal that grows and falls out of the cloud, a lot of them so that the bases of the clouds are partly obscured by falling snow.

Also there was no radar echoes around at this time.  This was to change.

Here is a 30 h loop of the radar imagery for the whole US.  You’ll have to get a microscope out or zoom in a lot to see our area of SE AZ here, but, it’ll be worth it, he asserts.  Also, turn the loop speed up to the highest level at left on this web page, or you’ll get upset over how long things are taking to view. Don’t want any “Web rage” out there!  Too, I thought it would be fun for you to see all the echoes and the things they do over a long period, in a fast loop.   You’ll see here that around AM in our area of SE AZ there is a patch of echo that develops and then kind of hangs out over us until mid-afternoon or so when it disappears.

Here’s what the sky looks like when there is widespread ice forming in the clouds and falling out, MOSTLY as virga, and when we had that little patch of radar echo over us:

Note how “smeared” the sky looks now!  Also my apologies that a bird was going by obstructing some of the sky…

Well those heavier patches that are hanging down a bit and trail off to the side is what “virga” is.  Its often more spectacular than this, I have to say.

What happened to cause this rather sudden transformation of this layer, this sky; why did all this ice begin forming in that cloud layer when it had little or no ice over most of it around dawn?

I don’t know.  End of blog.

That would be a little too honest, and so I will guess. If you’ve worked in science, and this kind of thing is your specialty, its REALLY not good to say you don/t know something.

There was a disturbance aloft that was about to come through, and I will GUESS that the tops of this layer got a little bit higher and colder as it approached.  If you saw the clearing later in the afternoon, for example, your instincts would have told you about this event.  However, after it went through, and when tops were definitely falling in height, the Tucson sounding at 5 PM MST also indicated they were slightly warmer than they were on the first sounding in the morning, and so, I am, in effect, filling in a blank, hypostulating that there was a hump in the tops that was not observed.  Oh, well. If nothing else, you might now know the difference in the appearance of the sky when ice is not present (first two pics) and when it is falling out at you (last pic).

If you want to see an action shot of all the happenings described above, here’s a movie from the U of A Department of Atmospheric Sciences rooftop of the Catalina Mountains.  My location is under the leftmost portion of this view, beyond Pusch Ridge.  This movie will take a couple of minutes to load, and is only available today (for yesterday).

The Colorado connection:  Wintertime clouds in Colorado are generally as cold as our clouds yesterday, and are constantly producing falls of ice crystals and snow when present, and so to me, a six year resident of Durango, it was a “Colorado” wintertime sky over Arizona yesterday due to the really cold air over us.