Category Archives: Aerosols

Another big Cirrus streamer from the Equator heading our way!

Well, when there’s no rain in the model predictions for 15 days, you have to get excited about something….

Like a solar flare, there has been another massive ejection of high clouds from the equatorial region and its heading toward Catalina, AZ.   Here, from the Washington Huskies Weather Department1,  is a 24 h loop of the event.  Hope our cell phones still work.  Here’s the latest still image:

Satellite image for 3:30 AM AST supplemented with various interesting annotations, some of which are correct.


What are the ramifications of ejected Cirrus coming all the way from the Equator to Catalina?  Pretty skies, sunsets and sunrises, which is quite important to us humans.  Also, when it starts arriving today, we’ll have milder nighttime temperatures.  Yes, even Cirrus clouds cut down the outgoing longwave radiation leaving the earth’s surface at night, and of course, moderates the incoming visible (shortwave) radiation (sometimes called “sunlight”).  We don’t want to dumb this down too much.

After 9-11,  when all the aircraft stopped flying for a week some guys at a small university, one so small I don’t think it even had a football team,  found that the daytime and nightime temperatures were affected by the lack of contrails.  Daytime temperatures were a spec higher and nighttime temperatures a tiny bit lower, suggesting that even CONTRAILS have an effect on the weather and climate.  It was an important finding.  Of course, without a football team I am clueless, as are you are,  concerning what university those findings came from.

You know what gets a lot of us scientists about that contrail study after 9-11, is that something simple and important was done that I (we) could have done had only we thought of it.  We’re kind of bitter about it.  Might have got a raise, too, got the name out there.   Citation index fluffed up some.  We’re dealing with a lot of loss here.  Heck, you probably could have done this, too, it was that easy.

The study of contrails is a pretty big topic these days, though the effects are deemed small for the present.  Here’s a short article for you.  Here’s an unrelated one, one about smog’s effects on clouds, but one you should read, anyway.  Might be true.  Reading the second one is like doing an extra pushup.  Its good for you.  And me since one of the authors of the second article (Danny Rosenfeld) criticized me (and Pete Hobbs) royally in print in the late 90s only because we said his work was invalid.  Show’s I’m magnanimous, following the ideals of science meaning that as scientists we have no personal feelings about our detractors.

Yeah right.  Check the climate blogs and those ones who refuse to allow other scientists to even comment on their work!  Its a hideous situation out there now, far from the ideals of science where one WELCOMES criticism.  But, I diverge….getting worked up when I should be concentrating on clouds.

BTW, that little blob of clouds north of the ice cloud mass coming at us, is due to a little disturbance that will hit the coast of Cal in a few days.  With it, the clouds here will get pretty thick, probably as will happen later today or tomorrow with the ice clouds, causing the optical depth to exceed 4.00000000000 (4).

What does an optical depth of four mean?

That means that the sun’s position is not discernable.  (Also, can’t be a Cirrus cloud, BTW, but rather Altostratus if its an all ice cloud).  Optical depth is usually something used by the smog folks.  A really clean sky has an  optical depth of 0.05 or even less.  Smog laden skies, such is the coastal areas of southern California, or back East on humid days in the summer, have optical depths of 0.2-0.5 at times, horizontal visibility might only be a mile or three; the leaves are gray and the sky is brown, as the song says.    Aren’t we happy we don’t have that kind of smog?

Looking way out, just now, I saw this in the ensemble of spaghetti, thought you should see it, too.























While no weather beyond warm breezes and high clouds is portended here, where would you really like to be in the West in two weeks or so for some really heavy rains?  Can you tell?   What’s a place I mention too many times when comes to Cal rains?  Yes, the King Range around Shelter Cove, between Frisco and Eureka.  This plot gives high confidence to major flooding in northern California in the 10-15 day window.   Why?  Because so many of the blue lines (564 dm height contours) dip down toward the tropics in the eastern Pacific in support of the actual forecast from last night (represented by the yellow lines). Remember that the blue lines result from small errors put into the model runs at the beginning to see how robust a forecast is.  The wilder the spread of the lines, the less reliable a forecast is.  The more they group together, the more robust, more reliable a forecast is. They look fairly bunched up in the eastern Pacific, and this is the reason for having this plot here today.   I suspect we’ll be reading about heavy rains in Cal during that 10-15 day window.  It will be fun to see if we can make such a call so far in advance!

