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April 27, 2008
:: Melt, melt, melt!
So, Meggisi 2008 is approaching fast, and the lakes up north -- whilst trying to melt -- just got 6"+ of snow dumped on them yesterday in a very powerful and freak late-April snowstorm. WTF?! I have been keeping my eye on several webcams up on lakes of notable interest over the last couple of weeks, including lakes around Ely, MN, Rainy Lake, and Lake of the Woods. All locales - pre snowstorm - had black ice and major "candleing" working on the ice conditions. In fact there was also quite a bit of open water. I emailed some contacts north of the border and one of them said not to worry and that the snow might help to "weight down" the ice. Hmmmm.
I am not too sure about that. Chemists agree that ice does not sink "on its own", but what if there was something with higher density on top of the ice? The density of ice is less than the density liquid water (remember that ol' chestnut from high school Chem?) so what if there was water on top of ice? Another mystery is the phenomena of water's highest density - it occurs at 4°C, just before the freezing point. It gets less dense in the final cooling degrees before it freezes to its solid form (ice). This causes convection currents in lakes, which help agitate either the freezing or thawing process. Technically, lake ice melts from the bottom-up. As the sun's rays penetrate the snow and ice from above, the energy stimulates the convection currents from the bottom of the lake where water warms to somewhere above 4°C. That water rises up to the ice layer, transfers its heat energy to the ice as it cools back down to 4°C and then sinks back down to the bottom where it can be warmed again. The cycle continues (in circular, convection currents) until the ice is melted to a point where other forces from above the ice can take over. This may include snowmelt and surface currents. Remember, snow is simply ice crystals in a very loose, air-filed structure. Snowflakes are less dense than ice, but snowflakes enmasse (snow pack) may have enough combined density to cause a less-dense ice structure to be overcome by gravity.
I am not certain the snow that fell up at Meggisi and across other parts of northern MN and southern ON accumulated enough to "weight down" the ice. More probable to me is the possibility that the snow melts on top of the lake ice, and the liquid snowmelt (which is more dense than the ice) has some effect on "weighting down" the ice. Some other surface effects may come into play here as well: snowmelt may come off of land (where it is sure to melt somewhat faster) and rush into lakes, causing more warm water to move in and amplify the lake ice melting process. If warmer runoff currents flow under the icepack, the convective process described above could get a nice boost!
So, in the long run, I think the rest of MN and southern ON's lakes will melt in due time for opener (and Meggisi 2008)...given the following:
- Sunny days, with highs pushing or better than 10°C every day
- No more accumulating snow
- Low temps only hitting 0°C at their lowest points of the overnights, so there is not a prolonged time at or below the freezing point in the atmosphere.
Posted by pierre at April 27, 2008 9:25 AM