Image credit: Purdue University photo/Shepson Lab
Snowflakes, we have seen, are beautiful and diverse but they are not inert byproducts of cold clouds. In fact, some scientists believe that snowflakes, and the specific shapes they form, could have a significant impact on our atmosphere's ability to clean itself.Researchers at Purdue University have been growing snowflakes in the laboratory, using a refrigerator-sized chamber, to gain a better understanding of the ways temperature, humidity, and atmospheric chemistry influence their shape. They have found that increases in temperature and especially humidity have a direct effect on crystal formation.
Image credit: ViaMoi/Flickr
How a Snowflake Forms
Snowflakes begin as a small nucleus in the atmosphere. A droplet of water vapor surrounds a tiny particle of dust or other particulate and freezes.
As the nucleus bounces around the atmosphere, water vapor diffuses onto it, causing it to grow in size. As bumps and irregularities form, they reduce the distance vapor must diffuse and, as a result, attract more condensation particles. This causes them to grow into branches and the same process causes sub-branches to form.
When smoother crystals form, faceting outpaces branching to produce plate-like snow crystals.
The Role of Humidity
Travis Knepp, a doctoral candidate at Purdue University who is conducting the research, explained that:
On the surface of all ice is a very thin layer of liquid water...Even if you're well below the freezing point of water, you'll have this very thin layer of water that exists as a liquid form. That's why ice is slippery. Whenever you slip, you're not slipping on ice, you're slipping on that thin layer of water.
This quasi-liquid layer is also present on the surface of snow crystals and it plays a key role in crystal formation. Knepp's research has found that at temperatures between 27-32 degrees Fahrenheit, the quasi-liquid layer, which is highly sensitive to temperature, allows crystals to expand rapidly, leading to plate-like shapes. Between 14 and 27 degrees Fahrenheit, crystals take on a prism or needle shape.
It is humidity, however, that has the most dramatic effect on snow crystal formation. As the humidity increases, the quasi-liquid layer thickens and more branching occurs.
Image credit: Muffet/Flickr
Impact on Polar Ozone Levels
Knepp's adviser, Paul Shepson, believes this research can help understand the thinning of ground-level ozone in the Arctic and Antarctic.
Snow, Shepson explained, has a powerful influence over the lower atmosphere. The water of the quasi-liquid layer reacts with the gasses near the earth's surface, creating chemical emissions that reduce levels of ozone.
The greater the surface area of the ice crystals, the more reactive they are. Increased humidity, Knepp's research has found, produces more branched crystals, which have a greater surface area than their plate-like cousins. This more reactive snow could have a significant impact on ground-level ozone.
Though ozone, a major component in smog, is toxic to humans and other life in high concentrations, it is vitally important for the atmosphere's natural cleaning process. Shepson commented that "as the impact of emissions from human activities continues to grow, we need to be able to understand the impact of global average ozone."
Developing a more precise knowledge of snow and ice—and how it is changing—is an important part of that research.
The Unbelievable World of Snowflakes (Slideshow)
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