The End.






1Nobody knows your university by its scientific accomplishments, but only by its athletic accomplishments.  Its been written up. I certainly wouldn’t.  If online universities could have football teams, it might be the end of “brick and mortar” universities.

July 2012: 15 days with measurable rain (with three traces); adds up to 4.45 inches!

And we were one of the drier areas!  Many places around us were between 5-10 inches this July making it a pretty memorable one and the 6th wettest July in Catalina since 1977.  Here’s the list of those wettest for Catalina:

1.  July 2006    7.22 inches   11 days with measurable rain

2.  July 1984    6.54 inches   17 days with measurable rain

3.  July 1990    5.25 inches   12 days with measurable rain

4.  July 1983    4.94 inches     9 days with measurable rain

5.  July 20081   4.73 inches   12 days with measurable rain

6.  July 2012    4.45 inches   15 days with measurable rain

1The record begins in 1977 down at Our Garden garden on Stallion Ave and in July 2008, is a mix of their records for the first half of the month, and here on East Wilds Road for the second half, and from East Wilds Road thereafter.

Here is the NOAA 30-day radar-derived rainfall map, ending July 31st, for Arizona to provide an overall picture of how well our State did this July.  Also shown is the percentage of normal map.  You can find these maps here.

De-briefing for yesterday

A disappointment for sure, since the day before, the models had indicated a pulse of rain moving through in the afternoon.  But then they changed by yesterday morning.  The rain pulse was gone.  That’s what weather computer models do;  they say they like you and then they say they don’t, in a sense.

However,  if one looked far enough to the south in the evening, there was a Cumulonimbus way off in Mexico, maybe a 100 miles or so,  that was exactly headed this way. It lasted about 20 minutes and never crossed the border.  Oh, well.  Rain hope springs eternal.

7:14 PM  Down Mexico way, a Cb!

Closer ones suddenly popped up to the NE and ENE, quite pretty, but they weren’t moving this way.

7:12 PM.  A closer Cb but may as well have been in New Mexico because it wasn’t moving this way.

There was too much haze yesterday, too.

7:03 PM.  “Deliquesced” (fattened up by humidity) smog aerosols show up as the cloud-free whitish areas at cloud base.  Really looks awful from an aircraft flying at that level since you’re looking at that layer on its “side.”

 The Weather Ahead

Seem to have entered a break in the showers now.  U of AZ mod (here), crunching data from 11 PM last night, doesn’t think the rain will be back until the 3rd.

We can hope its WRONG again!

Dusty sunset

In case you missed it, due to all the dust, yesterday’s sunset:

Moreover, if you go to the NWS Tucson site and the forecast for Catalinaland, you will see icons showing pretty much the same thing for today and tomorrow as is in these photos. It will be interesting to see how deep the dust gets.

Where did all that dust come from, that plume of dust that moved into Tucson and environs during the mid-afternoon?

The Mexican Sonoran Desert NE of the Gulf of Baja.

You can see rivulets of dust being raised in this visible satelllite imagery loop (this will take a LOT of band width!) if you look hard at the desert regions southwest of the AZ border.  Also check out the U of A’s loop here, but that one will soon be overwritten, so good luck, and there’s nothing I can do about it.

Below, a still from that loop for 6:30 PM AST, near the time of the photos, with the dust origin region annotated with a red circle, something I have only recently learned to to with Apple’s Preview photo viewing software.  Dang.  The plumes are oriented SW-NE.  So, a few tons of Mexican desert came across the border yesterday to make AZ that bit higher in elevation.

