| Re: A reverse thermocline,any ideas about the physics behind it |
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Group: aus.sport.scuba · Group Profile
Author: Bryan HeitBryan Heit Date: Aug 19, 2008 07:20
On Aug 18, 7:31Â pm, "dechucka" vomithotmail.com> wrote:> Next door neighbour stupidly dropped his car keys into his farm dam while
> out on his kayak. Knowing I was a diver he asked if I could try and find
> them. OK we are 670m but I have dived at higher altitudes and the dam was
> only 7-8ms deep so no problems.
>
> Interestingly the water temp at the surface was about 5C and than I hit a
> distinct thermocline at 3ms and the temp was 8C. Surely even with not much
> wind the colder heavier water should sink below the warmer water. The dam in
> question is quite large probably 1/2 a hectare and has got good inflow from
> a small stream. Wondered how the warm water could sit on the bottom even
> knowing that Sydney's bad smog is caused by an inversion layer where cold
> air traps the warm air and pollution in the basin.
>
> Anybody else experienced this and any theories on why it can happen
>
> PS I soon got out of the water without his keys, far to bloody cold and he
> can afford some new ones. Only had a 5mm Farmer John with hood
Most likely explanation is that the lake was undergoing a turnover.
Here in the north this usually occurs in the spring and fall, and it
plays a central role in the ecology of most lakes.
Basically, what happens is over the summer you get a layer of warmer
water over the layer of colder water. By the end of summer this warm
layer tends to be quite thick. When fall comes (or a cold front, etc)
the upper layers of water cool, faster then they sink. This creates a
layer of colder water at the surface. Eventually the sinking of this
cold water will mix the whole lake, but for brief periods you can have
a colder surface than the layer immediately underneath it.
Bryan
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