The freezing point of water and air pressure

Dear Tom,
The boiling point of water varies with air pressure. What about the freezing
point?
Russ Scott, DeKalb
Dear Russ,
Changes in air pressure do affect the freezing point of water, but the
effects are vanishingly small.
Dr. Christopher Grayce, Professor of Chemistry at the University of
California (Irvine), explains, "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 pressure."
That is, the difference is air pressure values between sea level and the
farthest upper reach of the Earth's atmosphere, say 500 miles aloft,
provides a freezing temperature increase of only 0.01 degree. For all
practical purposes, the freezing point remains unchanged.