The Twelve…rain drops in Catalina, that is

Well, maybe there were about 27, but anyway….not very many; still, those drops were to be treasured after not seeing a single “hydrometeor” display in SE AZ in so–ooooo LONG A TIME!

———————————-

PG-13 advisory; DRIZZLE is discussed

I have to warn you at this point. That rain event yesterday WAS NOT DRIZZLE!!!!!!!!!!!!!!!!!!!!!!!!!!! I will be ROYALLY PO-ed if I hear someone in my social network or a TEEVEE weather presenter say that it “drizzled” yesterday!

Why make a BIG THING out of the correct type of precipitation?

I have to tell you a true story (well, I don’t have to, but I am going to anyway) about the importance of drizzle (i. e., fine, close together drops that appear to FLOAT in the air). This event happened during my cloud seeding “vigilante” adventures (see Publications for samples). A well-known professor of cloud seeding in a foreign country asked me to leave his office and never come back after I told him it had been “drizzling” outside, “10s per liter” in the air.

Drizzle is a profound indicator of cloud structure overhead, and the presence of drizzle falling from the clouds in that professor’s region’s meant his numerous reports of how clouds were, ripe for cloud seeding, were in substantial error. So you can understand why a report of true “drizzle” would naturally be upsetting to that professor. Man, am I digressing here! Yikes. My apologies. (BTW, those reports WERE in error, confirmed by aircraft years later! (Spiking football now, with a proper amount of decorum, of course!)

——————————-

OK, back on task….

With the sky full of low (“boundary layer”) clouds by mid-day (f you’ve forgotten, that was yesterday, May 10th, 2011) and with RW— in the air (“triple minus”, extremely light rain showers) by 1:30 PM, with gusty winds, temperatures in the mid-60s, it turned out to be quite a “storm.” It just as well could have been but a mostly sunny day with just a scattered Cumulus clouds here and there the way some models were “telling it.”

Here’s a pictorial on how it went, from a Catalina, AZ, perspective:

1) 09:29 AM, itty bitty Cumulus (Cumulus “fractus”) starting to appear,

2) 12:03 PM, larger Cumulus growing up into Cumulus “mediocris” beyond Tortolita Mountains on the horizon,

3) 12:29 PM, virga and rain visible to the NW horizon! Now I am getting apoplectic since the best models in the world did not have this precipitation over thataway! But there it is, bigger than watermelons. The models have to be really red-faced about this! Not everything in the world is predetermined by numerical models; you can say things that might be right and those models are WRONG! Just like in the 1970s when a lot people thought global cooling was underway and that’s where we were headed! But they were WRONG! Who were those clowns anyway?! (hahaha, sort of).

4) 1:25 PM. Now where was I before all that excitement? Oh, yeah. Here’s some ice for you. See the frizzy top parts of this cloud in the center of this photo above the dead tree that the birds like to sit in? Well, them’s ice crystals, and likely snowflakes that have formed in that medium-sized Cumulus cloud (above the dead tree) and its in the upwind direction. Behind that is more ice and precip falling from a wide area of a Cumulus-Stratocumulus complex.

—–

Mini-diversion

Quiz. How cold does the top of THAT cloud have to be to look like that (have that much ice in it, probably a few per liter to maybe 10 or so, not a tremendous amount but significant)? Well, with bases as cold as they were, near freezing by this time of day at around 7, 000 feet above the ground or 10, 000 feet above sea level, around -15 C (or about 5 F). Amaze your friends with cloud trivia like this! Well, maybe not.

—-

5) 1:25 PM. Here it is, a band of precipitating clouds overhead. Now the ONLY question remaining, as you gaze upwind at Twin Peaks clearly visible through the precip and virga is, how much will there be? None? Or as much as a “trace”? Measurable is out of the question, looking at this scene below the clouds. Most of the visibility degradation is due to dusty air, not precip. Darn. (Amaze your friends with skills like this! Well, maybe not.)

6) 3:03 PM. The End is Near

7) 7:06 PM. Nice sunset with traces of Cirrus and Ac len on the horizon, driblets from a storm striking the Pac NW. Isn’t there always a storm striking the Pac NW? I digress again.

Man, I could go on about the weather maps of yesterday, but will quit here.

The end.

If you REALLY want to see how it went, take a look at the U of A time lapse video here.

