Let it Snow: Mystery Behind Many Snowfalls' Baffling Persistence Explained

Researchers from Michigan Technological University say they've cracked the code behind many snowfalls' seemingly odds-defying persistence.

In order to form, snow requires dust to which water molecules can attach and form a structure as they freeze. And yet in areas such as the Arctic where the atmosphere is impeccably clean, the snow seems limitless, leading scientists to question how the snow is able to carry on for such long stretches of time. The same question goes, albeit to a lesser degree, for places like Lake Superior, whose clouds release megatons of snow on surrounding areas.

"Within a few hours, you basically purge the atmosphere of all those particles," said Raymond Shaw, a physicist at MTU. "So how can it snow for days on end?"

Together with his colleagues, Shaw developed a model describing how ice crystals form, grow and fall, backing it up using data on Arctic clouds. The results were startling: As the number of snow crystals increased, their mass jumped by a power of 2.5. This meant that if the number of crystals tripled, the mass shot up by a factor of 16.

"Our first guess would have been that if you triple the number of crystals, you triple the mass," Shaw said. "It turns out to be a much stronger relationship than that."

Underlying all of this was the idea that ice crystals form on particles previously believed to be incapable of helping to make ice crystals.

"The key assumption we made was that there's a hidden source of ice nuclei that's always there, but they are just really, really low efficiency," Shaw said.

"The consensus in the research community has been that you need special pieces of dust to catalyze the ice. We thought, 'What if there was more stuff out there that would produce ice if you just wait long enough? Maybe when you put it in contact with a drop of water, it doesn't freeze immediately. But if you wait an hour, or two hours, it does. Our model assumes that the atmosphere is full of those really inefficient nuclei."

Such inefficient nuclei would come into play, Shaw explained, during those heavy snowfalls in which big crystals are formed.

"The mass of an ice crystal is related to its growth time," Shaw said. "The longer it's in the cloud, the bigger it will be." So, when there's an updraft that keeps crystals from falling, snowflakes that form on regular, snow-forming particles get larger and larger. During that time, many more snowflakes have a chance to form on weak nuclei."