Also in this sat image is the horrible fire near Silver City, NM, northeast of the red circle, the one that grew so much yesterday in the wind and heat.  You can see that the smoke plume was already reaching as far as Texas by the time of this image, 6:30 PM AST!

BTW, smoke particles are, in general, much smaller than dust particles are (hundredths of microns vs. a few microns) and the color of the setting sun can be used to help tell what you are looking at as far as aerosol particles go.  Smoky sunsets tend toward orange and red; dusty ones toward yellow, as above.

The End.

Cirrus altocumulus castellano-floccogenitus

We had a rare form of Cirrus yesterday, whose name I have made up in the title as a hint of where they came from, due to the very high altitude and low temperatures of some Altocumulus yesterday.   Those Ac morphed to Cirrus, hence the strange, unpronounceable  title.

Reminder,  weatherscience mavens, its more proper to say “low” temperatures; not “COLD” temperatures, FYI, though you constantly hear it.  (“Things”, like coffee, air, chairs in the sun, etc., are hot, warm, cool,  tepid, and cold; temperature is not a physical thing, and is high. moderate, or low, etc.))

Still bristling over some unexpected clouds yesterday, so I wanted to complain about something minor, bring some discipline to the field.

Mr. Cloud-maven person was not paying attention, asleep at the wheel, etc., when some Altocumulus castellanus and Cirrus castellanus came a truckin’ over the horizon and floated over Catalina after dawn yesterday, but had not been mentioned in this blog in advance.   I am sure, since they had not mentioned  from this keyboard, you may have been in some distress yesterday when they showed up and you weren’t sure what was happening.  My apologies.  It will almost never happen again.

Here are some photos of the interesting clouds that passed overhead yesterday.  I was quite excited to see them partly because I had not prepared myself mentally for them.  Now, there is something strange in the first caption.   But I wrote it that way on purpose because I REALLY want to know if YOU know WHERE the HELL you are, and where the mountains are around here.  Next, after that outrage,  some interesting banded Cirrus. Then a hint at where those Cirrus came from in the background of the 3rd shot.

First, this sunrise over the Tortolita Mountains with Cirrostratus nebulosus (vellum-like cloud) and a hint of Cirrocumulus (tiny, brighter, flocculent specs).
This banded Cirrus gave some hint as to its origin. Might be termed, Cirrus uncinus, or floccus, or fibratus, its a pretty complicated set.


Caption function not working now for this third shot in WP, so here it is:
3) A nice example of Cirrus uncinus in the foreground, tufted or hooked ice clouds trailing tiny ice crystals.  In the background, a clue to the origin of the patchy, banded Cirrus.
4) Another shot of the approaching Altocumulus castellanus (Ac cas) and (Ac floc) floccus clouds as they arrived overhead, some of which have morphed completely into ice (Cirrus) clouds, such as that larger element over the house in the foreground!  In the upper left quadrant of this shot are Ac clouds that, to this eyeball, are still liquid.
Droplet clouds have more sharply defined edges because droplet clouds have MUCH higher concentrations of particles in them than ice crystal clouds (which tend to make them “fuzzy”, ghost-like, striated, fibrous, etc.
Why this visual difference, which I want you to learn, to see for yourself and impress your friends?
There are more cloud droplet condensation nuclei than there are ice crystal nuclei.   For example, liquid Altocumulus clouds might have 100,000 to 500,ooo drops per liter in them, while ice crystal clouds may have only tens to a few thousand per liter  (and then only in newly formed elements) of ice crystals.  In general, there are more cloud condensation nuclei than ice nuclei, too.


While “Joe” is spinning up into his little hurricane-like self in some kind of weather tantrum off the California coast today before heading to Oregon, our skies over Catalina will be marked by various forms of Cirrus clouds, ice clouds well above 25,000 feet above the ground, and not much else.  BTW, you can follow Joe’s progress here from the U of WA, if interested.

