Freezing Rain Is Supercool Physics
Every winter, we southerners wish for snow. Sometimes, just when we think we’re going to get our wish, the conditions are slightly off and we get something weirder called freezing rain. Much of the continent has experienced this odd precipitation over the past month. It’s an icy nuisance, but it demonstrates something unusual: water dropping below 32 degrees without freezing!
Three vertical atmospheric layers are required to produce freezing rain: cold air high up and near the ground, with warm air sandwiched between. Snow forms in the uppermost cold layer. Upon falling lower, the snow encounters the warmer air and rapidly melts into liquid droplets of rain. (Without this warmer layer, the precipitation would reach the ground as snow.)
As the melted liquid droplets fall out of the warm layer and into the cold layer just above the ground, something unusual can happen. If the temperature of the ground layer of air is roughly 29-31 degrees Fahrenheit, the droplets become supercooled. This means that, while they are cold enough to freeze solid, the structure of their molecules remains liquid. How can this be?
When matter changes from one state to another, physicists say that a phase transition has occurred. Phases of matter include the familiar liquid, solid, and gas. There are also more exotic phases: plasma, Bose-Einstein condensate, superfluid, glass, and amorphous solid. Supercooled freezing rain is a strange state between liquid and solid.
The phase transition of water from a liquid to a solid is a clear change in the position of the molecules. Liquid water molecules move and flow around one another. They are all pressed together, but with no particular order; stirring them can rearrange the order easily. Solid water ice, however, is a rigid structural arrangement of atoms. This is called a crystal: trillions and trillions of atoms all line up in harmony to form a nearly perfect repeating pattern.
How does a crystal begin to form? Nucleation triggers the process. After a few atoms form into a crystal structure, the remaining atoms snap into position almost instantly. If a water droplet cools to roughly 14 degrees F, nucleation occurs spontaneously. Otherwise, at warmer temperatures, it needs some help. This can be a vibration to shake the atoms into place or contact with a solid particle.
Supercooled droplets are falling through relatively still, clear air, so nucleation isn’t occurring. Thus, they fall all the way to the ground as liquid. However, when they hit the cold ground, nucleation suddenly occurs. The water freezes instantly, often in interesting shapes.
The aftermath of freezing rain is beautiful: Clear, flowing, liquid-like ice coatings. Of course, it’s also an inconvenient and potentially dangerous mess.