Re: Does Ice have a constant temperature?

To answer the first part of your question, it is very possible for ice to 
exist at lower temperatures than 32 degrees Fahrenheit.  For example, it 
is almost always colder than that temperature in Antarctica, and the ice 
there would be approximately the same temperature as the surroundings (at 
least at the surface).  Ice has been studied at temperatures down very 
close to absolute zero.

You are almost on the right track, however, because there is an ice-
related temperature that is constant.  That is the temperature of a 
*mixture* of ice and liquid water.  If ice and liquid water are coexisting 
at normal atmospheric pressure (provided you are at an equilibrium 
condition, which would take some time if, for example, you dropped an ice 
cube into warm water), then that means you are *exactly at* the 
freezing/melting transition, and the temperature is 0 degrees Celsius (32 
degrees Fahrenheit).  Scientists sometimes use this invariant temperature 
to calibrate thermometers.  It doesn't matter whether you have 99% ice and 
1% water or vice-versa, since the temperature doesn't change during the 
transition having the 2 phases coexisting means you are at the unique 
transition temperature.

As for those citrus growers, I'm not certain, but I think what they are 
doing is trying to maintain coexistence between liquid water and ice on 
the exterior of the fruit.  As long as you have that coexistence and it 
doesn't completely freeze, the temperature there will be 32 degrees 
Fahrenheit.  Since the juices inside the fruit are solutions of sugar, 
etc. in water, their freezing point is a little lower than 32 degrees F 
(you can look up "freezing-point depression" for an explanation), so the 
water/ice coexistence on the surface would protect the fruit from freezing.

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Dr. Allan H. Harvey, Boulder, Colorado, steamdoc@aol.com