If you’re interested, instead, of following our Cirrus clouds as they approach and go overhead today, go here, also from the U of WA.  You see the Cirrus clouds pealing off the main frontal band in the Pac NW and then fading as they head this way.  (I would increase the speed of the loop for maxium excitement.)

The End.


What’s up? Smoke, most likely

You may have noticed, if you’re from around here, that we did not exactly have an “Arizona Highways sky” yesterday.  The sky was an awful whitish color tinged with blue (I’m fussy here).  Below are examples of smog from the ground and from satellite.  from yesterday afternoon.  (Where there are clouds, BTW, the satellite can’t tell if there is smog under them.)

As you can see, the sky is not good over Catalinaland in this mid-afternoon shot from yesterday.  And, it was not aloft in a layer as we have sometimes seen, but is partially obscuring the hills and mountains in the distance.  It is also what we would call a well-mixed smog, which indicates its not generated locally.  In that case of a very local source, there would be obvious thick and thin regions.  Yesterday, it was homogenous, pretty much the same looking smog in every direction.  (Tends to look worse toward the sun.)

In the aerosol satellite image, you can see we are in a blob of smog as indicated by the light turquoise area over Arizona and extending into northern Mexico.   BTW2, dark blue in this graphic is a good sky, while a red one is awful.  Our smog level here, as indicated by a parameter we call, “aerosol optical depth” (AOD),  was around o.3 or so according to the satellite image below (probably on the high side).  AOD measurement doesn’t know the height of the smog;  its just how much is in the air between the top of the atmosphere and the ground.

For comparison, if the sun is NOT visible due to smog, the AOD would be about 4.0.  Yikes!  A really clean sky has an AOD of less than o.o5; its a really blue one and on such a day, from here you would be able to see Mt. Humphreys near Flagstaff if it went up to 40,000 feet or so, it is THAT clean.  (You can actually see the tops of Cumulonimbus clouds near Flagstaff in the summertime from here in Catalina on these kinds of days with no intervening clouds.)

So where is it all coming from, I’d like to know?

To get an idea, we go to our trusty NOAA Air Resources Lab and their “HYSPLIT” model and run back trajectories for the air at the levels we estimate the smog is at.  Of course, you can do this yourself; in fact, I have to say that it seems like I have to do everything for you.

Anyway, below are a couple of plots for various heights above the ground from ARL, 500 m (1500 feet), 2000 m (6600 feet), and 3000 m (10,ooo feet) for the past 4 days before the air arrived here.

You can forget about the one that starts at 3000 m (green line) above ground level in the Pacific.   These plots also estimate the amount of lifting and descent the air went through, and you can see that the  green parcel was caught up into a low center, got spun around in a circle, thrown upward to above 20,000 feet (7 km) and then descended on the back (west side) of the low.  In going upward so much, clouds and precip forming in the upward moving air would have removed all untoward aerosols, so we  can pretty much rule out something from Asia having crossed the Pacific, as happens from time to time.

Looking at those lower level trajectories (500 and 2000 m above ground level, blue and red lines in the plot) the likely culprit is found.   Those ones represent back trajectories in much slower moving air that arrived over us, air that apparently spent a day or two in northern Mexico before getting here.  That the of smog over us extended into northern Mexico and you start to think that the smoke probably was captured there and then eased northward across the border. This seems to be hinted at in the cloud wind field available through the University of Wisconsin and the US Navy’s Research Lab here.  There are a couple of “wind barbs” (yellow colored ones) in northern Mexico that suggest southerly and southeasterly winds at levels where the smog was.  Not an airtight case, but it probably drifted up from Mexico.

Enough about smog!

Yesterday’s clouds, dust, and smoke; virga ahead

It was zero visibility in Parhrump, Nevada, yesterday afternoon with wind gusts to 85 mph, as the cold front was about to crash on by.   I guess we were lucky to only have 40-50 mph puffs of wind here in Catalinaland overnight, and not so much dust (yet).  A sharp, but dry cold front is bearing down on us, but the low center that was so intense yesterday over Nevada, then moved across Utah, has faded trying to move through the Rockies.  This means that the winds will be much less than yesterday.

