Snow What?

January 20, 2010 06:00 PM

Pam Smith, Farm Journal Seeds & Production Editor
It's snow joke. No two snowflakes are alike. But do you know why? Ice chemist Travis Knepp, a doctoral candidate in chemistry at Purdue University, explains that it has to do with the chemistry that occurs on ice surfaces.
"On the surface of all ice is a very thin layer of liquid water,” says Knepp. "That's why ice is slippery. When you slip, you're not slipping on the ice, you are slipping on that thin layer of water."
This thin, or quasi-liquid, layer of water exists on the top and sides of a snow crystal too and it influences the general shape the snow crystal will take as temperature and humidity change. "Even if the temperatures are well below the freezing point of water, you still have this very thin layer of water that exists as a liquid form,” he notes.
"The crystal's shape is ultimately determined by the environment in which it is grown. Since each snow crystal follows a different path as it falls in the atmosphere, each one effectively grows under different conditions,” Knepp says.
For example, the sides of a crystal growing at 27 to 32 degrees Fahrenheit expand much faster than the top or bottom, causing it to take on a platelike structure. However, between 14 and 27 degrees Fahrenheit, crystals look like tall, solid prisms or needles.
"As you increase the humidity, you'll get more branching,” he observes. Snow crystals transition to other shapes as temperature and humidity change. "The bottom line is that the thickness or the presence of this really thin layer of water is what dictates the general shape the snow crystal takes.
"Much like twins separated at birth, they both have the same starting point and very similar genetics, though they live different lives, have different experiences, and can ultimately become very different people,” says Knepp.
This is more than just gee-whiz work. Understanding these frosty flakes is helping Knepp and his colleagues understand ozone depletion and what it means to both humans and the plants they grow.

You can email Pam Smith at

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