Tuesday, April 26, 2011

Sable Lake Ice Melt

So how does ice melt on a frozen lake anyway?  I have never actually seen the final moments when the ice on the surface of Sable Lake in the Pictured Rocks National Lakeshore goes away.  I have been watching it over the last few days.  The ice is breaking into sections and getting darker in color. 

I decided to go onto the Internet to see if there is an explanation of how ice melts.  From various web pages, I have pieced together the following explanation.  To understand how a lake thaws, it's helpful to know how it freezes.
  1. As water cools, it gets more dense. That's why colder water is at the bottom of a lake in the summer -- it is denser and sinks.  But when water cools to 39 degrees it gets as dense as it's going to get. As it gets colder still, water actually becomes less dense. So the coldest water remains on the surface of a lake and eventually starts to freeze.
  2. Since ice is even less dense than water that is between 32 and 39 degrees, it floats on top.  That is why ice cubes float in a glass of water. If this were not true and if the colder water and ice sank to the bottom,  lakes would freeze solid from the bottom up.  This would not be good for fish, other aquatic animals, aquatic plants, and fishermen!
  3. The lake loses heat to the atmosphere, and then on a day or night when the wind is not blowing, ice forms. Lakes tend to start freezing first along shorelines, where water is shallow and calm.  The ice starts forming and grows away from shore and gets thicker as long as the lake can continue to lose heat.
  4. In most Januaries and Februaries, snow both reflects sunlight and insulates the lake. With a thick snow layer, the lake neither gains nor loses heat. The bottom sediment is actually heating the lake water slightly over the winter, from stored summer heat.
  5. In the spring, the ice starts to melt in reverse order of how it formed.  Around March as the air warms and the sun gets more intense, the snow melts, allowing light to penetrate the ice. Because the ice acts like the glass in a greenhouse, the water beneath it begins to warm, and the ice begins to melt FROM THE BOTTOM. The shallow shorelines melt first, because those waters warm first. The water beneath the ice warms and pushes up against the bottom of the ice, eroding it.  Ice also melts from the surface, and rain can help the process. The ice loses its structural integrity as it warms. It's weakened by the heat it's absorbing.
  6. When the ice thickness erodes to between 4 and 12 inches, it transforms into long vertical crystals called "candles." (See pictures taken from the Internet below.). These crystals that develop in columns perpendicular to the surface of the lake conduct light even better, so the ice starts to look black because it is not reflecting much sunlight.  Candle Ice makes a clinking sound when the candles are broken apart and floating in the water, bumping up against each other. The ice still might be 18 inches thick, but it won't support weight. It's soft and flexible -- ''rotten'' some say. That's why when the ice goes away in the spring, it's not a thin layer that goes out.  
  1. Warming continues because the light energy is being transferred to the water below the ice. Melt water fills in between the crystals, which begin breaking apart. The surface appears grayish as the ice reflects a bit more light than before. Wind breaks up the deteriorating ice and speeds up the melting process. The lake turns into a big slurpy, with the ice crystals tinkling in the waves. Then it's just a matter of time before the ice disappears.  Usually this happens in a matter of hours.

The following series of shots show pictures taken from the overlook on the northeast side of Sable Lake.  I took similar shots 24 hours apart.  The first picture below was taken on April 24th and the next on April 25th.  Notice how the ice is much darker on the second day.

Another series:  day 1 and then day 2.

Here is a close up of some of the swirls from day 1.  I am not sure if these are left over from snowmobilers or if the swirls form from wind blown snow drifts.

From one day to the next, the amount of open water along shore at least doubled. 

This is a shot from the east end of the lake.  There is a lot of open water here, probably because of the shallow depth.  It looks like in some cases the ice is melting in sections.

We have had a couple of good sunsets in the past few days.  Here is one.

1 comment:

  1. How about a column on lake "turnover"? :-) It was my favorite subject in micro ecology, many long years ago :-)