Category Archives: Altocumulus clouds

0.07 inches

It was beginning to seem like measurable rain could not fall again here!  But then those Stratocumulus clouds because closing in in the afternoon, then soon after that some snow virga began to trail down from them here and there, and the next thing, large regions of the sky were suddenly shedding virga and rain at the ground.  Also, as with the day before anmd at the same time, a windshift line moved across the Tortolita mountains and into Oro Valley helping to augment those Stratocu.  Here’s the pictorial record:

Your thoughts at this time: "Look, the clouds are quite thin, and the sun is going to be out soon! It will be a nice day after all." Background: the morning overcast of low clouds has dissipated and you are responding to new conditions.
The lower clouds gradually fatten up on solar calories, and because there is a disturbance aloft approaching that you can't see, the clouds begin grouping into darker regions that cover ever larger portions of the sky. The upper air disturbance is the "conductor" of the cloud "orchestra" and its getting closer to passing overhead at the time of the 2nd photo. However, you may still not be too concerned, but after all, there is no precip drooping down from all these clouds that you now see. BTW, I went into the Tortolitas to get this 2nd panaramic shot looking downwind toward Oracle and the Oro Valley for you (hahaha-just kidding).

 

Quickfire quiz:  Why isn’t there virga trailing down from those clouds in the 2nd photo, except in the very far away clouds where you can see just a tad????

(Answer, printed upside down if I could):  “They are not cold enough yet to have ice form in them, and ice in clouds is necessary for rain at the ground, as a rule in AZ.”

So those Stratocumulus and some of them are starting to look more like Cumulus clouds at about this time (2nd photo), have to get colder by deepening upward some, and/or colder air must move in aloft to chill them down.

Weird factoid about ice in clouds:   A cloud will form MORE ice for the same top temperature as its bottom gets warmer!  That could be a whole 100 page discussion.  Now, if you’re really pedantic, you can go here to read about all the mysteries that we (those who make a living studying clouds) are going to try to solve in a project in the Virgin Islands this summer, called “ICE-T” here.

In the 3rd and last photo, the nose of the windshift coming across Oro Valley is marked by that highlighted shred cloud in the center of the photo.  It was moving from R to L, undercutting the  higher based clouds that were still moving from the SW.   As you can see by the obscured bases all around, there is widespread areas of rain at this time, augmented by taller clouds with thicker, darker shafts of rain, probably leaning toward a “weak” Cumulonimbus classification if you could see the whole thing.  Note how the distant hills and mountains are obscured in rain.  Yay!

It was after this shot that Catalina got its little amount.

 

Next “storm” this being 2 of the model foretold six days of rain some time ago?  Wednesday.   However, the mods are unimpressed with the moisture in this and none that I have seen have a drop as this upper air disturbance goes over us.   At the LEAST, we should have some nice Cirrus, and probably Altocumulus clouds.   And you know what that means now….   The possiblity of a trick sunset due to a parhelia (aka, sun dog, mock sun).

Also, I really like the the Canadian model run today that calculated where the highs and lows are going to be for the next several days based on last evening’s weather measurements around the globe.   I really, really like it because it shows  a huge storm here and in the SW six days out.   My preference has nothing to do with objective science.  The models have been fluctuating on how this next storm is going to be, minimal or gigantic, sometimes that’s just the way the models are when there is a lot of uncertainty about things upwind of us. Its one of those that is accompanied by an Arctic blast down the West Coast.  Can’t wait!

The end.


 


More about holes-in-clouds while we’re waiting for the AZ rain in a few days

There have been a coupla comments on that aircraft effect in clouds blog of a coupla weeks ago and so I thought I would follow up with this sequence from the Atmos Sci Building rooftop at the University of Washington where I spent most of my time instead of at my desk.1

Here is a rarely photographed sequence of the effect of an aircraft on a supercooled cloud.  The first photo, right after a contrail-like feature was seen in these Altocumulus clouds.

Look at what seems to be a dark contrail-like line in the middel of the photo. The Altocumulus (perlucidus) cloud layer, mostly comprised of supercooled liquid drops, is probably around -20 C, though I did not get a PIREP on this day for some reason.

In the minutes after this first photo, the aircraft trail seems to disappear as it widens and the shadow lessens.  This stage is not shown because I didn’t realize what was going to happen until minutes later.   This second stage is almost impossible to pick up visually because there are no ice trails yet, nor is the cloud opening up at this time.  This “invisible” stage might last 5 minutes before you see the hair-like signs of a fallout of ice crystals.

This second photo is about ten minutes after the line in the first photo. Now it is clear that ice has formed, the crystals are growing and falling out as "virga", and a clearing is starting to open up.

