WASHINGTON—Your fourth-grade teacher wasn't lying when she said that no two snowflakes were alike. Yet some snowflakes look identical.
The explanation depends on how closely you look.
"Look at identical twins. When you get up closer, you start to see the small differences," said Brian Swanson, an earth and space sciences professor at Washington State University. "It all depends on how much you magnify it."
A magnifying glass is a good starting point to see similarities and differences in snowflakes, technically called snow crystals. But zoom in even more with a light microscope, such as the kind found in high school laboratories, to about 10 microns (a hundredth of a millimeter) and you'd have to sift through tons of snow crystals to find two that look identical.
The differences become even starker if you use a scanning tunnel microscope to look at the water molecules in a snow crystal. At that point, each crystal will be different because of the way water molecules stack together, Swanson said. With some crystals containing hundreds of thousands of water molecules, the chance of finding icy twins at this level is nearly zero.
They're almost like fingerprints, said physics professor Ken Libbrecht of the California Institute of Technology in Pasadena.
A snow crystal starts as a dust particle that the wind has launched into the atmosphere. Water vapor freezes around it and forms a droplet. As the frozen particle falls, it starts taking on more water vapor, which also freezes, causing it to grow.
Most snow crystals grow symmetrically in a hexagonal shape, but humidity, temperature, the wind and how long they fall all contribute to their final appearance.
More complex crystals grow fast as they fall through the clouds, and they thrive in humid and calm weather. Some sprout six arms and can reach several inches in diameter with dozens of tiny branches on each arm. Snow watchers call these crystals "stellar dendrites," which means treelike. "Sectored plate" snow crystals have complex ridges on rounder, bulkier arms. The differences can be visible to the naked eye if the snow crystals are large enough.
"Each crystal has a different path through the clouds, but ... because each arm experiences the same conditions ... the arms tend to look alike," Libbrecht said. "And since snow crystals all follow slightly different paths through the clouds, individual crystals all tend to look different."
Simpler snow crystals grow the same way, but more slowly, because they exist in low-humidity environments such as Antarctica and the Arctic. These crystals form hollow columns, tubes and needles. Put them under a microscope and many will look identical, until you start zooming in to, say, 10 microns.
But to enjoy the beauty of snow crystals, you need only a simple magnifying glass.
"Keep your magnifying glass with you all the time, because you never know when good crystals are going to strike," Libbrecht said.
(For more information, visit www.its.caltech.edu/(tilde)atomic/snowcrystals.)
HOW TO LOOK AT SNOWFLAKES:
_Any magnifying glass will do, even the fold-up kind. Test it by looking at a penny. If you can see Abraham Lincoln's face pretty well, your optics are fine.
_You don't need a fancy backdrop. A jacket sleeve or black cardboard works fine.
_A snow crystal looks best when its six arms are still growing outward into spiky or round branches. You'll have 10 minutes at best before it melts.
_ If conditions are really wet or windy, forget it. The best viewing comes during cold, quiet and light snowfalls.
_ Look for 12- and three-sided snow crystals. They're rare and still a mystery.
_ Prime temperature is 0 to 10 degrees.
_ Don't expect the snow to be white. The clear crystal reflects light, tricking our eyes into thinking it's white.
_ If you're planning a ski trip, take your magnifying glass. "It's a great icebreaker on the chair lift," physics professor Ken Libbrecht said.
(c) 2005, Knight Ridder/Tribune Information Services.
PHOTOS (from KRT Photo Service, 202-383-6099): SNOWFLAKES
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