Category Archives: Stratocumulus clouds

Aerosols, Cloud seeding, Definitions, Drizzle, Ice crystals, Stratocumulus clouds

So Seattle

Seattle nostalgia part

Got a little homesick yesterday with the weather the way it was here in Catalina.  Well, after 32 years in Seattle it seemed like home.  The dank, gray, low-hanging, lifeless, overcast skies of Stratus and Stratocumulus clouds hour after hour (spoken with emphasis on each descriptor using a whiney kind of emotional voice) reminded me of all the great times in Seattle, the friends, the occasional unsatisfactory girl friend (hahaha), the sports glory days in the Western International League semi-pro baseball league (well, there was one day in particular), the days as a Mariner batting practice pitcher in the equally dank Kingdome, an edifice that mimicked the skies outside, the 25 years or so of bike trips to and from the University of Washington under those same kinds of dank, gray, lifeless skies, and piddling rain we saw here yesterday.

And with people here driving around in the middle of the afternoon with their car lights on, it was the perfect replica of a Seattle winter day. Even the temperature cooperated here to mimic Seattle;  it only changed a couple of degrees from dawn to the mid-afternoon high of 47 F, some 20 degrees or so below normal.  Sometimes in Seattle, the temperature is the same all day because the sun is so weak and the layers of clouds, piled one upon the other, seemingly impenetrable to light and heat.

The Seattle mimicry was completed with some light rain in the morning here, raining just like it does in Seattle, barely accumulating hour after hour but there to annoy you.  We only had 0.02 inches after 7 AM LST, but it stayed wet all day here just like back at my old Seattle home.  The grass in the backyard there might be dry sometime in April, otherwise your shoes and your dogs come in wet everytime.

Yep, that’s what Seattle is like.   I loved it there….  Hmmmm.

Well, what I really loved was being at the University of Washington during the glory days of the Atmospheric Sciences Department when all the “big guns” in meteorology were there it seemed;  Holton, Hobbs, Wallace, Reed, Charlson, Fleagle, Bussinger, Leovy, Houze, and the job of flying around in clouds in the “CARG” aircraft, etc.  Their kind will not be seen, clustered together like that, anyway, again.  As a kid I tried to get autographs of the big stars in meteorology like Jacob Bjerknes at UCLA so that’s why being at the U of WA was so exciting.  They were to me like major league ball players were to other kids, though I did like Ted Williams and the Bosox in those halcyon days. Egad, enough!

A review of yesterday’s clouds below.  The first photo toward the Tortolita Mountains (obscured) and a near perfect example of Stratus, followed by two shots of Stratocumulus.

Weather and clouds part

Yesterday was interesting in several ways.  First the rain fell from clouds topping out at a modest -10 C, something we don’t see much of in Arizona.

Second, it was so dark during the day even as the clouds became shallower as the day wore on. Below are the two soundings from TUS for yesterday from the U of AZ, the first at 5 AM and the second 12 h later at 5 PM LST.  Where the green and white lines come together mark where the clouds were.  From that description you can see that they were topping out around 700 mb, or around 10,000 feet above sea level in the morning, and maybe 9,000 feet by evening at about -6 C.  No echoes were being observed by the afternoon.

But why?

Well, the cloud savant will know that there must have been a lack of ice crystals in those supercooled clouds in the afternoon (ones whose tops, at least, were below freezing.)  But why no ice?  Almost certainly it was because the cloud drops in those clouds were smallish, less than 25 microns in size, there were no “ice nuclei” active at those higher temperatures (a normal situation).  The lack of drizzle (fine misty rain that appears to float in the air) is evidence that the drops in those clouds did not reach sizes larger than 30 microns in diameter. Drizzle drops (those between about 100 and 500 microns in diameter, have often been associated with the onset of ice in clouds at temperatures of -4 to -10 C over the years.  But not always.

BTW, the formation of drizzle helps eradicate clouds by draining them of their liquid water, causing clearings.  So the absence of drizzle due to smaller drops helped “seal the deal” on a long day of overcast.  Also surprising to me was the amount of underlying haze yesterday, again evident in very strong crepuscular rays late in the afternoon.  See below, along with the rare example of liquid cloud mammatus formations.  That haze suggested to me that the drop concentrations in yesterday’s clouds were also pretty high, and helped keep the sizes of the droplets down by spreading the “condensate” on more particles. And guess what, those likely more numerous and smaller cloud drops helped make those clouds darker than clean clouds.   More sunlight is scattered back into space by dirty clouds than by clean clouds, so the bottoms of dirty clouds are darker even given the same thickness as clean clouds.  (This is a HUGE issue in global climate models, drizzling and not drizzling clouds because it effects the cloud cover, and the more cloud cover you have, the more CO2 warming is offset.)