Ice grows rapidly in the presence  of the supercooled drops.  Ice represents something of a low pressure center in the middle of all those droplets and that attracts the vapor from them, causing them to evaporate.  That vapor deposits as ice on the newly present ice “germs”/crystals created by the aircraft.   Since the drops are disappearing, before long, you get a hole or ice canal in the cloud where the droplet cloud used to be.

In this third photo, there is no longer any doubt about what's is going on. The hole is there, and its only a question of how much larger it will get.

The ice crystals shown above are clearly falling out (ever-so gradually because they are so small still, perhaps a few hundred microns in width).   Becasue they are so small, they usually evaporate well before any precip reaches the ground.  However, recently it has been shown that in deeper clouds and more moist conditions, that an aircraft can actually produce a bit of rain/snow at the ground due to this effect.

Here is last photo I took that day.

The ice crystal induced hole in the Altocumulus layer has gotten closer to exiting the liquid cloud (has moved to the edge of it) as well as expanding some.  This suggests that the ice cloud was moving faster than the droplet cloud, something that happens when waves in the atmosphere are producing the droplet cloud.   It was also getting closer to the observer, however.

Sometimes, if the cloud layer is lifting enough, the original Altocumulus clouds will gradually fill back in because all of the ice has settled below the liquid cloud layer.

For history buffs, holes in clouds with ice in the center, or ice canals were seen in the 1930 and 1940s, but as you can see, unless the observer saw the original trail (which they usually didn’t) no one knew what caused them.   Eventually an ice canal was was photographed in 1946 that was so convoluted it was realized that ONLY an aircraft could have done it.

Furthermore, that report in 1946 preceded the discovery of modern cloud seeding with dry ice by Vincent Schaefer in 1947 who performed his most convincing, and could be seen as ironic, demonstration of seeding with a similar convoluted ice canal as was seen in 1946 in a supercooled Altocumulus cloud layer  Its interesting in retrospect, as so many things are, that Schaefer did not have to drop dry ice on his clouds that day in 1947 in which he made history, but rather only had to fly his prop aircraft through them and likely would have gotten the same effect!

BTW, there is a lot of new interest in this topic, a new article recently appearing in Science mag.

I mention this cloud seeding benchmark since these aircraft events represent inadvertent cloud seeding, and in a sense they demonstrate that you CAN get something in the way of precip to fall out of a previously non-precipitating or barely precipitating cloud by seeding.  When clouds are already naturally precipitating, what happens when you do cloud seeding is subject to question; the science domain in this murky world is highly polarized.

The “Story of APIPs”–Aircraft-produced Ice Particles)  is told (in the usual “style” you will often find here) in the gigantic powerpoint “show” on this website under Sci Talks toward the middle of the show, around slide #472 (hahaha).

This ppt “show”, BTW,  is WAY overdone, but, what the HECK!  Why tell only “the whole 9 yards” when you can tell 12 or 14?

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1Hahaha, sort of.   Sometimes, looking at those several thousand film shots from the rooftop of the Atmos Sci Building, I do wonder about that.  But then again, since I used 1/100 of a second exposures with my film cameras, these photos would only PROVE that I had been on the roof, maybe 30 seconds in 30 years there at the UW.

Feeling better now.

What’s Up with This?

Got pretty mad yesterday when I saw this overhead in some Altocumulus perlucidus clouds.  You’ll have to hold your monitor or Ipad, or cell phone, or whatever, over your head to see it EXACTLY the way I saw this because it WAS overhead;  straight up.  (Actually, doing 3 sets of 12 might be good for you.)  Also, click on images to get the full view.

As you can see, the white strip below in these clouds is a contrail caused by an aircraft, but a special one that occurs in “supercooled” clouds.   Supercooled clouds are clouds that are composed of drops, yep, they’re still liquid, even though the temperature is FAR below freezing.  Here, the clouds were likely colder than -20 C (-4 F) and yet there is no ice forming in them!  (You don’t see trails of snow coming out, do you?  No.)  Run of the mill contrails occur at cirrus levels at temperatures below about -35 C  (-31 F).

Note that except for being much whiter than the surrounding cloud, the elements are exactly the same size and texture as those around it.  That is going to change, because this white strip is composed of “horrendous” concentrations (probably thousands per liter) of ice!   You can only know this by what happens later.

In the next shot below, is an example of what happens later, trails of tiny snow crystals fall out leaving a hole in the droplet cloud, so called, “hole punch” clouds, a form of inadvertent cloud seeding by aircraft.  Note the delicate strands of ice crystals falling out of this cloud from the hole, so pretty because they are so delicate looking.  Note, too, I am one of the “trailing authors” of the journal article above, like one of those itty bitty ice crystals in the second photo which are almost evaporated at the bottom of those fine strands.

