Category: Ice crystals

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.