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<< Previous   of   Next >> Courtesy of Mark Stoelinga  (click to enlarge) This type of snowflake, called an unrimed dendrite, forms at very low temperatures and has little moisture. Courtesy of Mark Stoelinga  (click to enlarge) An unrimed dendrite snow crystal (left) and a rimed snow crystal (right). The rimed snow crystal, common in Western Washington, forms when droplets of moisture attach to an unrimed crystal and then freeze. Courtesy of Mark Stoelinga  (click to enlarge) An unrimed assemblage of plates, a type of snow crystal that forms temperatures between 25 and 32 degrees. Courtesy of Mark Stoelinga  (click to enlarge) This snowflake is called a rimed needle. Needles are snow crystals that form at 21 to 25 degrees.   ADVERTISEMENT   Snowflake guide More information on snowflakes is available at a Web site operated by Kenneth Libbrecht, a professor of physics at CalTech University: http://www.its.caltech.edu/~atomic/snowcrystals/   Related Stories • Animated graphic: How Snowflakes Happen 12/17/08 • Animated graphic: Snow, Sleet and Freezing Rain 12/17/08 • That'll be $25,000 12/17/08   CONTACT THE HERALD Robert Frank, City Editor frank@heraldnet.com   Published: Wednesday, December 17, 2008 Lumpy, clumpy, crunchy: Some snow is perfect, but not ours By Bill Sheets, Herald Writer Our snow gets no respect. Often, the flakes that fall from the sky in Western Washington are not classic, symmetrical, miraculous masterpieces of crystallized art. Yes, snowflakes are here, but ours often are encrusted with lopsided, lumpy globules of frozen water. Their points are rounded, blunt or broken off. They're like the Mona Lisa with a severe case of warts. The elite, drier, fluffier snow common in the Rocky Mountains is called "powder." The snow here is often referred to as "Cascade concrete," "cement" or "crud." "I sometimes refer to it as oatmeal," said Mark Stoelinga, a snowflake expert and associate professor in the department of Atmospheric Sciences at the University of Washington. This sounds appealing compared with another type of snow we get, the one with the name that sounds like another breakfast cereal: graupel. This is Cascade crud at its ice-caked worst -- so bad it's not even considered snow any more. It's called "snow pellets." Skiiers are notorious for preferring powder to pellets, or any other type of local snow. "It's like skiing on Styrofoam," said Mark Moore, director Northwest Weather and Avalanche Center in Seattle and a longtime snow lover and skier. By contrast, with more powdery snow, skiing is so smooth it's "where you're almost like falling," he said. "It's like if you're in a hang glider, or you're floating in the wind." It isn't that we never get the powder here, Stoelinga said. It's just that it happens a lot less often than places with drier air, like the Rocky Mountains. When that kind of snow falls here, it's usually not for long, because it doesn't stay cold. "We tend to get more liquid water clouds just because of the maritime environment -- the temperatures are warmer, there's more moisture content," Stoelinga said. Snowflakes are actually tiny ice crystals. When we crunch snow under our feet, we're cracking thousands of those crystals at once and mashing them down. If it's warm enough, those crystals stick together as they melt. That gives our snow in the Puget Sound lowlands at least one advantage: It often can pack nicely into snowballs. Nice hard ones. "It hurts if you have one thrown at you, if you don't have a heavy coat on or something," said Derek Smith, 16, of Everett. Snow is more complex than it seems. There are many kinds of snow crystals, each with a different name. The simpler types, formed at warmer temperatures of about 21 to 32 degrees Fahrenheit, have self-explanatory names such as plates and needles. Columns and sector plates are found in midrange temperatures, about 10 to 21 degrees Fahrenheit. Flakes that form at zero to 10 degrees Fahrenheit. -- the intricate, sharply defined designs -- are called dendrites. Several different types of crystals can form and fall in any particular snowstorm. Sometimes the flakes run into each other and form conglomerate flakes, Stoelinga said. Or, around here, droplets of moisture attach themselves to the flakes, freeze and form what is called rime. "It's all about the personality of the storm," he said. Riming actually happens more consistently in the Cascades than it does in the lowlands, Stoelinga said. This is because storms that produce our lowland snow often come from inland, and some powder will fall, he said. Then it tends to warm up quickly afterward and it's slush in no time. "The frustrating thing around here for people who like snow, is when we get these cold air outbreaks, it's very common to get (rain) either at the start of it or at the end of it," Stoelinga said. That didn't happen with Saturday's lowland snow. It stayed cold for three days. Saturday's snow was a mix of rimed and unrimed snow, Stoelinga said. It wasn't cold enough for dendrites when the snow started falling, but the temperature quickly dropped, keeping graupel and other flakes from sticking together after they fell, he said. This gave some of the snow the feel of powder when it really wasn't. Whether the snowflakes are perfect or not, it's pretty much true that no two are alike, experts say. Some of the simpler forms, such as plates and needles, are alike in basic shape, but are microscopically different. Among the dendrites, the shapes themselves always differ at least slightly when examined close up. "I guess we can't actually prove the statement, we can only express it from a probability perspective, that's it's extremely unlikely," Stoelinga said. A useful analogy is to consider how many ways there are to arrange 15 books on a bookshelf, said Kenneth Libbrecht, a professor of physics at CalTech University in Pasadena, Calif. Libbrecht has studied snow intensively for about 10 years and authored six books on the subject. There are more than 1 trillion potential combinations for the books, Libbrecht says on his Web site, www.snowcrystals.com. Given that dendrites have more than 100 different types of features, the possibilities are astronomical. "Thus it's unlikely that any two complex snow crystals, out of all those made over the entire history of the planet, have ever looked completely alike," Libbrecht wrote on his Web site. He has set up a lab at CalTech where he creates and studies snow crystals. Libbrecht has traveled the world studying snow. He's rigged a microscope with a light and a camera, and picks up each snowflake with a small paintbrush. He places it on a slide and takes its photo. The optimum temperature for perfect flakes is about 5 degrees Fahrenheit, he said. "I spend a lot of time looking for photos of the really nice ones and discarding the ugly ones," he said. "I'm always out looking for the supermodel of snowflakes." Reporter Bill Sheets: 425-339-3439 or sheets@heraldnet.com. READER COMMENTS Be the first to comment. You must be a registered user and verify your e-mail address to post comments to blogs or articles on HeraldNet. To register, click here. To read other terms and conditions, click here. Log out

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