Answers: Supercooled water

By Jack Williams ©2015

Q: How can water that is below 32 degrees F (zero degrees C) remain in liquid form and not freeze? That is, become “supercooled”?

Matt, LaGrange, Ga.

A: The best way to begin understanding how supercooled liquid water forms is to forgot what you’ve been told about water “freezing at 32 degrees F.” This is really the temperature at which ice melts.

The statement that water freezes at 32 degrees F oversimplifies what happens as water changes among its gas (water vapor) liquid–most of the time we just call this “water”–and its solid phase, ice.

The real difference in water in its different phases is not its temperature, but the speed at which the water molecules are moving. The higher the temperature, the faster the molecules of anything move. But, molecules are not vibrating in a solid or moving around in a liquid or gas at the same speed at any particular temperature. Some are moving faster than average, others slower no matter what the temperature happens to be.

To see what this means for water, let’s look at the glass of ice water that’s on a table next to me as I write this. The glass contains water in all three phases.

Supercooled liquid water drops hitting this airplane wing formed a heavy layer of rough ice. NASA photo.

The air in the glass above the liquid water and ice contains water vapor–molecule of water molecules that are moving at an average speed that’s fast enough to overcome molecular attraction between molecules. Some of these molecules are going slow enough to remain with the liquid water if they happen to  hit it.

The molecules in the liquid water are, on the average, moving too slowly to break away from molecular attraction with neighboring molecules. Some, however, are moving fast enough to fly into the air as water vapor if they reach the top of the liquid.

The water molecules in the ice cubes are moving so slowly, on the average, that they are locked into six-sided crystals. Instead of moving around, they are vibrating.

If I put the glass of ice water in the freezer most of the water molecules will slow down enough to change into ice.

Cooling below 32 F isn’t enough to make water molecules form into ice crystals. For this they need an ice crystal template to latch on to. As the water grows colder some water molecules will naturally form into ice crystals. When the water is close to 32 F, faster moving molecules will break up many of these crystals, but in a relatively large container, such a section of an ice tray in a freezer, enough ice crystals will form and serve as templates for ice to form rather quickly.

Enough of the needed spontaneous ice crystals needed to turn small cloud drops of or even larger rain drops are less likely to form. Impurities in the water or in the air–known as freezing nuclei–that are shaped somewhat like ice crystals–speed up the process of freezing. But, if the air is very clean tiny water drops can cool to minus 40 degrees before they freeze.

(Minus 40 happens to be the temperature at which the Fahrenheit and Celsius scales are the same.  But this is just a coincidence. Both temperature scales were invented long before scientists knew that liquid water could cool to approximately minus 40 on both scales.)

However the small drops that make up clouds or falling rain are less likely to have enough ice crystal templates to freeze all of the water in the drop. If the air is clean with few freezing nuclei around, the drops can grow as cold as minus 40 degrees before the drop freezes.

All of this plus an explanation of why ice is lighter than liquid water–this is why ice floats–is explained in Chapter 4 of my AMS Weather Book: The Ultimate Guide to America’s Weather.

If you want to learn even more about the science of water a good starting point is the Explorations: Water a Unique Molecule on the supplemental Web site for the AMS Weather Book. If you want to learn a great deal about ice I highly recommend the book Ice: The Nature, the History, and the Uses of an Astonishing Substance, by Mariana Gosnell, which the Explorations page briefly describes.

This doesn’t have anything to do with supercooled water or ice, but Gosnell is the author of Zero 3 Bravo: Solo Across America in a Small Plane, a book that I think any pilot will enjoy.

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