Hawai’i in Arizona

Yesterday, in the wake of TD Odile, it was about as Hawaiian a day in Arizona as you are ever likely to see. First, the high dewpoints, ones that replicate those in HI, mid and upper 60s (69-70 F in HNL right now), cloud base temperatures of around 60 F, and with misty, even drizzly warm rain around at times. The only thing we didn’t see was a rainbow, so common in HI they named a sports team after them.

If you thought the clouds looked especially soft-looking yesterday, I thought they were, too.  That soft look that also characterizes clouds in Hawaii and other pristine oceanic areas arises from low droplet concentrations (50-100 per cc),  characteristic of Hawaiian clouds.1   Both low updraft speeds at cloud base, and clean air result in low droplet concentrations in clouds.

The result of these factors?

The droplets in the clouds are larger than they would be forming in air with more aerosols (having “cloud condensation nuclei”, or CCN) and stronger updrafts at cloud base.  Yesterday, you could have remarked to your neighbors late yesterday morning,  as the rain and true drizzle began to fall from that Stratocumulus deck out to the SW-W, that the droplets in those clouds, “….must’ve exceeded Hocking’s threshold” of around 38 microns diameter.  Lab experiments have demonstrated that when droplets get to be that large, which isn’t that large at all, really, that they often stick together to form a larger droplet, which in turns, falls faster and bumps into more droplets, and collects them until the original droplet is the size of a drizzle (200-500 microns in diameter) or raindrop (greater than 500 microns in diameter) and can fall out the bottom of the cloud.2


6:10 AM. Light drizzle or rain due to collisions with coalescence rather than due to the ice process falls from yesterday morning’s Stratocumulus deck (fuzzy, misty stuff in the center and right; eyeball assessment).  Quite exciting to eyeball.

9:46 AM. So clean and pure looking, these clouds during a brief clearing yesterday morning. These might well have been seen off the coast of Hawai’i.

10:23 AM. One of the Hawaiian like ambiance of yesterday was both the low clouds, the humid air, and the green texture on the mountains highlighted by the occasional ray of sunlight. Fantastic scenes!



11:31 AM. Being a cloud maven, I wasn’t too surprised to see drizzly rain start to fall from our Hawaiian like Stratocumulus clouds, but I was excited!


11:41 AM. Stratocumulus clouds mass upwind of the Catalinas. Hoping for a few drops at least.



12:07 PM. An especially tropical looking scene I thought, with the very low cloud bases, the humid air; the warm rain process likely the cause of the rain on Samaniego Ridge.


6:08 PM.  As the day closed, this fabulous scene on Samaniego Ridge.
6:08 PM. As the day closed, this fabulous scene on Samaniego Ridge.  Clouds might be labeled Stratocumulus castellanus.



6:25 PM. As the air warmed in the clearings to the southwest and west of us, King Cumulonimbus arose. Expect some today around us.

Mods still coming up with cold snap at the end of the month, even with rain as the cold front goes by.  How nice would that be to finish off September?  Still have a couple of days of King Cumulonimbus around as tropical air continues to hang out in SE Arizona.  Hope trough now along Cal coast can generate a whopper here before that tropical air leaves us.  Am expecting one, anyway, in one of the next two days, probably our last chances for summer-style rain.

Speaking of Odile….

the thought that inches of rain might fall in Tucson, something we all heard about two eveings ago WAS warranted by the gigantic amounts that occurred as Odile slimed its way across extreme southeast AZ.  In modeling terms, the error in its track was pretty slight, but the predicted amounts that we COULD have gotten were pretty darn accurate.  I did not see these amounts until after writing to you yesterday.  Note those several four inch plus values around Bisbee, and the one in the lee of the Chiricahuas.  That one 4.45 inches over there suggests to me that the Chiricahuas like got 4-6 inches.  Check’em out:

24 h rainfall ending at 7 AM AST yesterday for SE AZ (courtesy of the U of AZ rainlog.org site).
24 h rainfall ending at 7 AM AST yesterday for SE AZ (courtesy of the U of AZ rainlog.org site).

The End

1Except those affected by Kilauea’s “VOG” which have much higher concentrations, and look a little “dirty.”

2A thousand microns is a millimeter, in case you’ve forgotten, and that’s only about 0.04 inches in diameter.  Most raindrops are in the 1000 to 3000 micron diameter range, though the largest, measured in Brazil, the Marshall and Hawaiian Islands, can be about a centimeter in diameter.

By Art Rangno

Retiree from a group specializing in airborne measurements of clouds and aerosols at the University of Washington (Cloud and Aerosol Research Group). The projects in which I participated were in many countries; from the Arctic to Brazil, from the Marshall Islands to South Africa.