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

Tuesday, February 1, 2011

Frazil ice: truly cool!!

A layer of granular frazil ice floats on water in Yosemite
Photo: National Park Service
Geology.com has a really interesting article today about Frazil ice.  The National Park Service video that you can access from this site has an excellent description of this form of ice. Since we're in the midst of two days of ice storms here in central Illinois, it's appropriate to feature ice!

Water normally freezes at 273.15 K (32 F), but can be supercooled down to almost 231 K if there are no nuclei for the ice crystals (that is, the water needs to be very pure). Frazil ice forms in turbulent, slightly supercooled water. It consists of small discs of ice 1-4 millimeters in diameter and 1-100 microns in thickness.  It is estimated that sometimes there can be one million ice crystals in a cubic meter of water. As the crystals grow, they will stick to objects in the water and tend to accumulate on the upstream side of objects.  This can cause ice dams and serious flooding.  In the NPS video mentioned above, you can see Yosemite Creek change directions in response to the movement of the frazil ice.

 Frazil ice in rivers can be a serious problem if there are hydroelectric facilities because it can block turbine intakes, or can freeze open gates. It's also hard on the fish! In the ocean, frazil ice forms around the edges and within open water within ice packs.  Here it has become of concern because of oil and gas development in the Arctic. A review article on this by Sellye Martin can be found in Annual Reviews of Fluid Mechanics, v. 13, pp. 379-397, 1981.) Other posts on ice on this blog are Boiling water turns to snow, Freezing rain in southeast U.S., Comet Hartley 2 and a cosmic snow storm, ice stalactites, and Hail, hail.

As of today, this site has had visitors from 66 countries!! Many of you live in places that are a lot warmer than here, so I thought that I'd post a couple of pictures below of our winter weather!  "Trees plus fire hydrant" was taken about a week ago.  Yesterday and today we've had a storm of ice pellets. The world is covered with about a half-inch of them! Since we also had freezing rain, the pellets have solidified into a solid mass that I can easily walk on top of. The photo shows these little spheres in a sheltered place on my front porch. This form of winter precipitation forms when there's a layer of air above-freezing point between one and two miles high in the atmosphere, and sub-freezing air both above and below it. Snowflakes formed above this warm layer melt when they fall into it, and then refreeze as ice pellets when they hit the subfreezing layer below it.  If this lower warm layer is too small, the droplets may not completely refreeze and the precipitation then falls as freezing rain.  Nasty stuff!

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