This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Monday, January 14, 2013

What should I call this "snow"? Rimed? Ice pellets?

This morning when I went out to early to wheel my trash can to the curb, the temperature was about 25 F (after an afternoon/evening of freezing rain/sleet). The low sunlight was reflecting off a monolayer of brilliantly-shining "snow" on top of the garbage can. The most striking thing about this photo to me is that the pellets, which were a couple of millimeters in diameter, were rimmed with tiny crystals; I think that's what gave the layer such a sparkling brilliance. The only other hint that I can offer is that the trees were all coated with a fairly thin, but definitely noticeable, layer that we would normally associate with freezing rain rather than sleet. I've tried to figure out what to call this snow (ice pellets? rimed pellets?). I should have poked around in the pellets to see if the small crystals were on all sides, or just growing out of the top--oops, big mistake...  A quick Google search on "sleet", "ice pellets" didn't turn up any photos like this one! Viewers comments on what to call it and what it says about its history of formation are most welcome!

ADDED: Emboldened by Mr. Lockwood's comment that this looks like "graupel," I add some information about the formation of graupel. From that impecable source, Wiki:

Graupel (also called soft hail or snow pellets) refers to precipitation that forms when supercooled droplets of water are collected and freeze on a falling snowflake, forming a 2–5 mm (0.079–0.197 in) ball of rime. Strictly speaking, graupel is not the same as hail or ice pellets, although it is sometimes referred to as small hail. However, the World Meteorological Organization defines small hail as snow pellets encapsulated by ice, a precipitation halfway between graupel and hail.
The term graupel is the German word for the described meteorological phenomenon. Its METAR code is GS.

And, indeed, if you Google "graupel" and then "images", you'll find some with the overgrowths like I was seeing above. Mystery solved, thanks to Mr. Lockwood!


Lockwood said...

Try graupel- that's what it looks like to me. It's one of the most common forms of cold weather precip around here, but is often mistaken for sleet. http://en.wikipedia.org/wiki/Graupel A Google image search matches pretty well, too.

Anonymous said...

Wow, that was fast (I'm having trouble figuring out how to answer a comment, so my reply might appear twice). I should have mentioned that I did look at graupel, and I agree that it's the closest, but the photos that I found didn't have the overgrowths of small crystals on the bigger grains. So, I was wondering what they tell us!

Lockwood said...

I can think of two possibilities: first the clumps of snowflakes didn't completely collapse, so the overgrowths may represent remnant fragments of broken flakes. The second (and it seems to me, more likely) possibilities is that the pellets formed high enough that as they completed their fall, still more ice crystals, "frost," grew on them by direct vapor to solid crystallization. Not sure, but either seems possible. Here's an even more extreme example: http://www.top-wetter.de/lexikon/g/graupel.htm (Text in German.)