As rain-free skies finally cleared sufficiently to let the sun shine for more than two or three days in a row this month, our average air temperatures have increased, and our local lakes’ surface temperatures have likewise increased.
As the water near the surface heats up, deeper water, insulated by the water above it and shielded from the sun, remains cool. Warmer water is lighter than cooler water so the water begins to separate into a warm upper layer (called the epilimnion) and a cool bottom layer (called the hypolimnion). There is a separating layer of water between these called the thermocline. The thermocline is typically several feet thick, and within this horizontal band, the greatest change of temperature takes place, with the water getting colder as the depth increases.