Cirrus uncinus display; the tops of storms made visible

First, some instructional material: You should be looking for your camera now, as seen in the first shot! Those Cirrus clouds to the SW are moving at you rapidly (95 kts, 115 mph at 30-35 Kft ASL!), and so there’s not much time! In this first shot you can already detect some Cirrus uncinus, Cirrus clouds with hooked, or tufted tops in the center, with long icy strands trailing to the left. At this point perspective makes them bunch together so that they may not appear that “photogenic.” However, just wait! And, it was worth waiting just a few minutes for.

Take a lot at which they looked like passing overhead in the second and third shots, only about 7 minutes later. Just magnificent, some of the best Cirrus uncinus examples I have seen.

What is interesting about these clouds is that you are getting a glimpse of the structure of “stratiform”–that is, steady rain and snow storms that happen every day around the world, except that here in these photos, you are only seeing examples of the very tops of them. Those widespread rainy/snowy storms are usually packed with thousands of these kinds of clouds in a solid overcast up there, each “cell” shedding tiny ice crystals which then waft their way down, growing, perhaps merging into “aggregates” of ice crystals we call snowflakes, and, that most of the time except in Wisconsin in the winter, melt into raindrops as they fall below the melting level. Chances are, our little snowstorm of a few days ago had tops just like this.

Sometimes, clouds like these, and returns from vertically-pointed radars that can detect clouds like these, are referred to as “generating cells”, for obvious reasons. The trails you see here are clearly visible on very sensitive “cloud sensing” radars–they are not visible on “First Alert” Doppler style radars and such used by the NWS because the ice crystals are too small at this point to produce a return on “normal” radars. These cells form in a relatively shallow layer that usually lacks wind shear, likely mixed out by the little up and downdrafts in it. Its only after the crystals fallout that they encounter wind shear and end up being stretched out into “tails” as here.

Falling from heights of 30,000 feet takes a long time, for an ice crystal falling at only around 0.5 meters per second or around 1.5 ft a second or even less. It will take LONG time for anything to reach the ground, perhaps 2-3 hours to reach the melting level. So the little generating cell that produced a ice crystals at the top of major storms that grow and merge into snowflakes is likely over Alamosa, CO, and points northeastward by the time that flake landed on you at the ground with upper level winds such as we had yesterday.

I think its kind of interesting, but I may be the only one!

The end.

Complications in the sky

First of all, let me assure quesy readers that the jet leaving the contrail at left was not “flaming out” and about to crash as it traversed the sky at this time, as the staccato nature of the contrail at left might suggest. The staccato nature of the contrail is due to vagaries of humidity at flight level. Where it was quite dry, the contrail disappeared immediately behind the jet.

Its a little unusual to see such short segments like this, however.

What was really interesting and a bit inexplicable (again) except via a LOT of hand-waving is the crossing patterns seen in the clouds at left and in the next shot. Pretty darn remarkable. In the second photo, icy strands of cirrus (“fibratus”-the strands are straight in this variety) are seen running SE-NW, “whilst” above there are Cirrocumulus clouds with ripples and little cloudlets oriented from SW-NE! All of these clouds were moving rapidly from the SW.

Maybe its too complicated; we should forget about it and go to the movies. Well, the cloud movies…

Here is yesterday’s cloud movie presented by our own University of Arizona Wildcats. This will help.

First, if you take the time to load this time lapse movie, you will enjoy the tiny cloud “tumbleweed” that goes by at 12:02 PM. It really acts like one, as you will see! Its an entertaining sight, and one that illustrates the wind shear in the atmosphere yesterday.

And that’s what these remarkable crossing cloud configurations are about, changes in wind direction and speed with height, something we “met men” call “wind shear.”

Some of the clouds in the second photo passed within the viewing area of the U of A time lapse movie between 1:59 and 2:03 PM LST and you can just make out the cirrus clouds going by with their lower portions draining off to the right very slowly. In these blog photos, looking as those clouds approached from the SW, the trails appear to originate on the right and “drain” to the left, a mirror image.

Ice crystals in cirrus clouds often form in sudden appearing, tiny flocculent specs like the Cirrocumulus (Cc) clouds. In these photos, in these photos, the Cc clouds are above the icy cirrus clouds. What normally takes place is that the ice crystals grow and the largest ones fall out producing strands below the “head” of the cloud. The slower they fall, the more apt the fallstreak is to be drawn out over a long distance away from where the burst of ice formation took place, and if there is a sharp wind shear, the more angled and contorted the trail will look. Still, at right angles? The Tucson sounding for yesterday afternoon doesn’t reveal much wind shear in the moist layer where these clouds were forming, 23, 000 to 27,000 feet above sea level. On the other hand, balloon soundings would not show extremely sharp changes in wind direction/speed over very small height increments. So, it remains somewhat of a mystery. In view of the orientation along the jet airway upwind of this site that runs in the direction of the line cirrus in these photos, it is possible that these are contrail remnants. There jets that flew at low enough altitudes at times to glaciate some of the Cc-Altocumulus clouds and one short ice trail produced by an aircraft in them can been seen in the U of A movie at 2:43 PM.