Does that mean no dust around Catalina today?  Nope.  Those strong winds in Nevada and western Arizona yesterday raised a lot of fine (as in tiny) dust particles that are likely to be suspended for a day or two, and so we’ll likely see dusty skies today, without so much wind anyway.

We had some nice Altocumulus/Cirrocumulus lenticular clouds in the afternoon yesterday.  I wonder if you saw them?   They weren’t around for long.  Here’s what they looked like.

The last shot is of a cigar-shaped flying saucer with multicolored lights, OR, a Cirrocumulus lenticularis showing some slight iridescence (which are those rainbow colors near the edges of the cloud due to diffraction).    You have to look very hard to detect coloration in this cloud shot, but its there.  Diffraction is the bending of sun’s white light as it passes around the tiny (micron-sized) drops in the cloud and that leads to a separation of the white light into its components of reddish, greenish, blueish colors.

Take yer choice on what was photographed, but it is true that clouds such as lenticulars have been reported as flying saucers that “hover” then disappear.  This cloud was completely gone in one to two minutes after this photo AND was stationary, as lenticulars are in the face of strong winds aloft and at the ground, which also influences the observer’s reports of unexplained “hovering.”  This little cloud had been much larger ten minutes before reaching this size.

You can probably understand why such reporting might happen when you look at how smooth this little lenticular was.  And sometimes, when nearer the sun’s position, the colors caused by diffraction are quite vivid. 

Wildfire smoke

Also, in the late afternoon some smoke from the Nogales wildfire headed our way. It gave a great example of what young smoke looks like, that is, smoke recently emitted from a fire nearby. Its always got lots of gradations of the smoke in it because it hasn’t been around long enough for turbulence to mix out the smoke into a homogenous layer.   This happens when smoke has been around for days and days and has traveled thousands of miles, and so its one way of telling that a smoke layer has come from a long distance.

Sometimes high, smooth, long-range transport smoke layers can be mistaken for Cirrostratus, hold yer hat, “nebulosus”, a completely smooth ice cloud without much internal detail.  Below, the smoke from the Nogales area as it headed northward toward Catalina.

Rain possible?

It seems as dry as this system is, about all we can hope for is a trace.

It does appear that there will be enough moisture by tonight and for a couple of days as this cold air over us hangs around that we will see high-based (that is, probably based at or above the top of Mt. Lemmon) Cumulus and Stratocumulus clouds, and with the low temperatures aloft, ice should be able to form in them–which as you know, means virga, snow falling out and melting on the way down.

However, it would appear that only sprinkles are possible at ground level here in Catalina.

What to do?

You won’t want to miss entering the fact that a sprinkle occurred in your weather journal, one that might only last a minute or two, and so its best if you keep, say, your car parked outside where a layer of dust can accumulate, and then, when the rain drops fall, they will leave impressions in the dust.

The full moon of last evening, FYI.

Factoid:  it is thought that the moon was originally part of the earth, the result of a gigantic (!!!) impact that sent part of the earth out into space which then became our moon.  This theory would explain the synchronization of the moon’s face with the earth, that is, having the same face toward the earth.  Hmmm.  Hope we don’t have another one of those soon.  Two moons would be mind boggling.

Nice temperatures; bad sky

That was an awful sky for Arizona yesterday.  Thank heavens those kinds of days are rare here.   Here are a few more shots of that rare smoky layer day yesterday, including one at sunset where you can see some undulations in the layer.  You may have noticed that as the day wore on, the smoke appeared to be getting lower, and starting to reduce the visibility toward our mountains.  At the beginning of the day, the layer was well above any mountains around here; there was no apparent haze whatever below that layer.  There is a tendency for layers of smoke to lower with time, much as middle and high clouds do before storms, and that was part of it, as well as a tendency to mix downward as the day warms up.