So several factors likely went into making yesterday “so Seattle”; lack of precip later in the day, small cloud drop sizes, likely due to high droplet concentrations in those clouds, at least later in the day.  Lets hope it doesn’t happen again.   The End

Addendum on cloud seeding potential for yesterday

Yesterday was the perfect day for seeing what you can do by seeding clouds with dry ice–dry ice is more effective on days with marginally supercooled clouds.  Would there have been some rain on the ground?  You betcha with those low bases.  However, it is virtually certain that any rain would have been but a trace, or barely measurable.  The main thing that would have happened is that clearings would have ensued in the seeding zones, seeded by the way, by aircraft dropping dry ice into those tops.  So, a little precip, and some clearings, and those clearing would have likely triggered a widespread natural clearing if you could get enough holes produced by your aircraft.  The last photo shows would you can do when you glaciate a relatively thin layer cloud.   Were yesterday’s clouds thin enough for holes?  I think so.


Altocumulus clouds, Cirrus clouds, Definitions, Lenticular clouds, Stratocumulus clouds, Sunrise, Sunsets, The weather ahead

Low spin cycle continuing off Baja; water being added

That enfante terrible now dawdles over the coastal waters of California and northern Baja today, adding some water to its central system as seen here from IPS Meteostar.  Note, too, a scruff of Stratocumulus clouds racing northwestward in the Gulf of Baja abouit the latitude of the border between north and south Baja.  This is good.  Still, this is a marginal storm as it trucks through on Sunday evening and I will be happy if we get a quarter of an inch.  Mods don’t think rain will get here until the upper center is upon us later Sunday.   Here is the whole map forecast sequence showing temperatures and winds aloft at 500 mb from the U of Washington unranked Huskies who play #12 Baylor in the Alamo Bowl –what a horrible bowl game that is for Baylor–on Dec 29th.  Still, as a spin off myself here into the SW like all of our lows this year, but from the U of WA, I will be rooting, of course,  for the “company team” along with my friends and former grad students with whom I worked.  BTW, these are pretty maps with lots of color as you will see.

You will also notice in this 4 day series of forecast maps for 500 mb that yet ANOTHER low drops into the Yuma area replacing the current one that begins to move toward us later today.  That new low develops via the back door from Colorado and nests over Yuma as a cut off at the end of the forecast cycle above.  That ‘s an unusual trajectory for a low! However, mods think its too dry to produce any rain as you might imagine.   But with another low center aloft ending up in the SW, it demonstrates again our characteristic pattern for this early half of the winter, having become something of a low magnet.  It certainly has been strange to see so many cut off lows.

BTW, the longer term picture after our little rain is a dry one for the next two weeks;  it may well mark the end of our cut off low fantasia.  Hope not.

Today’s clouds…

The marginal amount of moisture circulating around the periphery of this Baja low, I think will spin out some spectacular middle and high cloud streaks and patches over us such as Cirrus, Cirrocumulus with its tiny granulations, and Altocumulus lenticularis here and there, ones that often break up into small and interesting cloudlets downstream.  Get yer cameras ready!  Some of this began to happen yesterday afternoon.  Sometimes you see the most amazing tiny delicate patterns, very photogenic.

Here are a few shots from yesterday, beginning with that sunrise patch of Altocumulus and ending with a sunset shot.

Cumulus clouds, Definitions, Ice crystals, Stratocumulus clouds

The most interesting clouds–technical discussion ahead; skip now if you have a headache

Well, to me, anyway, those flat, thin ones yesterday (Stratocumulus, and Cumulus fractus, humilis, mediocris, if you want some names).  The thicker versions of these clouds were shedding ice and snow but weren’t very cold and that’s what made them interesting.  Take a look.  If your eye is calibrated you can see that light snow is falling on the Catalina Mountains in the first two photos, and in the third photo, one that better illustrates how thin these clouds were (probably 2,000 to 3,000 feet thick is all), snow is seen falling in the distance from the cloud above and to the right of the white roof of the shed in the foreground.  Quite remarkable.

Why?

