Ominous clouds over the Arctic Ocean.
Credit: NOAA Climate Program Office, NABOS 2006 Expedition
For snow to form, there has to be stuff in the atmosphere — microbes, specks of dust — for water molecules to freeze on and then form ice crystals. But in the pristine Arctic, where the atmosphere is very clean and the ocean is covered in ice, it can sometimes paradoxically snow for days on end.
Researchers at Michigan Technological University in Houghton set out to investigate the mystery of where snow in the Arctic comes from, and how it can fall so persistently in the region.
"Within a few hours, you basically purge the atmosphere of all those particles," Raymond Shaw, a physicist at Michigan Technological University, said in a statement. "So how can it snow for days on end?" [Winter Wonderland: Images of Stunning Snowy Landscapes]
It turns out that atmospheric particles that were thought not to play a role in the formation of ice crystals may actually be key drivers of snowfall in the Arctic, a new study by Shaw and his colleagues finds.
The team studied data on Arctic clouds, and developed models to investigate the characteristics of ice crystals as they form, grow and fall. They discovered a surprising relationship between the number of ice crystals that fall over the Arctic, and their mass.
"Our first guess would have been that if you triple the number of crystals, you triple the mass," Shaw explained. "It turns out to be a much stronger relationship than that."
The researchers found that tripling the number of ice crystals increased the mass by a factor of 16. This means that the more crystals there are, the bigger they will be — by a lot — the researchers said.
The results suggest there may be other atmospheric particles that can form ice crystals, albeit less efficiently.
"The consensus in the research community has been that you need special pieces of dust to catalyze the ice," Shaw said. "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."
These inefficient nuclei may also explain why some snowflakes are bigger during heavy snowfalls.
"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."
The researchers are unsure what the weak nuclei in the atmosphere are, or where they come from, but they are now searching for clues in lab experiments.
"By assuming they are there, we got this mathematical prediction that fits with the experimental data," Shaw said. "So there's indirect evidence that these inefficient nuclei are there. This could be a solution to the mystery."
The detailed findings were published in the July edition of the journal Geophysical Research Letters.