So why be upset?

Rather than looking forward to good things in the coming year, this happenstance yesterday reminded me of all the trouble we had in the early 1980s trying to get our paper published on this phenomenon; namely, that an aircraft could produce tremendous amounts of ice when flying through supercooled clouds, inadvertently seeding them.

In the SECOND rejection of our manuscript (with Pete Hobbs), the Editors words still burn; “(the reviewers) are still unconvinced by these controversial claims.”

We had to do a LOT of extra work on this to convince those reviewers.  The third version was more convincing, I guess, for intransigent reviewers, and got published.  In fact, one of the great scientists of our time as far as clouds and ice crystals go, John Hallett (yes, the same one as in the “Hallett-Mossop” ice splintering process),  speaking at the Peter Hobbs Symposium Day in 2008 called this episode, “an embarrassment for the airborne research community.”  “Hey”, he wasn’t referring to our paper!

He was referring to the fact that such a phenemenon had been overlooked and not accounted for in research studies of clouds by aircraft.  Actually ground observers had been reporting this kind of thing (ice canals and hole punch clouds) since, if you can believe it, the 1930s!

BTW, this hole is not the one from the first shot; I got distracted and forgot to follow it until it was disappearing over the horizon.

BTW#2,, this shows what happens when you introduce ice into a supercooled cloud; “stuff” falls out.  Proves cloud seeding works, though for sure in limited venues like these (non-precipitating, supercooled clouds).

BTW#3:  The second photo is a nice example of the difference between supercooled clouds composed of tiny drops (probably less than 20 microns in diameter), and cirrus-ee ice clouds, composed of much larger crystals (here probably 100 or 300 or so microns in maximum size) that tend to settle with time. (Hence, those strands in most cirrus clouds.

BTW#4:  Today’s title is cribbed off the world’s most viewed climate website, “What’s Up With That?”  Mr. Watts, host of the site, has made significant contributions to our climate network by pointing out flaws, but has no “credentials” beyond having been a TEEVEE meteorologist.  He is excoriated on this point alone by “credentialists”, as I myself was when I first began to reanalyze other folks’ cloud seeding experiments such as this one.

BTW#5, a movie about credentialism is now out, called,  “The King’s Speech.”  I highly recommend it.   In this documentary, which I just saw yesterday, it will be seen that the credentialists in the King’s Court were royally put out by the help the King got by his uncredentialed therapist.

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.

Colorful announcement of a storm

This glorious sunrise today about 7:30 AM announces in its way that a strong storm is on the way.

Why?

First of all the clouds, “altocumulus lenticularis” are the lower, rippled clouds, combined with a higher,  solid layer of altocumulus and altostratus clouds demonstrates that the air is moist to saturated over a great depth above those lowest clouds.

The lenticulars highlighted by the rising sun just above the Catalina Mountains generally occur when the wind speed at their level is at least 30-40kts.  Thus, lenticular clouds have always been a sign of being around and under strong upper level winds we sometimes call the “jet stream.” While lenticulars might hover over the same spot for minutes to hours, watching how fast the elements in it move, or other cloud movements can tell you something about how strong the jet stream is over you.  Today, the clouds were racing across the peaks, consistent with the very strong jet stream and storm systems that is about to pounce on us.  The NWS balloon sounding from Tucson this morning, a couple of hours before this shot, indicated the winds were 30-35 kts at the level of the Ac len clouds.

The second photo, kind of dull compared to our glorious sunrise, has something to say in it, too. The wind at cloud level is at the photographer and its strong,   What happens in these “pre-frontal” situations is that the air ahead of the cold front can be relatively stable, that is resistant to moving up and down, and in resisting doing that when hills and mountains are present in the path of the air movement, something akin to gigantic ocean swells are produced.  Here you see darker bases off in the distance that are PERPENDICULAR to the wind just like huge ocean swells might be.  A time lapse camera would show the movement of these “swells” beautifully as they peak and die, going through ridges and troughs, that is, slight rises and falls of the air in its movement toward the camera.

At the same time, those clouds, due to the Catalina Mountains and the higher terrain downwind from Catalina, also forces these cloouds to deepen, a process that will continue as the upper low pressure trough approaches.  As this happens, clouds such as these that are not precipitating now, start to precipitate.  The rain often doesn’t move in, but begins to fall from these deepening layer clouds overhead.  I think that is going to happen here in the next couple of hours (its about noon now).

Here in Catalina, AZ, the rainfall (17.5 inches per year) is considerably more than that in and around Tucson (12 inches per year), and this difference largely comes in winter storms like these that are subjected to the lift zone described above out ahead of the higher terrain here in Catalina,  and downwind of us.