According to our TUS sounding data, these clouds, at least at 5 AM LST, were topping out at only around -10 Celsius (14 F), a very marginal temperature for ice formation in clouds in Arizona.  By the afternoon, the TUS sounding was suggesting the clouds near the sounding balloon were as warm as -3 C (28 F), too warm for natural ice formation.  (Where the temperature and dewpoint temperature lines come together, right now, over me.)

If you go to the movies, the time lapse one from the U of AZ here, you will see that the clouds were coming toward TUS from the Catalina Mountains by the time of the afternoon sounding, and therefore its not valid for the heights of those clouds over the mountains–the air has sunk coming off them.  So, the best we can do is probably with the morning sounding and assume the clouds were topping out at the -10 C level with bases about 0 C to -2 C.  You can get base height from where they were intersecting the Catalina Mountains (the cloud, not the snow!).  That was at about 6,000 to 7,000 feet elevation, roughly 4,000 feet above ground level here in Catalina.

So, we had pretty cold bases, and not very cold tops.  When we see ice coming out of clouds like these (and I would estimate from airborne experience at the U of WA, that those concentrations of ice coming out of those clouds were a few to 10 per liter).  If that estimate is correct, then we would say that these clouds exhibited “ice multiplication” or “ice enhancement.”  That’s because we don’t expect concentrations that high in clouds with tops around -10 C.  That is,  unless something “extra” is going on to add to the very few ice crystals that might have formed without “multiplication”; perhaps only 1 in 100 liters at -10 C.

So wha happened?

Our most accepted theory is that the very few lead to the many in collisions with cloud drops.  Those cloud drops, when a bit larger than usual (24 microns in diameter for a number), shed ice splinters, and those splinters go on to populate the cloud.  It takes a long time because the itty bitty splinters have to grow to sizes where they can collide with cloud drops, too.  This ice enhancing mechanism, referred to the Hallet-Mossop riming splintering mechanism, named after two scientists named Hallett and Mossop (hahaha), is known to only occur in the temperature range of -2.5 C to -8 C.

Those temperatures were indeed found in our little clouds.  So, too much ice explained!

Not quite.

In little, cold-based clouds deep into the continent as we are here in Arizona, it would NOT be expected that the droplets inside those thin clouds would be large enough (as big as 24 microns in diameter) so that ice splinters would occur when colliding with the rare ice crystal, the 1 in 100 liters one.

So, that’s the mystery of yesterday.   These clouds DID look kind of “maritime” to me.  In maritime clouds, such as those found in onshore flow along our coasts, drops reach that critical size for riming and splintering just above the bottom of the cloud.  That’s because the air is clean over the ocean as a rule,  and so there are few particles for the moisture to condense on, leading to larger drops right away, and many fewer in a liter than in continental clouds.   A typical maritime cloud has less than 100,ooo drops per liter, whereas a “continental” cloud would have several hundred to a half a million drops per liter and even more in some polluted areas.  So, as a drop, its hard to be a big deal in a “continental” crowd like THAT!

Best thought is that the heavy rains of late cleaned the air hereabouts and those clouds that looked soft and “maritimey” yesterday REALLY were like maritime clouds found over the oceans away from land.  Maritime clouds, as ours might have been,  have large drops in them, and even drizzle and raindrops before reaching the freezing level.  Therefore,  ice forms in maritime clouds at the highest known temperatures for natural ice formation, -4 C to -7 C.

The End.  You might want to rest for awhile if you got this far.

The weather ahead?  More rain, continued below normal temperatures as another “cut off” low rolls off the jet stream table in the north Pacific and falls into the Southwest and Baja area in the next coupla days.


Cumulus clouds, Definitions, Ice crystals, Snow, Stratocumulus clouds

Cumulus with Stratocumulus; hold the ice

Mr. Cloud-maven person hasn’t said much about clouds lately, which is kind of ironic since he deems himself a “cloud maven” and not much more.  Rather, he has been obsessing about POSSIBLE storms in AZ 15 days away which is kind of futile anyway.

