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Name: David
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Question:
If water density is greatest at 4 degrees Celsius then the temperature at the bottom of a large body of water should be 4 degrees. How does convection work at temperatures close to 4 degrees? If the temperature of water drops to 3 degrees it would start to rise and mix with the above waters of 5 degrees? Would it thus mix and increase the mass of the 4 degree water? Also when there is a frozen lake does the bottom of the lake stay at 4 degrees because of its higher density and insulated from dropping because of the upper levels being a lower temperature and less dense and it cannot release its heat through convection?



Replies:
Very observant!! Water movement around 4 C. can be pretty complicated. The situation you describe is a "most optimistic scenario", since you have not included turbulent flow of the water. While usually negligible, it can become important at temperatures around 4 C. First, consider your setup that "the temperature of water drops to 3 C." creating a temperature inversion. Driven by density and temperature alone, how is the temperature going to fall below 4 C.? I do not see how. If the ground is somehow colder than 0 C., ice will form and "float" to the surface. Should the depth be sufficient that the ice melts along its trajectory to the surface, it will find itself in a zone of different density than liquid water at 0 C. In that case a zone of turbulent convection is started. Second, normally a lake freezes from the top down because wind and evaporation (at an energy cost of about 10 kcal/mol) with the heat of freezing gives off only about half that amount. Third, the bottom stays at 4 C. even when the temperature of the surface ice is below freezing, as you suggest. It is interesting that life as we know it would be very different if the density of ice were "normal" i.e. the lake froze from the bottom up. If that were the case then fish would be forced to more and more shallow depths, eventually all the fish would be left "flapping around" on the surface of the frozen lake.

Vince Calder


David,

Water at the bottom of a deep body of water is usually 4 degrees Celsius. Water at the bottom of VERY deep bodies of water is always 4 degrees. For more shallow bodies of water such a small lakes, it is possible for turbulence to push colder water toward the bottom. When the water at the surface is warmer than 4 degrees, the water at the bottom is often the coldest water. When the water at the top is colder than 4 degrees, the water at the bottom is often warmest water. If sunlight starts to warm 3 degree water at the top, it will start to drop. If cold air starts to cool 5 degree water at the top, it will start to drop. When convection has circulated the heat until most of a lake is 4 degrees, then the top cools to 0 degrees and begins to freeze. For oceans, the huge currents of water and variations of air temperature make the process more complex.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College



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