The dark crystal: The secrets of snow

(Photo by Dion Ogust)

Most folks assume that water turns solid when the temperature reaches 32 degrees. In actuality, water’s solidification requires 80 calories of extra energy for each gram the size of a sugar cube. Water needs a push, an extra bit of frigid encouragement, to turn solid. Moreover, ice is not a good heat conductor, which means that it is a poor cold conductor, too, so it thickens only gradually.

To use real numbers, if the air temperature stays at an unwavering 14 degrees Fahrenheit, studies show that ice will form and grow to four inches thick in two days. That’s the minimum recommended thickness for ice fishing or other activities pursued on foot.

How much time to double that to eight inches? Not another two days, but rather a full extra week. Ice starts fast, but then takes a go-slow approach. It requires an entire month more to achieve the 15-inch thickness that can support the weight of cars.

And an odd cloud dance has to happen for snow to materialize. First, several water molecules have to collide before a potential ice structure can begin to form. A single molecule cannot freeze.

Second, if the droplets are pure water, the ice-crystal process is reluctant to get underway at all. It won’t happen anywhere near the freezing mark. As if bureaucratic red tape is gumming up the process, ice won’t form unless the temperature reaches 72 degrees below the freezing point: 40 below zero. So for ice or snow to materialize at a more reasonable and common temperature, the cloud’s droplets need a seed or nucleus around which to grow. Since air normally contains lots of tiny floating debris, this is usually no problem.

But you’d never guess what the best ice-generating specks might be. Germs! A droplet readily freezes into a crystal around a living airborne microbe, a bacterium, at any temperature below 28 degrees Fahrenheit. They’ll form around a tiny speck of floating clay (kaolinite) a bit more reluctantly, and only if it’s colder than 25 degrees Fahrenheit. And if all they have are specks of silver iodide – the compound used in cloud seeding – they’ll start to make crystals below 20 degrees Fahrenheit. But germs are the most common snowflake starters and lie at the heart of 85 percent of all flakes.

So next time you gaze at a lovely snowstorm, inform your favorite germophobe or hypochondriac that living bacteria sit shivering in most of those untold billions of flakes. Then hand him or her a snowcone or organize a catch-a-snowflake-on-your-tongue party.

Once the ice-forming process is started, more molecules join the party, and the crystal grows. It can ultimately become either a snowflake or a rough granule of ice called by the odd name graupel. A snowflake contains ten quintillion water molecules. That’s ten million trillion. Ten snowflakes – which can fit on your thumb tip – have the same number of molecules as there are grains of sand on the Earth, or stars in the visible universe.

How many flakes, how many molecules fashion the snowy landscape out your window? (That was a rhetorical question. But if you know the answer, let me know.)