So, as an excuse to show more cloud photos from that gorgeous day of snow and cloud shadows on the Catalinas yesterday, will go into a cloud lecture, a post-mortem so to speak.   Here are some cloud shots from yesterday, most below the one at left.  Note, not one cloud shows any virga yesterday, and some of them got, at least moderately humped up.  A promiscuous cloud maven person might have called one or two of the cumulus clouds, a “Cumulus congestus” (though they would be WRONG).  Well, maybe not that wrong–see the 1987 World Meteorological Organization International Cloud Atlas that I can’t stand because they goofed up on their cloud designations as you will see if you could only find one of those yourself.  Still kind of bummed out by that atlas, but one member of that cloud selecting panel told me they were too busy in their Paris meeting going to the Eiffel Tower and such rather than paying attention to getting the cloud photos they had properly named.   Now, where was I?

Right, I was talking about yesterday’s clouds….   Well, here are some cloud shots, ones that I was going to post 15 minutes ago before getting upset again over the 1987 WMO cloud atlas.  (Really, I could have done a better job than the WMO all by myself; it was a real boondoggle, that meeting of “cloud experts”, yeah right.)   OK, photos!

Now looking at ALL of these, you see no fibrous material falling out, even though some of the clouds look pretty dark in these perty scenes.   I was so happy to be alive and live here yesterday, feeling very, very lucky.  So, remembering the University that Bullwinkle Moose went to play football as the “Frostbite Flash”, “Whatsamatta U.”, we might say the same thing to these clouds, “Whatsamatta U?”   How’s come there no precip falling out, and those who read this silly site will answer immediately, “Them clouds ain’t got no ice in’em”, which would be correct.

But why?  It was awfully cold yesterday, and even Mr. Cloud-maven person, who does not even have the Master’s Degree, was wondering.  So, off to the TUS “99 Luftballoons” sounding data for yesterday afternoon, posted by our great U of A Weather Department below (where the lines come together are where the clouds were located).  Didn’t seem possible to me, but those cloud tops were hardly as cold as -5 C (23 F).  Ice does not form in clouds, even though they are below freezing, at this temperature in the natural state except in very special circumstances.  Ice formation in clouds, still not WELL understood, is known to be a function of drop sizes AND temperatures.   Over the oceans where cloud drop sizes are large,  it happens.  Usually, someone can get a whole scientific paper out of a cloud that formed natural ice when the top has never been colder than -4 C!

Here in Arizona, what we would call a continental cloud forming environment.   Cloud drops “is” smaller because there are so many more particles for the drops to condense on, and so the concentration of drops is higher, meaning the drops have to be smaller to condense out the same amount of water as over the oceans where the air has fewer particles for clouds to form on.   In a nice cumulus off the Washington coast of the sizes we had here yesterday, the cloud drops would be as large as half the diameter of a human hair (“wow”, huge, he sez, 30-50 microns in diameter, for the sake of a number) here in AZ in those clouds yesterday would be lucky to have drops in them as big as 20-25 microns, too small to activate ice forming processes, known to be related to drop sizes.   Oddly, the bigger the cloud drops, the HIGHER the temperature at which ice forms, especially if drizzle drops have formed.  The drops in our clouds yesterday were too small to have an appreciable fall speed, so they don’t fall out either.

Since I have published a lot of critical work on cloud seeding, one might ask if these clouds could have been made to snow by artificial means?   Even as a long time critic, the answer is an unambiguous “yes.”   With a small plane, and a little dry ice, you could have made a little snow fall out of these clouds because the tops were cold enough for that.  Dry ice, the substance you would have used,  has a temperature of -78 C, and when pellets falling, they leave a jillion ice crystals in their path as they cool the air momentarily to -40 C and below, the spontaneous nucleation temperature.  And, with ice in these clouds, the drops would be evaporating and the water molecules depositing themselves on the ice crystals.   Ice crystals in clouds of water drops are like little low pressure centers; the water molecules leave the drops and goes to ice, and ice crystal gets big enough to fall out.  Our natural precip here is like this most of the time.

So, summing up this little cloud-ice lesson, our clouds did not get cold enough, and at the temperature the tops DID get to, the drops weren’t big enough to trigger natural freezing.  Tell your friends.

The End.

Altocumulus clouds, Cumulus clouds, Definitions, Lenticular clouds, Stratocumulus clouds

“Oh so pretty….”

You know the rest of the words to this song, the punch line,  “…pretty ugly.”  Yes, who can forget Johnny Rotten….?

