Re: VRD - ice freezing on a pond

Let's start with a few basic facts, and then use them to answer your 
questions.

Snow is a very good insulator, especially 'fluffy' snow. This is because it 
is mostly trapped air by volume. The little voids in the snow are too small 
to allow the air to circulate, which stops heat from being lost by what we 
call convection. In addition, the trapped air does not let heat travel 
through it easily (mostly because air has very little mass per unit volume) 
and thus heat conduction is also low.

It is also generally true that the quickest route for heat gain and loss for 
a pond is going to be from its upper surface. That is, gain or loss of heat 
from the surrounding soil, rocks, etc is likely to be far slower than the 
heat transfer due to winds, etc. This doesn't always hold true...especially
if the lake has springs in it.

Water is a very peculiar substance, in that it expands when it freezes. This 
expansion actually starts a little bit before the formation of ice, so that 
the densest water is at 4 degrees centigrade (appx. 39 degrees F). This is 
why ice floats, by the way, instead of sinking. Note also that the water 
below the ice is warmer than the ice itself.

OK...on to the questions:

1) We noticed that when snow fell on a frozen pond, it seemed
>to weaken the ice--lessen its thickness, even though the temperature
>remained below freezing and it was not sunny.  Can you explain why this
>happened?

The water below the ice is warmer than the ice is. If you put a blanket of
insulating snow on top of the ice, then the heat loss from the upper surface 
of the ice can become less than the heat gained from the warmer water below.
Thus the ice layer ends up absorbing more heat than before, and some of it
will melt.

>2)Why does a pond freeze around the edges first and the center last,
>likewise why does a frozen pond mel around the edges first and the center
>last?

Since the main route of heat loss/gain of the pond is thorugh its upper 
surface, we expect the temperature to change in the upper layers first. Now, 
if the water is shallow, then there isn't as large a heat reservoir behind 
the surface as when the water is deep. This will generally mean that the 
margins will be affected before the center. 

When the pond is losing heat, the water densifies as it cools, and this 
cooler, more dense water will tend to sink, being replaced by somewhat 
warmer water. Now, near the edges, it doesn't take long before all the water
is cooled to the point that it can't cool any more without ice forming. But 
in the center, I have a much larger volume of water to cool before running 
out of warmer water from below.  There may also be a nucleation issue. It 
turns out that it's harder to form ice in pure water than on the surface of
almost anything that happens to be in contact with the water. So the 
shoreline, and the aquatic plant stems, etc, near the shore may act to 
speed up the freezing process.

Now, as to the reverse, my experience is different than yours. At least the 
small lakes I knew in Michigan tended to melt first in the center, not the
edge. Maybe they had springs and the convection from those weakened the
central ice first.  What will happen is that the area of greatest heat gain
will melt first. If the pond in question has ice that has frozen all the
way through to the bottom out in the shallow edges, then I'd expect the 
edges to melt first. The reason is that there is no liquid water below to
'carry away' the absorbed heat. Out in the center, as the ice absorbs heat,
it will warm the almost-zero-centigrade water just below it. When it does
so, this water becomes more dense and sinks (remember, the densest water is
at 4 degrees centigrade). This acts to carry away heat and thus retard the
ice's melting.

>3)Does the amount of surface area have an influence on the rate at which
>pond freezes?

What's  more important here is not the total surface area, but the
surface area (through which most of the heat loss and gain occurs) relative
to the volume of the water, or, in other words, the average depth of the 
pond. That is, a 1 acre pond 6 feet deep will take more time to form ice 
than a 1 acre "pond" 6 inches deep.  But smaller area ponds also tend to be 
shallower ones, so it is very likely that you'd notice an empirical 
relationship between the area of ponds and how fast they freeze, with 
smaller ones generally freezing first.