Along with these interesting alignments were numerous optical phenomena yesterday, some of which I could not identify, such as in these three. I suspect the first one is something called a parhelic circle, if you happened to have seen this bright arc radiating away from the sun’s position. A bit more mysterious, at least to me, was the sudden brightening of the contrail segment above the Catalinas and the cirrus (uncinus) cloud, last two photos.

OK, enough. I tried my best to explain everything and I have failed.

The End.

Exit right (or to the east)

Here’s what happened on top of us yesterday, that gorgeous snow day with so many wonderful sights to see. These maps below, courtesy of San Francisco State University , for 500 millibar pressure level, about 18,000 feet above sea level, for 5 AM LST as the snow band moved through Catalina, and then 5PM LST, a little before sunset:

A visual on what the clouds did as this happened yesterday is below. Interpretative cloud statements on the following gallery: shallow, deeper (precip begins in distance), deepest (small, soft hail falls here and there from miniature cumulonimbus clouds), less deep (barely-able-to-precip stage again), shallow, nil. Pics 1,2, 3, 4, 5, and 6, respectively. If you want all the visual glory of yesterday, go to the U of A time lapse movie here. However, you’d better hurry, these wonderful films are overwritten each day. You can really see the clouds flatten out after about 3 PM LST here, and there are some spectacular snow showers going by on the Catalinas.

The end.

Virga anyone?

Mr. Cloudmaven person foretold certain cloud types would occur yesterday in conjunction with “storm” 3 yesterday (which was really only the passage of an upper level trough over us–see map for 5PM yesterday). Let’s see how he did, that is, whether he is an actual “cloudmaven”:

(0=not observed, 1 observed, -1, cloud observed, not predicted:

Cirrocumulus, 0

Altocumulus floccus virgae, 0

Cirrus, 1

Cumulus fractus, -1

Cumulus humilis, -1

Cumulus mediocris, -1

Stratocumulus, -1

Stratocumulus virgae, -1

Cloud score: -4 =s bad cloudmaven; credentials suspect.

Here are some of the cloud sights from yesterday in case you miseed them and want to fill in some entries in your cloud diary: 1) 4:42 PM, 2) 5:26 PM, 3) 5:53 PM, has band of cirrus with trough passage, and 4) 6:12 PM, with some virga/ice fallout showing (darkish veils below clouds).

However, I did opine that there might be a sprinkle with this trough (see map again for example of yesterday’s “trough.” (Sounds like the stockmarket in ’09.) Having been specially trained to recognize virga and precipitation when a forecast of precipitation is on the line, I found it easy to recognize just how close I came to getting that sprinkle as evidenced by some virga trailing down from some of the patches of Stratocumulus clouds. See above hills in photo at left.

You now know, if you have been reading this blog and thinking about it all day, that ice formed in these clouds because they had crossed a temperature threshold, had gotten cold enough to form ice. That “virga” was, if you flew through it, snow flurries. Where it melted into raindrops closer to the ground is not visible. However, it is unlikely that virga of this magnitude reached the ground.

You might now even guess the temperature which the tops of the clouds reached. My guesstimate from the TUS sounding at 5 PM yesterday is somewhere between -15 to -20 C, a threshold for ice (precip) formation in shallow clouds such as these. Estimated depth of the thicker clouds seen here? About 2000 feet or so is all.

BTW, if you noticed these very subtle virga/ice in these Stratocumulus clouds that began to show up late in the afternoon, you are a cloud observer supreme!

Yesterday’s weather map:

The end.

“Little snowstorms in the sky, I think I’d like to have some pie”

You’re probably smiling now remember singing this little ditty as a kid, maybe singing it with your friends on the bus, whenever you saw “Altocumulus floccus virgae” clouds such as are pictured in the first photo. Wasn’t it great when you saw these kinds of clouds while on a vacation trip and mom and dad had to stop somewhere to get you some pie after you sang that song? Well, I nostalgiate here.