Gorgeous clouds yesterday, but no rain is going to follow them (the “ugly” part)!  I had really hoped for a splotch of glaciating Stratocumulus clouds this morning after the great display of….the tongue twister, Altocumulus (Ac) perlucidus undulatus, a mid-level cloud with a honey-comb of elements (“perlucidus”), and those elements also aligned in rows (“undulatus”).  If you looked off toward Twin Peaks, you saw that the the back edge to these clouds was very smooth looking and did not advance toward us.  That was an Ac lenticularis cloud that started the whole shebang.  That lenticular cloud, as often happens, devolved into little cloudlets and rows;  its smooth lenticular form devolving into cloudlets that trailed downwind over us here in Catalina.    See pretty pictures below. One has the crescent moon in it.

 

Now it looks “pretty ugly” here for rain since there are no clouds this AM!   “Dang”, as a friend would say.

Why get excited about the chance of a sprinkle, or at least some pretty virga this morning because of the two layers of clouds yesterday afternoon?   No model indicated any rain.  First, it doesn’t happen often, but to HELL with models, they can be WRONG.  Don’t bet against them too often, though.  You will lose everything.

So, if you were just eye-balling the movement of those two cloud layers later yesterday afternoon and using the crescent moon as your fix, you saw that those mid-level Ac clouds were jetting along at a tremendous speed as they passed it.

How fast?

The NWS balloon sounding indicated that at the height of those pretty Ac clouds, about 20,000 feet above ground level, they were blowing along at no less than 80-90 mph (70-75 knots)!   This is a really strong jet for May!  And it indicated that the jet stream must be right over us, or darn close, and it was blowing from the SW.  If you no doubt know,  Buys-Ballots Law, in the northern hemisphere means that a low or trough is to the west of you, and in this case;  above you, not at ground level since you’re looking at higher clouds.

Also, the small Cumulus were beginning to cluster into Stratocumulus over the Catalina Mountains.  Getting pumped because there movement was showing more southerly now; the wind was more southerly at that level than it had been in the morning, also suggesting the influence of the trough to the west.   Here they are.  If you really want to relive yesterday’s clouds, particular in the afternoon and evening, our friends at the U of Arizona Wildcat Department of Atmospheric Meteorology have captured them

here.  For the really sharpies who DO go here, you’ll see that these cloudlets were further devolving in this time lapse to “ghosts” of their former droplet selves in the form of barely visible, icy little veils as they exit the area.  So, can you guess the temperature of those clouds?  Piece a cake:  probably -20 C or less (-4 F or less).  Sounding indicates -20 C, BTW.

Perhaps, I mused yesterday afternoon in a bout of wishful thinking,  that in the core of that trough heading for us, there’ll be a smidgeon of Pacific moisture left within its interior, enough for some thicker lower clouds than now, and those clouds will be cold enough, too, so that they will form ice inside them and we’ll see some virga (trails of snow fall out of them) or get a sprinkle (in spite of what the models were saying)!  Many of you will remember that according to Willis and Rangno (1971–Final Report to the Bureau of Reclamation by EG&G, Inc) that rain can only fall in the wintertime here when you are in the interior of a trough.  I’m sure many of you have this report, and can look it up jf you don’t remember.

Well, we should still see a few isolated Cumulus around, small ones, maybe as big as “mediocris” stage (1 km thick or so, 3300 feet).  And, with the coldest air over us this afternoon and evening, I am going to stick with an expectation of some ice in those clouds!  And you will here about it tomorrow if there is one crystal up there!

Finally, for cloud technicians, how cold will it have to get at cloud top to have ice in those small clouds around here in Arizona today?  Well, between about -10  and -15 C (14 to 5 F)–this is somewhat higher than for those mid-level clouds.  So we will check that out tomorrow, too!

 

“The end”, unless I think of something else later.

 

Footnote:  It now appears that last night’s model run of the “Beowulf Cluster” at the U of A has some precip on the Catalinas, between 3-4 PM LST today.  Interesting that after I had this thought based on a crude conceptual model, that the “Cluster” would now have that thought as well….  Hmmmm.

Cumulus clouds, Definitions, Ice crystals, Snow, Stratocumulus clouds, Sunsets, Virga

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.

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Cirrus clouds, Cumulus clouds, Definitions, Ice crystals, Stratocumulus clouds, Virga

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.

Altocumulus clouds, Definitions, Ice crystals, ICE-T science project, Stratocumulus 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.


 



Definitions, Frontal passage, Ice crystals, Stratocumulus clouds

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.

Definitions, Drizzle, Stratocumulus clouds

First “storm” in the series, a trace of rain!

