Expert: James J. Kovalcin - 10/30/2009

Question
upper layers of water in a deep well are cool but those in a pond are warm and the lower layers of water in a pond are cool.Why?lro8x

Answer
First of all, the assumptions made in this question are not universal.
As for the well, if it is very deep the bottom end will begin to approach the depths where the molten core of the earth starts to have an effect.
For a surface lake the sun will tend to warm the surface waters which will NOT tend to mix with the lower layers since warm water is slightly more buoyant and will remain on the surface leaving the deep waters cooler. This effect is so strong in many areas of the oceans that the temperature of deep ocean waters is pretty much independent of season or latitude since water at about 4oC is the most dense and therefore sinks to and stays at the bottom of the oceans.