Zoom: From Atoms and Galaxies to Blizzards and Bees: How Everything Moves by Bob Berman Page A

from liquid to solid requires eighty calories of energy for each gram of ice the size of a sugar cube. Water reaching thirty-two degrees Fahrenheit isn’t enough to do the trick, though. Water needs a push, an extra bit of frigid encouragement, to turn solid. Moreover, ice is not a good heat conductor, which means it is a poor cold conductor, too, so it thickens only gradually. To use real numbers, if the air temperature stays at an unwavering fourteen 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 fifteen-inch thickness that can support the weight of cars.
    Resembling souvenir snow globes, the scene in Fairbanks can look Christmassy even in May and September, for the city has just three snow-free months. But here and everywhere else, an odd cloud dance has to happen for snow to materialize. Water droplets don’t simply freeze just because the temperature falls below thirty-two degrees Fahrenheit. 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 under way 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 seventy-two degrees below the freezing point. Forty below zero.1 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 twenty-eight degrees Fahrenheit. They’ll form around a tiny speck of floating clay (kaolinite) a bit more reluctantly and only if it’s colder than twenty-five 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 twenty degrees Fahrenheit. But germs are the most common snowflake starters and lie at the heart of 85 percent of all flakes.2
    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 snow cone 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 fashioned the snowy landscape I was observing as I drove east? It numbed the brain.
    The white surface stretching off into the distance was of course cold, but even beneath it the ground is permanently frozen here. This permafrost is ubiquitous north of the Arctic Circle, just over a hundred miles from Fairbanks. In the southern two-thirds of Alaska it’s common yet spotty. It may not start until a depth of thirty feet in some places, while a few yards away the permafrost lies at the surface.
    Residents have no choice but to build their homes, roads, pipelines, and schools on this permafrost. The results are often disastrous. A drive along many Alaskan roads reveals houses sagging