While taking the dogs out for their daily jaunt over the equestrian trails hereabouts yesterday morning, I was fortunate enough to experience several tiny drops of rain at 7:10 AM.  Rubbed a couple off the dusty car windows to be sure it was happening, it was THAT slight!   It lasted for maybe 15 minutes.  How tiny were the drops?  Oh, maybe 500 microns in diameter, practically “drizzle-sized.”  Recall this is ONLY THE BEGINNING of “stormy weather” in SE AZ, to quote a song title by Harold Arlen and Ted somebody.

What is “drizzle-sized” you ask?   Well maybe you didn’t ask,  but I am going to tell you anyway as part of a harangue about folks mis-indentifying drizzle occurrences.  It happens a LOT even with your local TEEVEE weather presenter (can’t refer to anyone who doesn’t know what drizzle is as a “meteorologist”!)

Drizzle drops are defined by official weather folk who know what they are talking about as those drops between 200 and 500 microns in diameter.  A typical human hair is 100 microns in diameter.  So, they are darn small and have so slow a fallspeed that they appear to float in the air. If you ride a bike, you will know that no baseball cap will keep drizzle drops off your glasses.   The World Meteorological Organization’s description, is “fine, close together drops.” Furthermore, unlike yesterday when those drops were incredibly sparse, drizzle occurrences are often noted by lots of tiny drops in the air, 1os per liter of air if you want a number, which I doubt.  This often causes the visibility to be reduced, but it is also close to saturation in most drizzle, sometimes the visibility is reduced further also by fog.

A common misconception today by those ignorant of what drizzle is, is to refer to a “sprinkle”  as “drizzle.”   A sprinkle is a smattering of much larger drops that fall rapidly; never appear to practically float in the air.  In the parlance of the old teletype weather reports, they used to be described in “hourly” station reports, such as at Tucson, as “RW–“, that is, “very light rainshower”, and NEVER as “drizzle”!  But for some reason, not sure why, there has been a corruption of the word “drizzle” to more often indicate “sprinkles.”  These have even crept into the official reporting records, particularly at military stations such as Davis-Monthan for some unknown reason.

Since repetition is important for learning things, I have copied the two paragraphs above and have pasted them below:

Drizzle drops are defined by official weather folk who know what they are talking about as those drops between 200 and 500 microns in diameter.  A typical human hair is 100 microns in diameter.  So, they are darn small and have so slow a fallspeed that they appear to float in the air. If you ride a bike, you will know that no baseball cap will keep drizzle drops off your glasses.   The World Meteorological Organization’s description, is “fine, close together drops.” Furthermore, unlike yesterday when those drops were incredibly sparse, drizzle occurrences are often noted by lots of tiny drops in the air, 1os per liter of air if you want a number, which I doubt.  This often causes the visibility to be reduced, but it is also close to saturation in most drizzle, sometimes the visibility is reduced further also by fog.

A common misconception today by those ignorant of what drizzle is, is to refer to a “sprinkle”  as “drizzle.”   A sprinkle is a smattering of much larger drops that fall rapidly; never appear to practically float in the air.  In the parlance of the old teletype weather reports, they used to be described in “hourly” station reports, such as at Tucson, as “RW–“, that is, “very light rainshower”, and NEVER as “drizzle”!  But for some reason, not sure why, there has been a corruption of the word “drizzle” to more often indicate “sprinkles.”  These have even crept into the official reporting records, particularly at military stations such as Davis-Monthan for some unknown reason.

Finally, I would like to insult you by quoting from the Bill Nye the Science Guy program, one sponsored by the National Science Foundation, BTW, and his science ditty, The Water Cycle Jump:   “Your brain is on vacation, if you don’t know about precipitation.”

So there.

Finally, here are the clouds that produced the drizzle as it was happening.  You are looking at the west side of the Cat Mountains and you can see that those “Stratocumulus” clouds are quite low based, necessary for drizzle drops to reach the ground.   However, I have to say that they were still a surprisingly great height above the ground for a drizzle drop to have reached me.  That drop HAD to be much larger coming out of the base to have fallen, say 3,000 feet and reach me as drizzle sized.  An alternative explanation that would go with the very sparse nature of the drops was that the clouds reached below the freezing level and a few ice crystals formed, which then grew into raindrops (bigger than 500 microns in diameter).  TUS sounding doesn’t support this, but, BUT, the bank of clouds over Catalina was NOT over TUS at the time of the sounding release, so I think this is still a viable explanation.

The end.