An acquaintance of mine recently discovered these odd discs of ice.
The discs floated freely in an eddy behind some fallen logs on the Nooksack River’s south fork. See other photos and a short video at
What we think happened is this: Foam floating about on the water, which you can see in the video, started to freeze, probably at night. Bits of frozen foam got pushed around in the eddy, and in the ensuing collisions, became roughly circular. Perhaps each disc grew radially when smaller pieces of unfrozen foam struck the disc, adhered and spread out a bit along the perimeter, and then froze in place.
The raised rims are undoubtedly due to the collisions. One sees such rims on “pancake ice”, which is also said to be due to collisions, but what about the concentric raised regions inside the discs? None of the images of pancake ice on Google seem to show these inner lines.
Here’s an idea about those lines. The air temperature oscillated during the growth of these discs: colder at night due to the clear-sky conditions and resulting radiative cooling, but warmer in the day. The discs may have grown at night, collecting new foam, then during the day, when the discs softened in the sun, softening particularly around the edges, the collisions raised up the rims. The next night, further radial growth followed by a new rim the next day at a greater radius. The pictures and video show 4-5 such rims, indicating growth over 4-5 days, which is roughly consistent with the duration of our cold snap.
During my last winter in Japan (2009-2010), I would walk around a neighborhood park on frosty mornings, looking for interesting ice forms. It was in this park that I found one rock (only one!) that on some mornings would sprout hair-like ice, arising from liquid water within (see the last images in my “Ice on the Rocks” post, Jan 27). This park also has a small, slow-flowing brook that, despite the relatively warm conditions, often freezes over. Usually, upon freezing, it shows an ice pattern consisting of many long (~ one foot) straight lines – a typical pattern you usually see on glaciated puddles and ponds. But on the morning of February 4, at one spot right before the water tumbled over a waterfall, the ice surface looked slightly different. It had lines, but the lines were short, thick, and bumpy (see photo below), looking a bit like black-blue caterpillars scattered about.
From the small bridge spanning the brook, I reached down, pulled some ice out, and was shocked at what I saw. This was not “solid” ice, but rather a bunch of very thin ice plates loosely resting on each other – somewhat like a deck of cards spread out in a fan, though in this case, each ice plate had its own size and shape. Also, unlike other thin ice plates I’d seen before, these ones resembled neither the fast-growing ice dendrites (the snow-crystal-like, branched forms) nor the slow-growing ice discs. The photo below also shows that the plates were big – the section shown being several inches across.
Over larger streams and faster-moving water, ice can develop into something called frazil ice. I’ve no experience with frazil ice, never having lived in a cold-enough region, but I this caterpillar ice may be different. For one, the water was extremely slow-flowing. I could not discern the flow, even in ice-free regions, whereas the descriptions I’ve read about frazil ice involve easily discernable flow rates, flow rates that introduce turbulence, crystal collisions, and mixing below the surface. Also, the ice crystals I saw were much larger than the millimeter-sized pieces I see reported for frazil ice. So, the caterpillar form might be a transitional form, between frazil and that over stagnant water.
As far as I could tell, nobody had predicted hail last night, yet there it was on the ground, the largest hail I’ve ever seen here in the northwest. It fell on wet ground, freezing to the surface and then growing hoar columns, making the ground white and crunchy.
The parking lot in front of our apartment was covered in ice, mostly clear (i.e., "black") ice, yet, because of the ice lumps, it was not slippery under my boots. I kept expecting to slip, but never did. On the curb, where it was a little colder, the hoar was much thicker, looking like an inch of snow.
Same story for the tops of cars, where it had been coldest. The temperature was such that the hoar was mainly columnar. Columnar hoar, indeed all hoar, grows just like snow – when an excess of water molecules deposit from the vapor – and yet columnar hoar tends to develop more in the “cup” (C1b) and “scroll” (C1i) forms (see my Feb. 14 posting for the meaning of the symbols).
But unlike the pencil-like columnar forms of falling snow, these cups and scrolls grow broader at their growing end, even branching out into a pattern a bit like coral.
And if you look closely at the above shot, you will see that the “branches” grow along the same axis as the crystal from which they sprout. (For the above shot, I put a piece of black cloth in back to show the boundaries more clearly.) Such branching contrasts with that for tabular crystals, such as the ones on the familiar six-branched snow crystal.
Why do the columns widen at their growing ends, thus making a cup-shape, why do the cup rims sometimes curl into scrolls, and why do they branch out? I suspect the reason for all three (though not a complete reason) is that the humidity next to the growing crystals is very high – higher than that surrounding the typical column form that drops from the sky. Unlike the latter case, where the humidity is too low for the prismatic faces to grow outward, at least at any rate near that of the basal, here both the basal and prismatic faces grow at comparable rates. Such growth behavior also depends on the temperature, for reasons that still elude us.
As always - click on an image for a larger view.
Hard to believe that one week ago today it was a balmy 53 degrees and warm gentle breezes were pushing the last fall leaves around on the sidewalks. The temperatures have dropped, the lake effect snow has begun, and here we are on December 6, with the first snow crystal shots of 2010/11. They aren't particularly interesting, but they hold the promise for more to come.
When I stepped out early Saturday morning, the air seemed relatively warm, particularly compared to the cold snap we had last week. Indeed, it was much warmer, and yet the parking lot in our apartment complex had a glaze that was much more dangerous than that during the cold snap.
But it was the frost that I noticed. Of course, the air was relatively warm, but the clear sky cooled the surface. Recently, snow melted, leaving plenty of open water to evaporate – perfect conditions for film frost and hoar.
I saw “striped-tail” film frost(1) on two car roofs, both times on the sunroof glass. Why only on the glass? Perhaps the glass, being a poorer conductor of heat, had a lower surface temperature. This lower temperature would have produced a thicker film of water, and the thickness of the film seems to influence the pattern. The bigger mystery though is the cause of the pattern. In a short online article(2) last year, I suggested a cause for the stripes on each tail, though I can offer no explanation for either the nearly uniform width of the tails or the meandering. In the case below, the pattern is dense with lazily wandering striped tails. When I look at it, I think of seaweed.
The Story of Snow has been nominated as a finalist in the has for the 2011 AAAS /Subaru Science Books & Films Prize for Excellence in Science Books.(AAAS stands for American Association for the Advancement of Science.) Nice to learn about this on the fall weekend when I cleaned out my garage to prepare for another season of snow crystal photography.
Read more and see the other fianlists at http://www.sbfonline.com/Subaru/Pages/Finalists2011.aspx .
A fascinating art exhibit dedicated to water in all its many forms will be opening June 1, 2010, at the Commerce Pointe Gallery in Battle Creek, Michigan. I contributed a couple of large snow crystal prints for the exhibit - which I believe are the only photographs of water as snow in the event. With a little luck, copies of The Story of Snow will be on hand at the gallery.
The opening reception will be Friday evening, June 11. The exhibit closes August 31, 2010.
For details, see a-mermaid-affair.net.
On March 14 the Cleveland Plain Dealer reviewed several children's picture books that deal with snow. The Story of Snow was on the top of their list, being described as a "dazzling nonfiction picture book." You can see the whole review here.