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The Lake Where You Live


Just melt already!

A lot of lake associations and lakeside taverns hold annual barrel-through-the-ice contests where people predict the date of ice-out. It's hard to get the date right because so many variables come into play: temperature, amount of sunlight, snow cover, amount of new snow, rainfall, peak ice thickness, and others.

Signs this year point to a late ice-out because the ice is very thick, the weather (so far) has not been very warm, and by the start of March the lakes were covered by two feet of snow, give or take.

If you've wondered why the ice takes so long to melt, remember that phenomenon called heat of fusion. Simply put, it takes 80 times as much heat to melt a given weight of ice (or snow) as to raise the temperature of that much water by one degree Celsius. So under the best of conditions melting takes hard work.

And these are not the best of conditions. Serious thawing generally doesn't happen as long as the lakes are snow-covered. Air at a few degrees above freezing, as we've experienced lately, doesn't have much impact. It's sunlight and the resulting radiant heat that mostly does the trick, and pure white snow is just great at reflecting sunlight back instead of absorbing it.

If you doubt this, look at how snow melts off your roof. It mostly melts from the edges in; the darker roof surfaces soak up the sun's heat, and that in turn melts the snow. The lake ice thaw begins in earnest when the snow has melted off and the sun can begin to penetrate the ice and heat up the sediments below. That in turn warms the water and melts the ice from underneath.

This process can take quite a while. Days of 35 to 40 degrees, with nights in the 20s or teens, don't bring much progress. A warm rain can help a lot; so can some days in the 50s; so can a warm wind. But what if you were to enter your local barrel-through-the-ice contest, and you knew the ice was, say, two feet thick? How would you make your prediction?

One helpful bit of data would be thaw degree-days - the number of degrees by which a day's average temperature exceeds 32 degrees F. By this measure, a day with an average temperature of 40 degrees would represent eight thaw degree-days. One scientist has estimated that ice thickness declines by 0.15 inch per thaw degree-day.

Another estimate I've seen indicates that 24 hours of a wind at 20 to 30 miles per hour and 50 degrees F could melt as much as two inches of ice. Still another says a heavy 24-hour rain could melt an inch of ice.

Of course, as best I can tell, these estimates assume the snow is gone. Then there's the effect of sunlight and radiant bottoms-up heat. So, if you're game, and if you have a good weather crystal ball, take these bits of wisdom and guess away. Right now, my sentiment amounts to three words: Just melt already!

Ted Rulseh, who lives on Birch Lake in Harshaw, is the author of the "The Lake Where You Live," a blog where readers can learn about the lakes they love - the history, geology, biology, chemistry, physics, magic, charm. Visit lakewhereyoulive.blogspot.com. Ted may be reached at trulseh@tjrcommunications.com.




 

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