Lake Michigan in Winter: Temperature, Freezing & Ice Fishing

[graphicer89]

Basically Lake Michigan is located in a relatively cold climate. Its around 494 km long and 190 km at its widest and with a depth of 281 meters...

From what i understand about water freezing in lakes...4 C was probably the temperature at the bottom of the lake , also winter isn't long enough for the whole lake to reach 4 C only just the bottom...

What i need help with understanding is...

If during the winter months the entire lake ISNT covered with ice, but near the shore, ice does form sufficiently thick for ice fishing ...Why is that??

...Is it because the edges are not really deep which allows for quicker freezing? ...or can you please explain why because i have no clue??


Does the lake need to absorb or lose energy for the ice to melt? how much of an energy change must the lake undergo for each gram of ice melted?

...My guess is that the lake needs to absorb energy but i an confused on the energy change...does it require 80 calories for ice to melt or does it require 80 plus 100 = 180 calories for the ice to melt into water?


Would the process of the ice melting change the temperature of the lake? Why or why not?

...I would give an answer but i don't have a clue...can you please help me understand any of these?

 

[Andre]

Indeed fresh water is most at 4 degrees Celsius and the amount of energy required to freeze one kg of water can be found somewhere on the net, googling for instance this.

So the surface water may cool off to around freezing, expands in the process hence gets less dense and hence tends to stay in the top levels of the water, that's why water freezes on top versus for instance, which that solidifies at the bottom first.

During the first stage of the winter, the water temp of the surface is probably above 4 degrees, so while it cools at the suface, it tends to sink, forcing deeper (relatively warmer) water up, which preclude a quick freezing. Obviously this process can take a while.

Eventually, perhaps when most of the water is colder than 4 degrees, this convective overturning would probably stop and surface freezing could start. But if natural currents and/or wind keep the water in motion (overturning), then the cooling surface water is continuously replaced by deeper and warmer water.

Obviously the deepest part of a lake having the most heat stored undertneath is likely taking th longest to freeze over.

 

[Ryker]

I know this is an old topic, but as a tangential question, why does the water only reach 4°C at the bottom of the lakes and doesn't drop below that? Is it because it's not cold enough for the lake to freeze down to the bottom and the densest portion of the water is always found there?

 

[niklaus]

Water is densest at 4°C (somewhat of an anomaly due to the molecular structure of H2O). So water at temeratures BOTH above and below 4°C will be less dense and float upwards. Therefore 4°C water will always be at the bottom of the lake.

Of course it is possible for the whole lake to freeze all the way to the bottom, but in a large lake that would take very long, especially since the ice layer on top isolates against the cold air above.

 

[Ryker]

Thanks, that's what I figured.