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How to decide if the ice is thick enough? Pose that question to blade runners and academics, and you get a host of different answers. Hi. I'm Dave Thurlow, and this is the Weather Notebook. As correspondent Claire Holman tells us, Mainers have developed creative ways to determine skating suitability. Claire: It's skating season up here in Maine, and skaters of all ages are venturing out onto frozen ponds and rivers. And to judge if the ice is safe, they use everything from sounds to bugs.

"I'm a novice, so I'm just gonna judge with other people's skating, and I don't go past their tracks."

"I haven't seen any water around the edges at all; it's all been ice, so I think it's pretty good."

"I hear some cracking out there, a little, and it's better to hear cracking than silence, because that means the ice is compressing, and it's a lot safer to have cracking ice than silent ice."

"If it's good... clear ice, you can look through the ice and see the stone fly nymphs on the bottom of the ice, with bundles of little sticks and twigs on them, walking on the ice upside down. Right now I can see one crawling, like a little tank, and I can see about 5 inches of ice, and I know that's fine."

Claire: Even two inches probably gives enough strength to walk on, according to Jerry Lasala an associate professor of physics at the University of Southern Maine, but the ice might not be the same thickness everywhere.

"Even fish swimming under it affect it, because they can stir up currents which will bring up warm water underneath, and therefore melt the ice a little bit."

Claire: The fact that we can skate at all, is because when water freezes, it expands -- forming microscopic, hexagonal crystals which are hollow. Thanks to this crystalline structure, ice floats -- if there is water under it.

"In a river or a lake that drains, the water call fall below the ice level, and then you are supported merely by the ice over air, and then it does become a question of the structural strength of the ice."

Claire: So to be safe, Lasala suggests cutting a hole in the ice and measuring.

Claire Holman prepared that report. The Weather Notebook is a production of the Mount Washington Observatory. It's made possible by generous support from Subaru, the beauty of all wheel drive, and the National Science Foundation.