Of course, if you’re from back East or from southern California, you’re wondering, “What’s the problem?” You see much more haze and smoke (smog) than this all the time. Heck, back East in the summertime, its so blasted hazy in the humid air before a cold front cleans it out, that you can barely detect that the sky is blue!

Here are examples of what smog looks like, first, over Virginia from on top (yuk), and second, on the ground at Chincoteague Island (also, yuk).

So what’s the problem?

We don’t like you’re kind of sky here in Arizona, even when its not that bad.











But heavy aerosol concentrations indicated in these last two photos help cool the earth off.  Here is more from NASA’s Earth Lab about that if you’re interested.  Would you like a cooler earth in which you can’t see the sky and mountains, or a warmer one with a blue sky? Well, a warmer one with blue skies is why I moved to Arizona!

Again, back trajectories for air arriving at 4 km and 6 km above ground level over Tucson (about 13,000 and 20,000 feet above us) suggest that the smoke is coming across the Pacific from Asia.  Here is another plot ending at 5 PM AST when the local photos were taken.

We can hope it will be gone today as the trajectories of the air coming over us begin to change.

The End.

Smoke layer aloft today

If you got up today, or even late yesterday afternoon, and got a feeling that the sky was “not right”, you were right!  We have a well-mixed smoke layer above us, “well-mixed” meaning you don’t see any gradations in it.  That in turn, means its been up there for a LONG time, and the gradations have been mixed away by turbulence. (If it was from a fire even as close as California, you would probably notice thicker and thinner portions of the smoke.)

Where did the smoke layer over us come from?

Probably from northern China or Russia.  Haze can be lofted to high levels and then can race across the Pacific to the West Coast and beyond in the jet stream.  Sometimes its even dust, but that’s a little more unusual than smog, that is, smoke and haze from regular pollution from urban areas and fires.

Below are backtrajectories for the air over Tucson ending at 5 PM AST yesterday afternoon just as the smoky air was arriving over us.   That was the latest global dataset available for this calculation from NOAA’s Air Resources Lab.   Estimating that the smoke layer is at 16,000 feet or higher, backtrajectories for three levels above the ground are shown, ones between for 16,ooo, 22,000 and 30,000 feet above the ground.  The calculation goes back four days (96 h).  The star is where the air trajectory ends, over us. (Note:  you can make these yourselves, BTW, at the ARL site using their Hysplit model.)

You can see that the air over us yesterday afternoon, and certainly today as well, came from Asia.  The model thinks the haze layer was already at a high level (look a the height of the air four days ago in the lower elongated graph) while exiting Asia.   So, this smoke layer is no doubt from even farther west east Asia

BTW, if you were on a plane departing or arriving in Tucson yesterday or on one today, you would doubtless pass through this smoky layer and see it as a thin  black or dark brown line.  Maybe you could ask a stewardess to ask the pilot or pilotess or his/her many helpers up front to tell you the level at which he passes through the thin dark line.  I would, certainly.  But usually nothing happens….  Oh, well.  Below some “just in shots” of the not-so-blue-sky.  The whitishness of the sky is caused by “forward scattering” of the sun’s light off the tiny aerosol smog particles.  “Back scattering” from these particles,  when you look at the sky opposite the sun, is pretty nil with smog particles, and so the sky doesn’t look quite as whitish as toward the sun, but its not the intense blue we should have.

Yes, we are a global community of smog producers, and here is one example.

 One caveat.  It is possible that it is dust from Asia rather than smoke.  It is a little difficult to tell for sure from the ground, but right now it looks like smoke to me.

The End.

Dark clouds but no rain yesterday. What happened?

Quick answer:  1) drops too small to coalesce and form ones bigger ones ; 2) no ice in ’em, for the most part.

Read below if you want a LONG discussion about yesterday; dull photos way below

Let’s talk about it, though probably more than you want to.  You’re probably a little down because it didn’t rain yesterday, hour and hour though it looked like it should, except for a couple of “sprinkles-its-not-drizzle” drops.    You probably had to use your headlights in the middle of the day like people in Seattle do.    It began to clear up some, gradually, in the afternoon. Here are a few of scenes below, beginning with the morning overcast, with the last two shots between 4 and 5 PM AST as the clearing was underway.

