There are a few areas that Roark is mistaken, though his dissertation is
reasonably close. One the point of maximum density of water is at 39
degrees, so as long as the temperature is warmer than 39, the warm water is
at the top, and when the temperature of the pond drops below 39 degrees, the
warmest water is at the bottom. The earth does continue to warm the bottom
of the pond with the heat being transported to the surface where wind,
evaporation, and cold temperatures rob the heat from the surface. When the
amount of heat transferred is not sufficient to keep up, then the
temperature at the surface reaches 32 degree water, and through continued
cooling becomes 32 degree ice. At the water ice interface, the temperature
is always 32 degrees. When ice forms, it expands about 9% which is why ice
floats with about 10% of the cube out of the water.
--
RichToyBox
http://www.geocities.com/richtoybox/index.html
"Ka30P" wrote in message
...
Roark did a bit on water and winter in response to a question and it makes
a
pretty interesting read.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~
From: Roark7
Subject: Bubbling bottoms and super cooling
Date: 1997/10/31
David Swarbrick wrote:
Bubblers are recommended for maintaining an ice free portion of a pond
over winter, and maintaining good oxygen levels. I have also seen it
suggested here that it should not be placed on the bottom of the pond,
but raised so that the circulation it induces does not disturb the water
at the every bottom because this merely results in the very cold water
by the ice being distributed all over.
I question this.
Yep... And I don't blame you one bit. I had the same doubts and voiced
the
same arguments that you shortly share because it *is*
counter-intuitive...
until you start looking hard.
First, warm water rises, and cold water sinks. The water at the bottom
will always circulate if there is a temperature difference. Frozen water
floats. I suppose there must be a point just before water freezes where
the situation turns on its head, but I assume also that this is only at
the point very near freezing.
I had major questions about the physics aspect of this whole "winter pond"
thingy as well. I did some asking and a bit of research and came up with
three
things which cause the inverse stratification effect. These are, in
ascending
order of importance:
- The native heat of the earth
- The physics of water going from a liquid to a solid
- Surface cooling due to winds and cold air.
I'll rattle through each contributing effect in detail below:
- Heat of the Earth:
It turns out that the earthern bottom of a 4-foot deep pond stays right
around
34-38 degrees even though the outside air temperature drops much lower.
The
simple reason for this is the Earth is slightly exothermic. As you dig
down
you hit a point where more heat is being released by the earth than can
be
pulled-away by wind, night sky, etc. This is why you bury water pipes
below
the "frost line". This heating effect is small in ponds, but it *is*
there.
The earth *is* pumping a bit of heat energy into it all the time. The
other
effects (below) tend to magnify this effect into something useful by
keeping
this slightly warmer water on the bottom.
- Physics of Water:
It turns out that water doesn't move upwards because it is "warm" nor sink
when
it becomes "cold". A little thought reveals this behavior is strictly a
function of density. Warmer water *tends* to be less dense so it
rises... but
this isn't gospel. An interesting kink in the water density -vs-
temperature
curve shows-up just prior to 32 degrees F. At the pre-freezing point
(32.8
F), water undergoes a major density change. As it cools it becomes
*less*
dense than water which is just a fraction of a degree warmer. This
difference
is fairly large. Being lighter than the surrounding water, near-freezing
water
*rises*. (This is one reason frozen pipes tend to burst. The density of
the
water decreases, the mass stays the same, so the result is volumetric
expansion which splits pipes with ease.) The degree of final bouyancy is
controlled to a large part by the dissolved oxygen content of the water.
The
more dissolved O2, the greater the expansion once the freezing point has
been
reached. Since water at the *top* of the pond tends to have a greater O2
concentration, this further contributes to stratification.
Near-freezing
water moves toward the surface, and then, having reached the surface,
freezes
completely.
- Surface Cooling Effects
This one is pretty obvious, but its worth restating to put it in context.
Given a sub-freezing day and a brisk wind, its a simple matter to pull
more
heat from the top layer of water than can be replaced by natural
convection.
Once the top starts to freeze, heat loss to ambient and basic water
physics
insures the top will *stay* cooler than the bottom. If this wasn't true,
you'd never see a thin coat of ice.... the pond would instead just hit a
point
where the entire thing suddenly became a solid chunk of ice.
It seems to me that if the air being pumped in is warmer anyway (in my
case, from inside an unheated shed), then the balance will be about
right.
Your idea about pumping warm air into the water isn't a bad one, but it
will
take a lot of warm air to make a dent in the ponds temperature.... far
more
than you could reasonably produce. Pumping a large amount of air in
would
also create currents which the fish would need to fight or at least
adjust
for. Hibernating fish are in no position to do this and forcing them
into
this situation uses energy they will need during the rest of the winter.
For keeping a hole in the ice however, you could likely use this warm-air
idea
to your benefit. Put an airstone a foot under the water and run warm air
to
it. Bear in mind that you will lose lots of heat in just a short run
between
your shed and the airstone.
Also if the problem with water under ice is the lack of oxygen and build
up of waste products under the ice, then the more chance the bubbler has
to oxygenate the water and take away the foul water the better.
The oxygen demands of fish near the freezing point are very, very low.
This is
a good thing because I've got a feeling there isn't much oxygen available
once
you near the peak of winter. Fortunately, decay and decomposition of
wastes
by bacteria has nearly stopped as well which relieves a decent portion of
the
oxygen load. Very little oxygen is needed in a winter pond.
During a really *cold* winter, I think the idea of a full-blown bubble
system
would tend to upset a natural balance which Nature clearly went out of
her way
to establish. Having said that however, I can see a very definite
*benefit*
to using such a system as the air temp starts to push into the upper 30's
and
40's. By introducing additional air and inducing water motion, you'd be
putting lots of needed oxygen into the water as well as helping the pond
to
absorb ambient heat. Fish coming out of their winter sleep wouldn't be
oxygen-stressed as well as being thin, worn and badly in need in of a
shower.
From what I'm given to understand, most fish have no problems during
the
actual *wintering*... its the *transition* from hibernation to normal
metabolism which gets them. Your bubbler could be a great tool during
that
transition period.
So.... thats my 2 cents on the subject. YMMV.
Roark --- wouldn't know a real winter if it bit him in the rumpus!
Ventura, Ca.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~
kathy :-)
algae primer
http://hometown.aol.com/ka30p/myhomepage/garden.html