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Strange and beautiful ice

Rupert the Bear, a staple of my early literary diet, was regularly very impressed by the window art of his visitors Jack Frost and his sister Jenny Frost. We can explain much of this art rationally, but that enhances rather than diminishes its strange beauty.

At the Earth’s surface, water ice crystallizes in the hexagonal system and snowflakes tend to have six arms or to grow as six-sided plates — which begins to account for their beauty. You can find beautiful snowflakes by the million in cyberspace. And of course when the weather is right you can just go outside, or even look at your window. But crystallography isn’t the whole story.

What got me thinking about the beauty and strangeness of frozen water was a recent article by Toshiyuki Kawamura and co-authors. They describe spray ice, derived from the freezing of spray onto trees and shoreline structures. In bulk, the spray ice resembles “very large monster-like forms”, as they put it.

They also describe ice balls, spheres (roughly) of ice, up to tens of centimetres across. The ice balls seem to form from lumps of slush on the surface of the lake. The lumps get rolled about by the waves and swell, frozen hard after a drop in temperature, then washed ashore by the waves and the wind.

The Kawamura photographs are technical rather than dramatic, but the freezing of liquid water onto solid objects can offer more drama than you need. Around where I live, we still remember the ice storm of 1998. Across eastern Ontario and Québec there are still trees, bent over in 1998, that haven’t yet got back to something like vertical.

That ice was technically glaze, with a density near that of pure ice. The Kawamura spray ice is technically rime, less dense because it forms in irregular masses. Hoar is the equivalent, usually of still lower density, that forms by the deposition of water vapour as ice.

Rime or hoar often forms on our glaciological instruments when they are left out over the winter. If the instrument is a simple mass-balance stake, you just end up with an interesting photograph, like the one taken by my co-author Marco Möller on Austfonna in Svalbard. We like this one so much that we are hoping to persuade the publisher of our forthcoming Glossary of Glacier Mass Balance and Related Terms to put it on the front cover.

Rime on a mass-balance stake, Austfonna, SvalbardA mass-balance stake photographed by Marco Möller on the ice cap Austfonna in Svalbard. The sun has done just enough work to loosen the rime (or hoar?) with which the stake had been coated, and the rime has fallen to the surface as an intact body. (I also like the waves in the middle troposphere, picked out at the thinning edge of the cirrostratus veil by ice crystals condensing at the wave crests and sublimating in the troughs.)

But rime can be a glaciological nuisance. If your instrument is an automatic weather station or AWS, it will have been built for ruggedness and is likely to be still chugging away, recording the temperature, wind speed and other variables, even while more or less buried in the rime. How do you know how to interpret these records, or indeed whether you should?

For me, the weirdest frozen phenomenon of all has to be ice spikes. These protuberances grow out of confined bodies of water that are freezing from the top down. Their base-to-tip length can exceed 10 cm, and can be 10 or more times the base breadth. They taper towards the tip and grow upwards at seemingly random angles. According to Kenneth Libbrecht, the best way to study them is in ice-cube trays filled with distilled water and placed in an ordinary freezer with an air temperature near to —7° C. About half of the ice cubes will produce spikes in these conditions. A fan to promote air circulation also promotes spike formation. Although tap water doesn’t yield as many spikes, plenty of spikes have been reported from out of doors, including the very first in 1921.

Water in a tray freezes at its free surface, starting at the confining walls and growing inwards. When the ice cover is all but complete, the water, under gentle pressure, has nowhere to go but up and into the little hole. If the liquid travels to the edge of the hole before entering the solid phase, you have a hollow tube that is evidently a working funnel for liquid water, which doesn’t freeze until it reaches the propagating tip.

But why? Evidently there is an exquisite balance at the tip between the arrival of liquid, the removal of heat, the release of heat due to the freezing, and probably other factors. Nobody has yet managed to write down the algebra describing this balance.

Upside-down and inside-out icicles are somewhere beyond the borderline of relevance, but a lot of science is like that. The fact that the irrelevant is also the unexplained has a lot to do with why people get excited about ice spikes. And although they can’t compete with Jack and Jenny Frost for beauty, they do show that strangeness can sometimes be strangely close to beauty.

 

By Graham Cogley

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