Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: 24hourprof , 5:41 PM GMT on December 19, 2012
Whoa Nellie! I guess I caused a stir by announcing that 32 degrees Fahrenheit is not "freezing." To repeat, only pure water freezes at 32 degrees (at a pressure of one atmosphere). Moreover, finding pure water is rather difficult. Yes, even distilled water contains some impurities (contamination by particles in the air, etc.). Even the water from the reverse-osmosis system that my wife and I use regularly for cooking and drinking isn't pure.
Craig Bohren, now a retired professor from Penn State, performed an experiment which I duplicated for my textbook. I took an empty tuna can, greased it with oil, and then placed drops of tap water on the can (see photograph on the left below). Please note that I first boiled the water in an attempt to remove any dissolved air. At any rate, I placed the can in my freezer (about -11 degrees Celsius, which roughly equals 12 degrees Fahrenheit) and waited ten minutes...tick, tock, tick, tock...
(Left) Water drops on the bottom of a tuna can. (Right)After ten minutes in my freezer (-11 degrees Celsius), four drops resisted freezing. Courtesy of A World of Weather: Fundamentals of Meteorology
I removed the can, and, lo and behold, there were several drops that did not freeze (see photograph on the right above). Back into the freezer. A couple more drops froze. Even after another ten minutes, two drops refused to freeze. I can only deduce that the drops lacked freezing nuclei. I plan to repeat the experiment using reverse-osmosis water. I'm expecting more drops to resist freezing.
And so it is with tiny water drops that exist in high, cold clouds. Despite the temperature being well below 0 degrees Celsius, they resist freezing because they are bereft of freezing nuclei. At temperatures below -40 degrees Celsius, however, all bets are off, and the tiny drops spontaneously freeze.
A short blog, I admit, but I hope I presented sufficient empirical evidence that 32 degrees Fahrenheit is not the freezing point of water.
Here endeth the lesson.
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