Whence graupel?

What a fantastically gorgeous, if uncomfortable day yesterday was!  Such skies!  Such odd temperatures for March 19th.  And another day with ice falling from the sky, mostly “graupel”, but also some snowflakes (aggregates of dendritic crystals) at one point, too, when “stratiform clouds” came by (flat, layered ones) about mid-day.  The total water equivalent, 0.08 inches, 0.75 inches for both days combined.   With a high of only 50 F, it was also the fourteenth lowest high temperature ever at Tucson in March.

It was great, too, that “sample” day yesterday of the Last Glacial Maximum, imagining what it was like thousands and thousands and thousands of years ago when humans and dinosaurs co-existed on this cold planet.  I could almost see the dinosaurs coming down out of the snowy Catalina Mountains, being chased by hunters, or vice versa.  I have to say I haven’t researched this, but I have seen some movies about it.

What is graupel, you ask?  A form of German wrestling?  “Die zwei Männer  graupeling sich auf der Strasse”?

Well, no my friend, it is what we weatherfolk call a tiny snowball that falls from a cloud.  You can also call it “soft hail”, because we call it that, as well.  You can easily mash it between your fingers because unlike hail, it has a LOT of air in it.  We had a LOT of that “graupel”-soft hail off and on yesterday, once, around 7:38:36 AM AST with a roll of thunder.

Here’s what they looked like, up close, along with a raingauge measuring stick for size (its one inch between labels).  You may have also noticed some, many at times, looked like little pieces of space debris, having a definite conical appearance.  This is called “conical” graupel.  Its quite common actually. The third shot shows, and this was somewhat miraculous, an element of conical graupel on the way down.   I was stupefied that I had gotten such a shot by accident!

On the right side of this third photo you will actually SEE a conical graupel in flight, on its way down, and how it falls, wide end first because that is the heavy end.  Also note the heavy shaft of snow/soft hail up against the Catalinas downwind.   The graupel that fell in the photo came from the back side of that Cumulonimbus cloud.  The second cloud shot shows the bottom of the cloud from which the graupel was falling.  Many of you know that I specialize in these kinds of photographs, the bottoms of clouds, hoping to have a show someday.

But, what do you see in the cloud base photo?  Not much.  The best eyes will detect that slight, slight striated look due to falling graupel.  Falls in strands reflecting the complex nature of the organization of liquid water and updrafts, wake capture in clouds.  The first precipitation falls out through the heaviest concentrations of liquid water (at well below freezing temperatures), and that’s what graupel does.   This the same as when the largest and heaviest raindrops in summer fall out from a cloud base with not much else going on.  And, like our graupel, they are spread around, sparse compared with the heavy rain that likely will soon follow.  So, graupel is the first thing that falls out of large Cumulus clouds, ones growing up to be Cumulonimbus ones.

Also you may have noticed that the graupel almost always was associated with a Cumulus clouds yesterday, localized clouds in lines with dark bases over you.  Cumulus clouds are loaded with liquid water, at least in the rising portions.  In those rising portions, a few ice crystals, or cloud drops might freeze.  Thereafter, they begin collecting drops that freeze on them when they contact the ice.

Cumulus clouds at below freezing temperatures are avoided by aircraft because this is where cloud drops can hit the airframe, freeze instantly, and weigh down the plane, as happened with our little graupel.  In Cumulus such as we had yesterday, a half an inch of icing can build up on the leading edges of airframes in just a few minutes while flying in their upper portions.  Near cloud base, the drops are too small to build up much ice.

With some of the graupel up there in those clouds, at some point early on, the freezing of drops on it as it collided with them produced a side that was slightly heavier than the rest of it.   That heavier side began falling downward, collecting more drops to make it more lopsided, conical.  You can then assume that graupel that are not conical, collected drops pretty symmetrically, something that would happen only if they were spinning on the way down.

Associated with the formation of graupel, as on this day, is a sudden burst of ice formation in the entire cloud leading to “glaciation”.  The liquid drops at below freezing temperatures are completely annihilated during this process in the turret initially spawning the graupel, and along with the remaining graupel, a dense shaft of precip drops out of the bottom. consisting of graupel and large snowflakes (aggregates of single ice crystals, sometimes hundreds of individual crystals in them).  So, on the back of this Cumulonimbus cloud raking the Catalinas, graupel, on the forward side where glaciation has taken out the liquid water, aggregates, probably huge ones.

Gads, I want to go on, but this is getting to be a little LONG!  However, here are a few more shots from that beautiful day. Some dessert after the heavy meal.

The End I think.