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As one descends in water, temperatures usually drop. Less light is able to penetrate, so there is less conversion of light to heat as depth increases. Generally, as water cools it shrinks. It becomes more dense. Unlike most other substances, water cannot get denser than when it is at a temperature of 4 degrees Celsius (39 degrees F), and for seawater -2.2 degrees Celsius (28 degrees F). If water at 4 degrees gets colder it expands, gets lighter, and starts to float on the 4 degree water. So, the minimum temperature a diver will encounter at depth in fresh water would be 4 degrees Celsius because all the lighter warmer or colder water would be above it.
To illustrate how this will affect the diver let's look at Lake Minnewaska in New York State. On a given summer day the surface temperature might be 24 degrees Celsius (24C) or 75 degrees Fahrenheit (75F). As the diver descends the temperature will drop. Since Minnewaska is usually clear, and the light penetration good, the temperature at 30' might be 18C/65F. All of a sudden a zone of water is encountered that is about 13C/55F at the top and 4C/39F just under the bottom! This zone might be 5-10' thick. The zone of a rapid temperature change is called a "thermocline," where the "thermo" refers to heat and the "cline" refers to slope. From the bottom of the thermocline to the bottom of the lake the temperature will remain at 4C/39F even if the lake were thousands of feet deep.
Many times the top and bottom of the thermocline is visible. Certain debree landing in the water may have a density of close to 1. Some of it sinks to the top or bottom boundaries of the of the thermocline where the colder water has a greater density than the object. So the object will float at that level. After a heavy storm Minnewaska may have particles added to the lake from stream runoff. Much of it does not penetrate the thermocline so the lake is clearer when the diver is in the denser water below the thermocline.
Another nearby lake in New York is Mohonk. The top of the thermocline is at about 18' deep in the middle of the summer. Even though Minnewaska and Mohonk are similar in most respects, the visibility in Mohonk averages about 10' and the visibility in Minnewaska is about 30'. Since light penetration is reduced in Mohonk, the heat produced from the conversion of light to heat does not reach as deep.
In the incredibly clear water near Alcapulco, Mexico and Caribbean Islands the ocean thermocline is about 300' deep. That is why ocean divers usually do not concern themselves with the affects of a thermocline like northern inland lake divers do. Typically the surface temperature in the Caribbean might be 28C/83F, and at 100' it is 27C/81F! The difference is barely noticeable.
Returning to Lake Minnewaska: As the air temperature cools due to seasonal changes, the thermocline begins moving upward until it disappears. The lake may be 4C/39F from the top to the bottom in November or December. As the surface continues to cool the water becomes lighter and floats on the top of the denser 4C/39F water. When the water freezes the ice has even less density and floats on the top of the 0C/32F water just below it which is floating on the 4C/39F that is the most dense. The opposite of this process takes place as the seasons warm. A thermocline will then reform and start its descent until sometime in August.
When a wind blows over a body of water, some the water on the windward side gets pushed toward the opposite side. In effect, the water is lowered on the windward side and is raised on the leeward side. If you blow your breath over the top of a cup of coffee, the coffee may be raised on the side away from your mouth so much it may go over the lip of the cup and spray coffee across the table. The amount of displacement is the direct result of the strength of the wind, the size of the body of water (fetch), and the duration the wind blows. At the Equator the Trade Winds make the water over a foot higher in Indonesia than in South America!
When the water piles up on the downwind side it is pulled down by gravity and travels under the surface back to the side where the wind is coming from. If there is a thermocline the water may travel along the top of it and be cooled as it travels. When the cold water rises to the surface it may have a dramatic effect on the surface temperature. This rising water is called an "upwelling."
On July 28, 1997 a weather report indicated the surface water temperature in Lake Ontario off Rochester was 43 degrees F! That was bone-chilling. As the report continued, it stated the wind in the State of New York was coming from the south. The next day the water temperature was 44 degrees F. The wind was still coming from the south. The next day there was a switch to a north wind and the temperature on 7/30 of the lake was 48 F. On the 31st of July the surface temperature rose to 67 F and the wind continued to come from the north. To explain the incredibly low temperatures, and the dramatic increase in temperature for such a large body of water, one must realize Rochester is on the south part of Lake Ontario. When the wind was from the south it pushed the warm surface water to the opposite shore in Canada where it got pulled down by gravity and traveled along the top of the thermocline back to Rochester. There it upwelled bringing cold water to the surface in Rochester. When the wind switched from south to north, the upwelling in Rochester stopped and the warm surface water was pushed in the opposite direction from Canada to Rochester.
There are short periods twice a year when there is no thermocline in New York State lakes, and the temperature will be the same from the top of a body of water to the bottom. During those times any wind blowing over the water will cause a circulation along the bottom instead of along the top of the thermocline. The water may pick up considerable sediment and nutrient causing an upwelling to be downright disgusting. The surface waters will become filled with bottom nutrient and sediment. There may be an algal bloom. That is what is happening when a lake is referred to as "turning over."
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