How long does it usually take for the water temp to change and the thermocline to rise once the weather starts to cool?

It was very hot in KY this year, water temp was between 85-90 most of the summer. Past 2 weeks the weather has changed. 70s, 80s and a few 90s for highs. But the nights have generally been upper 50s and 60s.

Wow, your question has been hanging out here for a while now - don't know that I can answer it completely, but at least you will know that somebody out there saw it.

Water is thicker than air, I know you know this, but that makes things like temperature changes slower.  But there are some other elements that can impact your question.

Several weeks ago, after a long hot spell in the mid-west, a friend and I hit the water at sun up to find that the water temps were 65* where they had been 85* the week before.  Wow, what a drop.  But then we considered that it had been raining quite a bit and the nighttime temps were falling pretty drastically.  However, as the day went on (blue bird skies, not a cloud in sight and virtually no wind) the water temps popped back up to near 80* after only about 6 hours.

There are probably people on this site that can give you more scientific answers to your question. but in general I would say that it has a lot to do with what is going on with the weather overall.  Sun, clounds, wind, rain (or even snow) can all affect the time it takes the water temps to change.

Hope this gives you at least something to think about.

It also depends on the lake itself.

How deep or shallow, the size of the lake, how much current is flowing, how many feeder creeks flowing into the lake, bottom composition is what rock, sand, and or mud.

All these factors can warm or cool off an body of water with given factors.

Just remember, as fall nears, the sun is moving further away, thus we don't feel the intense heat like summer, and the days are getting shorter, meaning longer nights and less daylight hours, that alone is enough to start the cooling process without a change in temps.