The Wonderful Sounds of Skating on Black Ice
Recently frozen lake ice has some interesting properties: it looks completely black and makes some interesting sounds when you skate over it. Swedish mathematician Mårten Ajne is a black ice skating enthusiast and he demonstrates his technique in this National Geographic video. You’ll want to turn up the sound or use headphones…the ice sounds like a cross between sci-fi phaser fire and getting pinged by sonar in a submarine.
The most desirable condition is virgin black ice, when a lake has caught its first ice cover and it has grown just thick enough to bear your joy. In favorable weather conditions this will take two days after freeze-over.
Five centimeters [2 inches] is often the limit [to how thin it can be]. If you’re close to shore, you can go thinner, up to about 3.5 centimeters, before it breaks. That’s for fresh, cold black ice. Brackish ice, which contains salt, needs to be thicker and is more difficult to assess.
At one point, Anje measures the ice with a caliper — it’s only 45 mm thick — but what really illustrates the crazy thinness of the ice is that you can see the surface flex as the water moves under it (wait for him to skate by the camera at ~1:42 in the video) and the ice cracking behind him (at ~2:02). Whaaaaa?!! (via the kid should see this)