We have had about two weeks of unusually low temps, with daytime (what little there is) highs never above -10C/14F and more recently down around -15C/5F. Although we had some snow almost 3 weeks ago and have done some xc skiing, the snow on the trails is pretty worn out, but in the meantime Jonsvatnet, a big reservoir, has frozen over, and the continuously cold conditions have led to blooming gardens of hoarfrost on the ice surface in some areas, while in others the ice has mostly stayed clear. This all has made for some very surreal ice skating through the biggest hoarfrost crystals I have ever seen. Here are a few favorite photos and a video.
Skating on black ice
Looking at the lake bottom through clear ice
In the Crystal Garden, an area of especially large hoarfrost crystals
Really great, BigRed! I used to love ice skating when I lived in Connecticut, but I never saw ice so clear and black, nor hoar frost like that. Thank you for sharing it with us!
Were is that? And what is hoar frost? My friend, those pictures are simply amazing. They literally blew my head off. In Quebec were I live it's been raining for a week now, let's just say them picture helps with the gloom of the lasting autumn.
Louis-Alexis: I live in Trondheim, Norway (but I used to live in Vermont, much closer to you).
Hoarfrost forms by sublimation, when water vapor in the air adds directly to the growing crystal without going through the liquid phase first -- snowflakes in the atmosphere grow the same way. It's common to see it on trees and shrubs during extended cold spells, you've probably seen something like that in Quebec. It can also form on the surface of the snow, where it usually makes platy crystals that reflect sun- or moonlight nicely, and if it's cold enough long enough the flat-fethery crystals can get to three or four cm long. Very cool to xc ski through. In that case I think the main source of water vapor feeding crystal growth is the snowpack itself, which is warmer at the bottom. This leads to the formation of depth hoar, a weak grainy layer deep in the snowpack that is an "invisible" cause of avalanches, especially in places like Colorado and Utah. The hoarfrost crystals at the surface can also create a weak, slippery layer that can cause avalanches if new snow falls on top of it.
But this is the first time I have seen hoarfrost growing directly on ice, and the amount and density and size is highly variable over the surface of the lake, there's a little "crystal history" for each part of the lake. I think the "Crystal Gardens" may be fed by moist air draining off the surrounding land, but I'm not really sure. Over just the past five days or so the main body of the lake, which was completely open, has frozen over and it looks like a continuous carpet of crystals has grown over the whole thing, many square kilometers worth. And where there's still black ice, clusters of hoarforst crystals spring up in old skate tracks or along the cracks that form as the ice contracts. I didn't go out on the new ice yesterday but I hope to go back tomorrow and skate the whole thing. It's all very delicate and short-lived -- it will warm up a bit (but still below 0C) this weekend and may snow, so I want to get out there at least one more time before that happens. I'm basically pretty obsessed with this while it lasts. Maybe the crystals will get even bigger!
I think the best is over now. Went out in beautiful weather on Friday and Saturday, on the latter day skated the full length of the lake and back, 20 km or so, with my older daughter, then went back with daughter #2 on Sunday for another long, fast tour, this time into the lake's western bay under clouds and with just a powdering of new snow. It's been spectacular: black ice so clear i't's scary, endless crystal prairies, areas where there are scattered clusters of crystals some up to 15 cm (6") long, all lit up by Norway's low midday sun. Some snow forecast for tomorrow, at least enough to dust over all the hoarfrost and black ice if not compromise the skating. I have put a selection new photos in my album here on Trailspace, copied a few of those below, and put even more on the Facebook album. I really think this may be a once in a lifetime "hoarfrost happening"!
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