Category Archives: Hoar frost ice crystals

Morning smog bank invades Catalina, smokes up clouds real good

I wonder if you noticed the blackish smog layer to the south and southwest of Catalina yesterday?  Usually it stays down that way, flowing peacefully toward the northwest from Tucson across Marana and Avra Valley, an area where a close meteorologist friend and his wife just bought a house even though they knew this happens in winter and not one in Catalina where we normally escape this characteristic Tucson smog plume. They must like winter smog overhead, but then as the sun heats the ground, it comes down to you. Go figure.

Here is yesterday’s Tucson smog plume exiting Tucson:

8:47 AM.  Smog plume exiting Tucson, moving left to right over Twin Peaks area.
8:47 AM. Smog plume exiting Tucson, moving left to right over Twin Peaks area.  This was one of the densest, most awful ones I’ve seen from Catalina.

But then, in the later morning hours, a southerly wind brought that smog bank to our normally clear air oasis of Catalina, infecting the shallow Cu fractus clouds that formed as the sun heated the ground.  This was a real disappointment since probably most of us were expecting the kind of pristine view of the Catalinas yesterday morning.

10:09 AM.  Smoke-filled Cumulus fractus clouds form along the Catalinas as the air begins to warm.
10:09 AM. Smoke-filled Cumulus fractus clouds.  The smog looks white here instead of dark because of “forward scattering”; the white light of the sun is being scattered in the viewer’s direction by the smoke particles.  (In the first photo, there was no forward scattering and so you can see the actual dark hydrocarbony smoke particles for what they are, dark and sooty.

Fortunately the smog was dispersed as the day wore on.  As the layer in which it is contained gets deeper, and without more smog being added to it, the amount of smog, say, per cubic mile diminishes and pretty soon it gets so thin you can’t detect it with your eyes.  Still,  exactly the same amount might be in the column of air between you and the higher cloud bottoms.   Here’s what it looked like in the later afternoon:

4:29 PM.   That's better!  Cumulus humilis dot skies.
4:29 PM. That’s better! Cumulus humilis dot skies.

BTW, while its easy to see that the Cumulus fractus clouds in the second photo are very low, in the 3rd photo above  it’s much harder to detect how high these small Cumulus are. The TUS sounding indicated that they topped out at 9,000 feet, or only about the same height as Ms. Mt. Lemmon! Top temperatures in these smoke-filled clouds were no colder than about -8 C (about 20 F), too warm for ice to form in them, especially when the cloud droplets are reduced in size by smog.   The larger the cloud droplets, the higher the temperature at which ice begins to form in them, and so smog generally reduces the chance of rain in shallower clouds.

This is why oceanic clouds in pristine regions lacking smog, even shallow ones,  rain or drizzle so easily.  The cloud droplets are much larger in those clouds right from the get go than those in smoggy regions.   So oceanic clouds can rain either because those larger cloud drops reach sizes where they can collide and stick together, forming larger drops that can fall out (“warm rain process”) or form ice at the highest temperatures known for ice formation, -4  to -5 C (23-25 F).  Usually both processes are work in those ocean clouds that rain so efficiently.  They’re pretty great,  really, such little clouds that rain.

Vacation in Hawaii if you’d like to see some up close (though not downwind of the Kilauea volcano plume and in the lee of the Big Island of Hawaii since that volcanic plume can smoke up the clouds real bad there and they stop being so darn efficient as rain producers.  Recall that the biggest drop in the world was measured in clouds in Hawaii (1 cm in diameter, Beard, private communication,  received AFTER Peter Hobbs and me got the Guinness record for the biggest drop ever measured, 8.6 mm in diameter–got a lotta publicity around the world, too, calls came from everywhere!).

You see, Beard didn’t publish anything about HIS BIG DROP; we published ours in a refereed journal. “Neeny, neeny, neeny”, I think is what you conclude here.  Immaturity:  sometimes I think its not valued enough in life.

That’s what its like in academia; you publish or die!  Die that slow death as an “Assistant Associate” professor of something, never reaching the exalted “Professor” status.

The “combo” ice seen yesterday morning

We had two forms of ice yesterday morning that you may have noticed, say, on your car if it was parked outside overnight.  There were originally rain drops left from the storm that froze in place during the cold night (was 30 F here yesterday morning), and then the deposited ice from water vapor on top of the drops.

The deposition process, as we call it, leads to hoar frost ice crystals growing in time as the molecules of water vapor add to it during the night.  This combo ice led to an unusual site on the car before the sun did away with it.  Here are a couple of shots of this unusual sight:

9:57 AM.  "Strange brew."
9:57 AM. “Strange brew.”


9:58 AM.
9:58 AM.

The weather ahead

After the “sunny malaise” for 5-6 days, with Arizonans statewide out doing things, its back to the Bowl, the trough bowl.  The period we’re in now might be called, “a sucker ridge”, a high pressure ridge that is.  You might well think, “Well, that’s it for winter in Arizona!” after a few days of the “sunny malaise”, but you’d be WRONG.  I can’t emphasize the word, “wrong” enough.  The Bowl comes back with a vengeance, too, when it reforms here in the Southwest;  there will be one storm and cold blast after another.  If you’re a snowbird, you might start to cry, and wonder why you didn’t go to Costa Rica for the winter.

Well, I am looking forward to storms and seeing more scenes of white mountains deep in snow, and green vegetation shooting skyward.  That’s the promise of the “Bowl” ahead, where storms collect,  in the weeks ahead right into March.

Taking a few days off now, likely without pay, to replenish mind, get out and do things like the rest of Arizonans will.   Will give you time to ruminate on all that’s been said here over the past year or so, correct and incorrect, mature and immature…

The End.