Does altitude change the freezing point of water?
By itself? If you put the water in a sealed, insulated container
and lofted it in a balloon? No. Altitude by itself is a measure of
how far away from the Earth's center you are, which in turn means how
strong gravity is. Gravity has very little influence on the freezing
point, because water is essentially incompressible.
However, you probably mean freezing in an open container, exposed to
the surrounding atmospheric pressure. In that case the answer is:
yes, for several reasons. First of all, the freezing point of water
increases with a decrease in the pressure applied to it. Hence the
freezing point of water will be less at high altitude and low
pressure. But this effect is small. The freezing point of water
rises a mere hundredth of a degree per atmosphere of decrease in
It is also true that water collected at high altitude, for example
water droplets in high-altitude clouds, is exceptionally pure. This
water will not freeze easily, because ordinarily water needs some
``seed'' to grow a crystal of ice around. That seed is commonly a
speck of dirt or the walls of the container, but there isn't either at
high altitude, so liquid water droplets exist up there down to
ridiculous temperatures, -40 or so I believe.
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Update: June 2012