Thermal Bar / Thermocline

Here's an excellent response by Tim Bromund on the LOTSA messageboard (7/15/05) to questions regarding the ‘thermal dynamics' of Western Lake Ontario:

“Guys, I thought Ron explained it (the thermal dynamics of the lake) pretty well last night. In the spring the entire lake is 39 deg or colder. Remember that water is at it's most dense at 39 degrees, so both warmer and colder than that are less dense, which is why ice floats, the 32 deg water is less dense than the 39 deg water below.

“As the shoreline water warms the 39 deg water gets pushed offshore towards the deeper water and thermal edges set up at the warm/cold water temp gradient. This happens on both shores and the 39 deg surface water gets pushed to the middle of the lake as spring progresses.

“At some point the warmer water over takes the cold surface water and the 39 deg dense water sinks to the bottom of the lake once that happens, the surface thermal structure is normally over for the season (with a few exceptions) and the thermal structure gose from a vertical orientation (surface temp break) to a horizontal orientation (stratification) and that is the summer fishing, finding and fishing the thermocline or horizontal thermal break between the warm surface water and the 39 deg cold dense water at the bottom. This transition area between the warm and cold water is usually a very tight band (10-20 feet thick typically) where the water will drop from the mid 60's to the low 40's. That is the fishes “comfort zone" that Ron was talking about. I believe the upper warm layer is called the Epilimnion (sp?) and the lower cold layer called the hypolimnion (sp?) with the thermocline sandwiched between the two.

“There is usually a pretty good change in water density in that thermocline with lots of plankton and stuff and a good sensitive graph can often pick it up (it displays a a faint gray band on non color graphs, not sure about color units, I've never owned one). That is the temp zone that is preferred by both the trout and salmon and the bait.

“The only time this changes during the summer is with a strong NE blow like we had last week. I don't pretend to understand the dynamics of it, but that causes the lake on the western south shore to “flip" or, in other words, it causes a cold water upwelling which pushes the warm surface water offshore and the cold deep water rushes into the shallows to replace it. Like Ron said, that cold water doesn't want to be in the shallow nearshore zone, it is denser than the water around it, it wants to be down at the bottom of the lake in the deeps, so as it tries to get back where it belongs, it sets up those wicked subsurface currents and a short term, vertical thermal gradient at the edge between the warm and cold water. Once the cold water works it way back offshore, the horizontal thermal structure stabilizes and it goes back to the normal summer pattern.

“This continues on into the fall until the surface water cools to the point that the thermal gradient breaks up and the lake does it's fall "flip" and the entire lake continues to cool more or less evenly to it's winter temps.

“Hope that made some sense to you guys...Tim"