"Billy" wrote in message
...
In article
,
"www.locoworks.com" wrote:
On Mar 20, 12:48?pm, Billy wrote:
In article
,
?"www.locoworks.com" wrote:
On Mar 19, 10:43?pm, Billy wrote:
Freezing is 30F. Did it get below 30F in you greenhouse?
--
Billy
Impeach Pelosi, Bush & Cheney to the
Haguehttp://angryarab.blogspot.com/http://rachelcorriefoundation.org/
Freezing is 32F, but it did get down to 30 in the greenhouse. ?I shall
start new seeds today and welcome the onset of spring.
Take notes. Water drops to 30F before it freezes (crystalizes).
The freezing point of water is 32 F and in practice it generally does freeze
at that temperature. Liquid water CAN get to lower than that (not 30F
specifically) but it would then be called supercooled.
Upon
crystalizing, it releases the "heat of
crystallization".
http://cancerweb.ncl.ac.uk/cgi-bin/o...f+crystallizat
ion
This heat warms the ice ?to 32F (any warmer and it would melt).
This is somewhat confused.
For liquid water at 32 F to transition to solid water at 32 F requires the
latent heat of crystalisation to be removed from it to an external source, the
water does not spontaneously give up this heat, it must be taken somewhere
else, usually by contact with something or radiation to something that is
below 32F. So the latent heat isn't doing any raising of the temperature but
it will stop the temperature falling for a while.
This is why orchards and vineyards use sprinkler system to protect their
crops. As long as you are making ice, the temperature will stay at 32F.
Yes the presence of liquid water will tend to keep things "warm" firstly as it
takes heat to cool it down to 32 F (specific heat) from whatever temp it is
stored at. This is significant as the specific heat of water is high compared
to most things and very high compared to air. So spraying water increases
thermal mass, which reduces the rate of cooling.
Secondly the latent heat must be given up during freezing before reducing the
temperature of water below 32 F. To give some idea of the relative
importance, in round figures it would take ten times as much energy to freeze
an amount of water at 32 F as to cool the same amount to 32 F from 46F.
But you are likely to get local spots where cooling is taking place faster
than the supply of liquid water and the temp drops below 32F. So while latent
heat has more effect than specific heat the plants may still get chilled below
32F. However since plant tissues do not contain pure water their freezing
point will be below 32F and they may not get tissue damage if they don't
freeze. All this is generalising of course and the degree of damage will
depend on the plant and the actual conditions.
Even placing a 100 watt light on the floor of the green house and having
a timer turn it on between 2 AM and 6 AM may have saved them. One of
these years, we won't have to say, wait till next year;-)
--
Another way to give some protection is to increase the thermal mass of the
greenhouse with big tubs or barrels of water. A third is to blanket it at
night to reduce heat loss. Quite thin coverings may reduce radiation loss to
a cold sky.
Billy
Impeach Pelosi, Bush & Cheney to the
Haguehttp://angryarab.blogspot.com/http://rachelcorriefoundation.org/
Say, Billy,
You might want to take some notes yourself:
http://www.straightdope.com/classics/a891215.html
32 F is so ingrained in popular culture.
There is a reason for that!
I may hae to go to my anciet
textbook. I'll keep looking for for a clean explantion.
In the mean time, as a place marker I'll leave you
www.wvu.edu/~exten/infores/pubs/ageng/sw4.pdf .
Forth paragraph, third sentence
Ice may not form when the frost is very light; temperatures
will usually be 1F - 2F below freezing before ice starts to form.
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This would be an example of supercooling. The degree of supercooling depends
on the conditions, if the environment is very clean, that is no dust etc, and
cooling takes place quickly supercooling is more likely that when dust nuclei
are present and there is time for the ice crystals to sort themselves out.
David