So, what kind of clouds are they?  Well, Stratocumulus in the first shot, in the second shot a higher layer of Altocumulus or water-topped Altostratus1 is underlain by Stratocumulus and Cumulus clouds  (when the bases are more isolated, we call them “Cumulus”;  when they are more connected together, we hedge the name toward “Stratocumulus”.)

The third shot, showing Stratocumulus looks particularly ominous, probably the darkest part of the daytime was here around noon AST.  A shot toward the mountains next shows the underlying Stratocumulus and Cumlus below the higher layer of Altocumulus/Altostratus.  These clouds can’t be “nimbo” this or “nimbo” that because there is no rain to speak of falling from them.  (“Nimbus” means rain in Latin.)  Note the good visibility under all of the clouds; no precip there.

Finally, with the breakup of the overcast, shown in the last two shots, we can get an idea of the thickness of the lower clouds, at least at that point, about 2,000 to 3,000 feet at most.  Also in those last shots you will notice that the higher layer has moved away or dissipated, and with a bit more heating, the clouds are tending more toward Cumulus rather than Stratocumulus.

The higher layer, located at Altocumulus level, about 12,000 feet above ground level, was actually the cloud layer producing the sprinkles, and was a key player in how dark it was; two layers, naturally, stacked on top of each other, will make it darker looking than just one, especially, in this case, when they are both pretty shallow.   And with a top at about -18 C, you can almost be assured that the top was composed of mostly droplets, not ice crystals.   A droplet cloud reflects much more sunlight back into space than ice crystal clouds like Cirrus and Cirrostratus.

Anyone still reading?  I’m doing my best here…

That last photo demonstrates that in spite of having a little rain overnight, and even during the day, there was a lot of haze/smog in the air.  It wasn’t washed out by rain.  And, the more clouds get bunged up with aerosol particles on which drops can form on,   the higher the concentrations of cloud droplets are in them, and the smaller they are as a result.   Smaller drops cause more sunlight to be reflected back into space, and when that happens, the bases look darker.  In Seattle, in our airborne studies, it was usually the case to have darker based clouds downwind of the city, and light gray clouds near the coast and offshore, even when both cloud layers were about the same depth.  However, there are natural sources, like volcanoes that can also affect clouds this way.   For example, “VOG” (volcanic smog) in Hawaii darkens clouds there because VOG has particles that can form drops in clouds.  I seen it myself and I know a dark, polluted cloud when I see one!

What happens when you get smaller drops in clouds, as smog produces in them?  It makes it harder for something to fall out the bottom in two ways.

First, in smog filled shallow clouds, drops don’t get big enough to collide and stick together to form larger drops (something that happens when they get to 30-40 microns in diameter (about a third of a human hair in width).  But, even in the event that could have happened yesterday, drops got that large, the result would have been only TRUE DRIZZLE, fine, close-together drops that go under your umbrella if there is a breeze of any kind.  Very tough on people who bicycle and wear glasses.

The more important key to not raining clouds, was that the clouds did not have, IN GENERAL, cold enough tops to form ice crystals.  The lower ones seem to have topped out around -5 to -7 C (23-20 F),  temperatures at which smoggy clouds with itty bitty drops cannot produce ice.  

The higher layer, seen in the second and fourth shots, was just cold enough, about -18 or so at top from the morning TUS sounding, to form a few ice crystals.  Also, being higher, it was probably not impacted as much by smog.

Quitting here, brain exhausted.  Hope this is somewhat comprehensible.

The End

1The smoothness of that higher layer is due to ice crystals falling out the bottom, obscuring an Altocumulus-like cloud from which they are originating.  Sometimes this has been called the “upside down” storm because the top is liquid like Altocumulus clouds where it is COLDEST, but underneath is all the ice, where the temperatures are higher.  (Man, this is getting way too complicated to comprehend!)

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.