Let it snow!
Many of the snowflakes you see in store advertisements or greeting cards are actually inaccurate renderings. The snowflakes are often incorrectly depicted with five or eight sides. The molecular building blocks of snowflakes are water molecules. There is only a certain way that water molecules can fit together, usually in a six-cornered shape. Water molecules occasionally form ice crystals with three or 12 sides — either half or double the usual number — but never five or eight. (1)
Dr. Kenneth Libbrecht, professor of physics and chairman of the physics department at Caltech University is a snowflake enthusiast. He provides a great explanation as to how snowflakes form.
"To see why snowflakes look like they do, consider the life history of a single snow crystal. The story begins up in a cloud, when a minute cloud droplet first freezes into a tiny particle of ice. As water vapor starts condensing on its surface, the ice particle quickly develops facets, thus becoming a small hexagonal prism. For a while it keeps this simple faceted shape as it grows. As the crystal becomes larger, however, branches begin to sprout from the six corners of the hexagon (this is the third stage in the diagram at right). Since the atmospheric conditions (e. g. temperature and humidity) are nearly constant across the small crystal, the six budding arms all grow out at roughly the same rate. While it grows, the crystal is blown to and fro inside the clouds, so the temperature it sees changes randomly with time. But the crystal growth depends strongly on temperature (as is seen in the morphology diagram). Thus the six arms of the snow crystal each change their growth with time. Because all six arms see the same conditions at the same times, they all grow about the same way. The end result is a complex, branched structure that is also six-fold symmetric. Note also that since snow crystals all follow slightly different paths through the clouds, individual crystals all tend to all look different". (2)
Activity for Kids
If you have a magnifying glass you can go snowflake watching the next time that it snows. You can test to see if your magnifying glass is good enough by looking at back of a penny. If you can see Abe Lincoln sitting in the chair, then your magnifier is good enough to see some of the bigger flakes. Unfortunately, the smaller flakes are more difficult to see without a microscope. Be aware that different weather conditions breed different snowflakes. There are two peak snowflake temperatures: right below freezing and then five degrees Fahrenheit.
Black construction paper makes a good collecting surface, since it's easier to see the crystals against a dark background. But you can also do fine just looking at your sleeve. Remember, however -- many snowfalls bring nothing but small, grainy snowflakes that look essentially like white sand. On such days, there isn't much worth looking at. If it's snowing, take a quick glance at the crystals on your sleeve. If they look nice, then grab the kids and go outside to give everyone a look!
Click here for more activities.
For more information
SnowCrystals.com - This site is all about snow crystals and snowflakes -- what they are, where they come from, and just how these remarkably complex and beautiful structures are created, quite literally, out of thin air.
text by Kenneth Libbrecht ; photography by Patricia Rasmussen
(1) Hamilton, J. (2009) What's wrong with this snowflake?. NPR. Retrieved from http://www.npr.org/templates/story/story.php?storyId=121827582.
(2) Libbrecht, K. SnowCrystals.com: Frequently Asked Questions. Retrieved from http://www.its.caltech.edu/~atomic/snowcrystals/faqs/faqs.htm.
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