Yesterday: a rare drizzle occurrence in the morning and later, gorgeous Cumulus-filled skies

First of all, Happy New Year to both readers! Thanks for hanging in there.

The weather ahead…

before a long diatribe about drizzle, followed by some pretty pictures with explanations:

Some picaresque Cirrus later today. Looks like next chance for rain is around the 10th-11th of Jan.

A rare drizzle occurrence

Cloud maven juniors were probably excited beyond description when they went out yesterday morning between 9 and 11 AM and intercepted a rare occurrences of brief drizzle here in Arizona falling from that low-hanging Stratocumulus overcast.  This happened after several very light RAIN (not drizzle) showers dropped another 0.02 inches, raising our storm total here in Catalina to a respectable 0.30 inches.

Drizzle is composed of drops barely large enough to cause a  disturbance in a puddle of water1, as though a large particle of dust had landed in it.  Drizzle drops nearly float in the air (should not be falling at more than about 3-4 feet a second), and in many cases of very light drizzle, the drops can float around like “desert broom” seeds. Visibility is usually lowered, things look fuzzy in the distance. Drizzle precip is so light it can’t produce even 0.01 inches except over periods of an hour or more.  Drizzle drops are also more uniform in size than the drops in rain, and are usually very close together.

While drizzle is common along the west coasts of continents in coastal Stratus and Stratocumulus clouds, its much rarer at inland locations such as here in Arizona, thus, the excitement over seeing it yesterday.

The reason why its rare in AZ?

Shallow clouds that drizzle must be what we would term, “clean” clouds; they don’t contain many cloud condensation nuclei and so droplet concentrations are low, maybe 50-200 per cubic centimeter (might not sound low, but for a cloud, it is). Clean evironments are found over the oceans and for awhile, in air coming inland along the west coasts of continents in onshore flow before it gets contaminated with natural and anthropogenic aerosols (smog).   Man, we are getting into a real learning module here!  Wonder if any readers are left?  Probably talkin’ to myself now.  Oh, well, plodding on. Nice photos below…far below.

Drizzle occurrences tell you a lot about the clouds overhead.  Not only are they low-based as is obvious (they have to be or the drizzle can’t reach the ground), but they are relatively shallow clouds no matter how dark they look.  Furthermore, and more subtle, they have larger cloud droplets (ones to small to fall out as precip) in them that must be larger than about 30 microns in diameter (smaller than half a typical human hair diameter).  When this larger size is reached in clouds, and those drops are pretty numerous, say 1000 per liter, they begin sticking together when they collide in the normal turbulence in clouds.  Those collisions with coalescence result in drops that fall much faster, bump into more drops, growing larger and larger until they fall out the bottom.

In a shallow cloud, those drops can’t get larger than drizzle drops, and that’s one of the ways you KNOW that they are shallow no matter how friggin’ dark they look.

Aside about rain not due to the ice process:
The largest drop ever measured, 1 cm in diameter, was observed in a Hawaiian Cumulus cloud that did not reach up to the freezing level!  Unfortunately, the authors of this finding did not publish their results and so did not get a Guinness record like me and Pete Hobbs did when we reported a smaller 0.86 centimeter diameter drop in Geo. Res. Lett., 2004–found them in Brazil, and again in the Marshall Islands–hit the pilot’s window like little water balloons.  Instead of being in a book with other famous people, like ones who can eat 47 hot dogs in 12 minutes, those researchers who encountered that larger drop in Hawaii sat on their finding! Unbelievable.

Strangely believe it, from lab experiments, drops bigger than 0.5 cm are not supposed to exist, but rather break up around 0.5 centimeters in diameter.  (hahahahaha, lab people). End of aside.

1Officially, 200-500 microns in diameter, equivalent to a couple or three of human hairs, maybe ONE or two horse’s tail hairs, to add a western flavor to the description.)

Yesterday’s gorgeous skies!:

Took more than 100 photos yesterday.  Was out of control, euphoric, thinking how great this earth is, maybe leaning toward thoughts of higher being and creativity therein, thus explaining the “creative” punctuation above.  Here are a few shots of those magnificent clouds and our magnificent, snow-covered Catalina Mountains.  First, those drizzle-producing clouds:

10:12 AM. Last of the drizzling Stratocumulus overcast. Patchy area of drizzle to west on the Tortolita Mountains here. The Stratocu gradually broke up after this time.


11:54 AM. Stratocumulus clouds still in charge, but lift here for a peak at the new snow on the Catalinas.
12:13 PM. First snow showers appear to the north-northwest as the stratocu deck begins evolving into Cumulus congestus and small Cumulonimbus clouds with large breaks.
12:35 PM. Snow showers from relatively shallow Cumulus race along the Catalina Mountains. These kinds of snow showers occurred right up until late afternoon.
12:37 PM. Snow and light rainshowers from shallow Cumulonimbus clouds also begin moving into Oro Valley before striking the Catalinas. Look at how similar these smaller clouds with their rain/snow shafts appear to our summer giants.
12:59 PM. Shower over the Oro Valley moves onto the Catalinas. Arrow points to a filament/strand coming out that is almost certainly composed of graupel (soft hail), something that was common yesterday from these clouds.
2:38 PM. HOWEVER, graupel often falls out of Cumulus congestus clouds on their way to being a Cumulonimbus without any sign of precip overhead, as here. This is because you are getting the result of the very first ice to form and fallout, usually those first ice particles are pretty rare in many of the shallow clouds as we had yesterday, and, because the updrafts are weak, they fall out as isolated little snowballs, too few to produce evidence of a shaft. But hang on, a shaft often, in the deeper clouds, imminent.
Also at 2:38 PM, looking northwest. A view of smaller Cumulus with the deep blue of the winter sky we love.
3:07 PM. “Congestus on the Catalinas.” You might ask, “where’s the ice?”, since yesterday all clouds reaching this size produced ice/snow/rain. Well, its on the other side (due to wind shear that carried the ice off toward the east. I think that’s the real reason why “the bear went over the mountain”, as we used to sing.

3:11 PM. Example of the medium Cumulus clouds (mediocris) that developed ice in them yesterday because it was so cold aloft, tops here colder than -12 C. (estimated).  Arrows point to ice, necessary for measurable precip here.
3:50 PM. Another modest Cumulus with plenty of ice (probably 10s per liter if you were guessing). Lowest top temperature likely lower than -15 C.

4:08 PM. I have no idea. This patch of ice cloud is left over, a “ghost” really, of a medium Cumulus cloud whose droplets evaporated. But what would it be called now? Altostratus translucidus cumulomediocristransmutatus? Cirrus spissatus cumulomediocristransmutatus? Silly, but I know of no name for such a patch of ice/virga
4:49 PM. You knew that on this cold day you would be treated to some of our finest scenes in winter, golden scenes of cloud-capped, snowy mountains, and later, those rosy under lit remaining small Cumulus and patches of Stratocumulus. What a fine day it was!

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.