To the right of the dead yucca stalk, Altocumulus tufts are shedding snow. The opacity of the virga is a give away that its snow, and not rain. In some of these little tufts, the water droplet cloud that preceeded the formation of ice has disappeared, and all that is left is falling snow. How much snow is it? Just a flurry, if you were up there, even though it looks pretty thick. Once in awhile in our research on ice in clouds at the University of Washington, we got to sample these from top to bottom. Because the ice crystal concentrations are usually pretty low in clouds like the ones shown, a few per liter and often less than 1 per liter, those delicate ice crystals don’t bump into each other much and break up, and you find gorgeous images of star-like crystals in these fall out streaks, the kind you see on Christmas cards (examples here). How do I know what from ten miles away and 16,000 feet or so below them. Its a funny thing, but ice crystals are differently shaped depending mostly on temperature. To get the temperature of these clouds you can get a pilot report (unlikely) or examine the humidity profile of the Tucson sounding for “00 Z” (5 PM LST yesterday afternoon and make an educated guess. The highest relative humidity on that sounding was at 525 mb ) about 15-16, 000 feet above the ground) with a temperature of -16 C (about 3 F), namely, darn cold. Continuing, we in this field have a well known chart by Magono and Lee (1966) that shows the temperature at which certain forms of crystals grow. At the temperature I am guessing those clouds were at, those crystals would have grown as stellars and dendrites, which grow between about -12 C and -18 C. Sometime I will show you some of these crystals, but for brevity will quit here on this topic.

The second and third photos show what one of these tufts looks like before the crystals have grown and fallen out. Top center, the largest raggedy tuft (Altocumulus floccus) show no fallout of ice. In the last photo, twenty-nine minutes later, there is a fine veil of ice crystals below it (upper right hand corner). Only now, with virga, are they “Altocumulus floccus virgae”! I’m singing right now! And, if you look really carefully you’ll see that most of those little guys have a little ice fall underneath them. Certainly, in those clouds you would find PERFECT crystal specimens!

I’ll end here on an exciting note. The Enviro Can model CONTINUES to show a very strong system moving into our area on Saturday afternoon, likely accompanied by winds as strong or stronger than we saw last Saturday, before the rain and cold air hits on Sunday. Snow levels are going to be really low and we might see some ice in the rain on Sunday here in Cat (alina) Land. Amounts are looking substantial at this point. Man, do we need it!

In the meantime, an upper trough off Baja passes over tomorrow. It has enough moisture with it to provide more “clouds for pies” (Altocumulus floccus virga, and, of course, Altocumulus castellanus virgae, which also qualifies as well for a pie). And, some cirrus will be around, too. However, I am going to stick my neck out and say there will be sprinkles tomorrow. Mods really don’t have a thing, so you’ll have to keep that in mind.

Be sure to keep you’re camera ready for sunsets like last night (see below)!

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.

Moist but mostly dry

Though HUGELY disappointing because only a trace of rain fell here as of 7 AM this morning, and only a little in the Canada del Oro wash watershed (amounts here), nevertheless, what a nice, classic passage of a cold front. A cold front, as it sounds, marks the advancing boundary of colder air that is displacing warmer air, and that went went by late yesterday afternoon. When it goes by, the wind direction changes almost instantaneously, the temperature begins to drop, often sharply at it did at 5 PM yesterday (see below), and the barometric pressure begins to rise.

But without measurements or satellite or other data, you yourself could have seen that invisible boundary approaching Catalina by the low, scruffy clouds that began to appear on the horizon to the northwest. Soon they were topping the Tortolita mountains, then the Catalinas. And you would have noticed that, unlike the clouds overhead, those lower clouds were advancing from the north. That evolving scene looked like this, finally ending up as a low overcast of Stratus clouds. The first shot below was at 4:40 PM, 20 minutes before the windshift and temperature plummet hit. The second shot is as the windshift was passing Golder Ranch Drive and shows the lower cloud bases associated with the cooler air racing south along the west side of the Catalinas. You can see that they are also connecting to the higher Stratocumulus layer. The third shot shows the Catalinas fully enveloped in the cooler air and lower clouds, and the last shot is of those much lower clouds (I would call them “Stratus”) over Catalina and Oro Valley, looking to the west.

You can also relive yesterday’s clouds and windshift from the vantage point of the University of Arizona’s timelapse film. You will see the windshift hitting there marked by puffs of dust from the NW and then those low scruffy clouds right behing beginning about 5:20 PM here.

So why didn’t it rain with all these clouds? What was missing? For almost every drop of rain that falls in Arizona, ice crystals are required to start the precipitation process going.

