FROSTS
Robt R Pries (Tue, 13 Nov 2007 12:58:22 PST)
Several people have mentioned that water gives off
heat as it freezes. If I remember my physics
accurately it is quite a lot of heat. 1 calorie is the
amount of heat it takes to raise one cc of liquid
water 1 degree F. It takes 80 calories to change from
ice to water and vice versa. In other words the amount
of heat it would take to raise the temperature 80
degrees is what is absorbed by melting ice or given
off by freezing water. Just imagine the tremendous
number of calories that are being absorbed presently
by the melting ice caps at the North Pole. Once that
ice is gone imagine how hot it could get without
something out there to absorb the excess.
Bob Pries, zone 5-6 having a rather warm November
day. And we our normal hard frost is about 20 days
overdue.
--- Jonathan Knisely <yale@email.med.yale.edu> wrote:
I tend to favor Jim McKenney's explanation.
What I have observed, and what I have read, is that
there may be a difference
among plants in their ability to tolerate
subfreezing temperatures based upon
the presence of 'antifreeze-like' soluble compounds
that can prevent the
nucleation of ice crystals in plant cells.
Differences between kale and
lettuce might be partially explained by something
like this. I don't believe
that the cells would 'want' to deplete themselves of
water--what would they do
when the temperature got higher at midday?
I wonder whether the anecdotal benefit of dousing
frosted plants (that are not
frozen) with water is because of the significant
heat capacity of water as it
undergoes a phase change from liquid to solid. The
splash of water would
prevent the nucleation of intracellular ice--all
that 'outside' water would
need to freeze before the intracellular water (with
those special solutes)
would freeze.
Jonathan Knisely
Coastal Connecticut, USDA 6a
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