The ice abides: How thick is it?

It has been cold. Very cold. For a long time. I am confident we have not seen a temperature above freezing since Thanksgiving, and now the forecast calls for 30 below zero on Sunday into Monday.

So, living on a lake as I do, I wonder: How thick is that ice? It’s not easy to find out unless you are friendly with ice fishermen. Two winters ago, when a friend came north in February to go fishing, we found the ice more than two feet thick. It’s been much colder this year, but it’s only January.

And of course there’s the matter of snow – a good insulator – on top of the ice. Just how much insulation does snow provide? It’s hard to get a good answer. One “fact” I’ve seen in several places on the Internet (though without a credible source cited) says 10 inches of fresh snow that is 7 percent water (and the rest presumably air) has about the same R-value (R-18) as a 6-inch layer of fiberglass insulation. That’s quite impressive, if true. We have at least 10 inches of snow on Birch Lake now.

Then there’s the matter of the ice itself. On the Internet I found statements (again unattributed) that ice a foot thick has an R-value of 9, “much higher than wood, newspaper, or rigid foam board.” I also found values (credible ones from university sources) for the thermal conductivity of ice. Without getting into a lot of scientific units, it turns out that ice is only about twice and thermally conductive as glass, which is a notoriously poor heat conductor.

So what does all that have to do with the thickness of this winter’s ice? Well, for the ice to get thicker, heat (such as it is) has to escape from the water immediately below the ice. To do that, it has to penetrate first the ice and then the snow. The thicker the ice, and the deeper the snow, the harder it is for that heat to escape. 

I am betting that 30 degrees below zero for a couple of days will meaningfully thicken the ice. But if you want to know how thick the ice is on your lake, you’ll likely have to talk to an ice angler. Or go out and bore a hole yourself.