The formation of ice in clouds is a continuing scientific enigma, believe it or not. However, we know that they didn’t form, with brief exceptions yesterday afternoon when a few sprinkles (NOT “DRIZZLE”, dammitall! Sorry, lost control there for a second)…..formed in the higher deck of Stratocumulus clouds, and again last evening when it rained again for a few minutes.

The first thing you would guess then, since we are talking about the formation of ice in clouds, is that the tops of the clouds did not get cold enough, that is, were not high enough above us and upwind of us, for ice to form. That would be my best explanation for those periods where it was not raining, we had low clouds and they looked rather threatening for much of the time between 5 PM and dark. (After dark, some rain did briefly fall.)

However, the Tucson sounding launched yesterday afternoon around 4 PM shows that the tops were plenty cold enough; the top of the moist layer was about -20 C! (Note: soundings do not measure “clouds”, but rather humidity, from which we INFER clouds). Normally a considerable amount of ice would be expected in clouds having a top temperature that low. Tiny echoes did occur over and downwind of the Catalinas all around the time of that sounding which means that ice was forming precip here and there in the clouds we saw, and measurable precip was recorded in the CDO watershed.

Sure wish I could have been up there in our former research aircraft to check this out more! But, will have to leave this in a bit of an unsatisfactory way.

My apologies if this got a bit deeper than you really wanted to get into.

Trick and treat sunset yesterday evening

Late yesterday afternoon, the sun appeared to be setting in the wrong location, about 20-25 degrees south of where it is supposed to be at this time of year. Perhaps something horrible had happened, I thought. Retirement with a happy ending here in Arizona was too good to be true, I thought, and now it was all going to come to an end. First, some perspective on where the sun was going down BEFORE yesterday. This first shot was taken just a few days ago (Feb 13th). Note where the sun is relative to the Tortolita mountains on the right, and Twin Peaks, the two itty bitty humps to the left. For further perspective, at the winter’s solstice, December 21st, and from this same location, the sun sets next to Twin Peaks. So, in this first shot you can also see how much the sun has moved in two months.

But then yesterday, something awful seemed to be happening. The next photo was one to send chills down your back, and in fact, if the sun was setting over there in the winter as a matter of routine, the northern hemisphere would likely glaciate down to about Blythe in the winter, due to the NH sunlight being so weak (that is, with so much tilt of the earth’s axis about which it spins). The days here would, in that case, be about the length of those in Seattle with daylight only from about 8-4 PM in the wintertime because the sun would be taking such a low trajectory in the sky; would rise late and sink early.

So, while I was concerned with the earth-sun system and some kind of apocalypse yesterday evening, I have feeling that most people were thinking, “Well, I guess we’re not going to have such a great sunset. Seems to be too many clouds over there where the sun is setting.”

Or maybe you were thinking about that important Washington Husky Arizona Cats basketball game today and how it might go.

But “No!”, a little later the sun underlit all those clouds, appearing to have sunk in its proper position for this time of year (3rd photo)!

I felt relieved and started thinking about that important Washington Husky -Arizona Cats basketball game today and how it might go. Then I also started thinking about how I might have been the ONLY person to notice something was terribly WRONG with that sunset. I feel pretty good about that part.

So, what happened? This “trick” sunset, followed by a treat of a sunset was caused by a parhelia (explanation by my friend, Bob, here) whose accessible name is “sun dog” or “mock sun”, which we CERTAINLY had in this case!

Parhelia appear, if you don’t go to the site above for a more complete explanation, when the ice crystals in the cirrus clouds up there are hexagonal plates, and fall with their faces down. The sun’s light is refracted (bent) as it passes through jillions of these plates and at about 22 degrees from the sun’s position, an observer on the ground will see a bright spot, sometimes with a little coloration. Sometimes there is also a “22 degree” halo along with the sundog.

I should add that the “trick” parhelia was being produced by ice crystals in the cirrus clouds above and behind the altocumulus cloud deck yesterday. Of course, as you know, altocumulus clouds are comprised completely or mostly of droplets and cannot, therefore, produce parhelia.

Finally, to end, the last shot is almost at the winter solstice, taken on December 26th, and has a parhelia, aka, sun dog, mock sun, at left so you can see what they usually look like and how far away from the sun they are near sunset. Yesterday’s, though, I thought was astoundingly bright and really made it look like the sun was going down in the WRONG place.

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.

APIPs REDO_078 — 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.

APIPs REDO_081 — 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.

APIPs REDO_076 — 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?

^{———————————————————————————————————————————————————-}

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.