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 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 increae 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 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."
