Snowflakes are one of nature's most whimsical and mysterious works of art. Though scientists have been studying the snowflake for decades, they are still stumped as to the science behind why these unique creations they look the way they do. One discovery scientists have made recently is that the presence of pollution in the atmosphere can dramatically alter the appearance of a snowflake.

Atmospheric chemist Paul Shepson of Purdue University says that pollution and other impurities in the air may contribute to whether ice crystals form in a flat plate, a spiky needle or other shape as it falls. Understanding how ice forms when it falls is extremely important for understanding our future climate, because many atmospheric chemical reactions take place on the surface of the ice (different shapes have different surface areas, affecting the rate of these reactions). Current predictions that our sea ice may be gone by the middle of the century could cause catastrophic changes to our atmosphere.

"The presence of ice and snow has a big influence on what is in the atmosphere," said Shepson. "We'd like to be able to predict how that massive change in the surface of the Earth will change the composition of the atmosphere," he added.

To properly use snowflake science to predict how climate change will affect our atmosphere, scientists first need to understand the actual formations of snowflake design, which they currently do not.

"We really don't understand why a snowflake has the shape that it has. We know really pathetically very little," said Shepson.

What they do know is that different shapes happen at different temperature ranges - for example, from freezing to about 25 degrees Fahrenheit, snowflakes tend to form plates. A little colder, and you get the needle-shaped flakes. And so on and so on.

"This has been known for about 70 years and no one has really figured out why yet," said physicist Kenneth Libbrecht of the California Institute of Technology in Pasadena.

Shepson and one graduate student recently performed an experiment to gauge the effect pollution has on snowflake. The two grew tufts of snowflakes on a string inside a very chilly chamber in a laboratory, comparing the shapes made from clean water with that of water containing acetic acid (a test substance that they expect mirrors the effects of other pollutants).

"The bottom line is that we learned the shape of snow crystals can be influenced by the presence of pollution," stated Shepson.

Libbrecht's findings suggest that impurities in the air that sticks to snowflake surface is crucial in determining the shapes that form at different temperatures. He found that snowflakes grew a remarkable 100 times faster under low pressure than normal, atmospheric pressure.

The impurities affecting snowflake shape aren't entirely man-made, he stressed - compounds produced by plants also play a role.

"One of the things I like about this impurity hypothesis is that one of the problems, for 50 years, is, how can just plain ice show all of this variation with temperature? How can you have so much variation with temperature if there's nothing there but ice? Maybe it's not just ice."

Though the evidence is preliminary, Libbrecht is setting up experiments that will test his hypothesis by growing crystals in clean air without impurities.

Looks like snowflakes serve a much bigger purpose than just looking pretty and being universally unique - they are a barometer of atmospheric activity, and hold a wealth of information that will help us better prepare for changes in our climate.

via Discovery