Answer:
General Phenomenon:
The top layer of water is normally warmed by the sunlight and has low density while the bottom layer has low temperature and is denser.
This different in density causes the circulation of water between upper and bottom layer of water due to which the heat transfers and becomes equal in top and bottom layers.
A large temperature difference, hence, indicates that the top and bottom layers have not mixed yet and there was no circulation of water. Otherwise, temperature difference would have continuously decreased.
Example of no mixing case:
In sufficiently large water bodies, if there is no disturbance due to wind on the surface layer of water, layers of water with different temperatures can lie on top of each other without mixing because differing densities. When the upper water layers warm in the summer months, they become separated from deep water layer by a transition zone known as a thermocline. The rapid decrease of temperature in thermocline with small increase in temperature is called water stratification.
When water reaches 4 degrees centigrade, the lower layer starts. The thermocline is what separates the top and the bottom layer. A large temperature difference is, hence, maintained between the top and the bottom layers of water.
