Category Archives: Hallett and Mossop

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!

Weather “stagecoach” full of storm presents set to arrive on Friday

Don’t really need me anymore.  Everyone’s on top of this ” incoming” now, set to begin in the area overnight on Thursday, the one you’ve  been reading about here since maybe last October I think.  So, feeling sad today, also because it looks like its going to be a bit too warm for snow, which I think I mentioned about a dozen times. Maybe I will take it out on you by boring you with a science story, one about ice in clouds…but one featuring such stalwarts as Sir Basil Mason, Stan Mossop, John Hallett, Pete Hobbs, Alexei Korolev, and others.  Interested now?

But first, a few nice cloud shots from yesterday so you don’t get too mad at me for boring you first:

2:07 PM.  CIrrostratus fibratus (has detail, not just an amorphous veil).
2:07 PM. CIrrostratus fibratus (has detail, not just an amorphous veil).
4:21 PM.  Cirrus spissatus patches and dog, Zuma (named after the acclaimed dramatic series, Baywatch, which took place at Zuma Beach, also where the author, whilst not storm chasing spent a LOT of time.
4:21 PM. Two dense patches of Cirrus spissatus patches and dog, Zuma (named after the acclaimed dramatic series, Baywatch, which took place at Zuma Beach;  also where the author, whilst not storm chasing,  spent a LOT of time.















5:22 PM.  Cross section of a Cirrus uncinus (hooked at the top).  This shows how the ice crystals forming at the top first get heavy enough to fall out, but if encountering drier air, start to evaporate, slow in fallspeed, and as in this case, form a flat layer of tiny crystals at the bottom of the head of Cirrus uncinus.  Likely a little moist again at that bottom location so the tiny guys don't away very fast.
5:22 PM. Cross section of a Cirrus uncinus (hooked at the top). This shows how the ice crystals forming at the top first get heavy enough to fall out, but if encountering drier air, start to evaporate, slow in fallspeed, and as in this case, form a flat layer of tiny crystals at the bottom of the head of Cirrus uncinus. Likely a little moist again at that bottom location so the tiny guys don’t away very fast.
5:29 PM.  Sunset in Cirrus (spissatus and others).
5:29 PM. Sunset in Cirrus (spissatus and others).























Cloud ice science story

(drink some coffee, maybe take an extra swig of an “energy drink” if venturing forward)

Kind of takes the fun out of it when other people are saying what you want to say by yourself, lilke today’s forecast for Friday’s storm.  Kind of like being second when you publish “new” results behind other researchers who “got in” a little a head of you (like Korolev et al.-with Hallett!) did in 2004 reporting the FIRST image of a shattered frozen drop they said.

Drop shattering during freezing; what about it?

It was thought not to happen in natural clouds after that embarrassing episode back in the 1960s when the Great Knighted, Sir B. J. Mason1 and his student, Swinbank (1960), reported drops exploded with they froze.  Liquid centers tried to get out of the ice shell as the drop froze from outside in, as you would expect, but then blew up when the freezing water expanded inside the shell.  Looked pretty good.

There was only one thing wrong, their findings weren’t valid for real clouds.

They put too much CO2 in their cloud chamber (that’s right, the very SAME stuff that’s supposed to make the earth warmer and warmer year after year but has been sitting around lately, about 15 years actually, not doing anything) and that CO2 in the experiments turned out to make the outer ice shell real weak, and also the CO2 came out of solution in the water in the liquid center to make matters worse by expressing gas through the shell.  I wonder how many people have done that?

This was found out by researchers in my very own group before I got there, Jim Dye and Peter Hobbs, a few years later.  When real air was used, the drops didn’t explode.  So, down that hypothesis went that exploding drops caused a lot of ice to form in natural clouds.

End of story?  Nope.

Later, Hobbs with grad student, Abdul Alkezweeny, repeated the experiments with freezing drops, but this time instead them just sitting there, had them rotate as they froze and they DID shatter some, but not a lot!  This was back in 1968.

But no one was reporting images of shattered drops.

In those days,  there was a HUGE amount of unexplained ice in clouds.  Cloud chambers on the ground and in aircraft, found that little ice formed until the air IN THE CHAMBER was at least as cold as -20 C (-4 F), but instrumented aircraft repeatedly found tremendous amounts of ice in clouds that had never been colder than -10 C (14 F).  Hence, an enigma.

But the explanation that a few drops exploded, sending out thousands of ice shards never gained any ground because there was never any observational evidence that it happened.  Instead, an Australian researcher, originally from South Africa, Stanly C. Mossop, with John Hallett, discovered in 1974 that a bar moving through a cloud chamber between -2.5 C and -8 C, caused ice splinters to eject from SOME of the little drops hitting the bar and freezing on it.  But the drops had to be at least 24 microns in diameter, fairly large for cloud droplets, or nothing happened.  Also, if they moved the bar too fast or too slow, nothing happened.  So, there were a lot of criteria involved in this process, temperature range, drop sizes, speed.

So, the Hallett-Mossop riming-splintering hypothesis was born.  They assumed the bar, moving at the fall speeds of soft hail, showed what soft hail did inside clouds:  multiply ice content!

It was an exciting time to see that the mystery of all that ice in clouds at higher temperatures was finally explained, not needing, shattered drops or anything else.

But there were some problems.  In the early days, it was thought that this process, to raise the ice concentrations in clouds much, would take as long as 1-2 hours because it was a “cascade” process.  The few first splinters had to grow to sizes there they fell fast enough to bump into drops and cause ice splinters to eject.  Well, that wasn’t right.  Natural clouds formed ice MUCH faster than that, as you here in Arizona know so well.

The experiments continued and it was found that shattering helped this process (assuming it occured, but even more important was the freezing of drizzle and raindrops.  When those froze, they became instant rimers, splintering objects, and so the time for a cloud, but one having drizzle and raindrops in it, and in the right temperature zone, just between -2.5 and -8 C, was cut down to minutes, something like 10-20, to get ice concentrations from about 1 per cubic meter, to tens of thousands per cubic meter, a real rain cloud.

Except for a single image of a drop half by a researcher using a cloud camera with a glider in the 1970s, no one had reported a shattered drop.  Then along come Korolev et al. (with the great Hallett!) in 2004 reporting shattered drop images in a Canadian frontal band using an advanced cloud camera.  They wrote that it was the FIRST images ever reported of shattered drops.  Rangno and Hobbs (2005) also reported images of shattered drops in clouds around the Marshall Islands, thinking at the time that they were going to be first in line, and then discovered the Korolev et al. report.  It was a sad day to find that reference, as a researcher that was thinking about the glorious days ahead, the keynote addresses to important conferences, that would result from being first in line with something and then other people would always have to reference you.

As Ecclesiastes wrote, their is hardly anything new under the sun if you’re slow going about it.

Published another paper on shattered drops back in ’08.  But, found they didn’t SEEM to be making a big contribution to the ice content in clouds, less than 10%.  You can go here to see that I didn’t make that part up. That was kind of sad finding, too.  You want what you find to be HUGE, and it wasn’t so huge as I hoped.

So, riming and splintering remains our best, most accepted explanation for the great amounts of ice in clouds that aren’t so cold, though the author and Hobbs, have mostly found it wasn’t powerful enough to account for the speed of ice development.  Only the author’s friends, Stith et al (2004),  have reported a lot of ice that couldn’t be explained by the riming-splintering mechanism as have  R&H over the years.

But it would be so great if others confirmed the Stith et al findings.

The End for now.



1Wiki doesn’t do a very good job, and doesn’t even list his outstanding updated, Physics of Clouds text published in 1972, the “bible” of cloud physics in those days!  Unbelievable.

2Riming: Think of what happens to an airframe in a liquid drop cloud at below freezing temperatures.  HELL, here’s a photo by the author from the author-occupied Lear Jet 35 flying in supercooled clouds over Saudi Arabia, 2006,  The weapon-looking things under the wings image precipitation particles like raindrops and snowflakes using laser beams with light sensitive diodes at the other end, one that when shadowed, give you a two dimensional image of what went through the laser beam.

ann DSCN1223 rime icing
8:01 AM, December 16, 2006.


11:02 AM, December 10th, 2006.  Had to land at Hail, a small, pretty town north of Riyad to pick up some supplies, ones for the randomized cloud seeding experiment underway.  It was interesting that we could carry these boxes labeled "Explosive" to the Lear Jet without any notice.   Hmmmm.
11:02 AM, December 10th, 2006. Had to land at Hail, a small, pretty town north of Riyadh to pick up some supplies, ones for the randomized cloud seeding experiment underway. It was interesting that we could carry these boxes labeled “Explosive” to the Lear Jet without any notice. Hmmmm.  They were there because that’s where another NCAR radar was besides the one at Riyadh, and a plane might have to land to continue seeding if it ran out of the seeding flares like the ones inside these boxes.