There's a scene in Harper Lee's To Kill a Mockingbird -- one of my all-time favorite novels -- where the little girl-narrator, Scout, sees pretty white snow flakes falling and assumes the world is ending. She's never seen snow before, since it's a very rare occurrence in rural Alabama. The world didn't end then, and it's not ending now, but it's just one more bit of evidence that weather is a very wacky thing.

Unless, like Scout, we've never experienced a genuine snowfall, we probably take snow a bit for granted. It's just another form of precipitation, after all, and we have a pretty solid grasp of that particular cycle. Just for the record, snow is not frozen raindrops; that would be sleet. Under certain conditions, water vapor can condense directly into tiny ice crystals, skipping the raindrop phase altogether, and usually forming the shape of a hexagonal prism (two hexagonal "basal" faces and six rectangular "prism" faces).

But that crystal also attracts more cooled water drops in the air. Branchings sprout out from the single crystals' corners to form snowflakes of increasingly complex shapes. And yes, for all intents and purposes, no two snowflakes are shaped exactly alike, at least according to Caltech physicist Kenneth Libbrecht, who runs this Website devoted entirely to snow crystals. But there are 35 different types of snow crystals, all of which he has carefully documented.

Libbrecht usually has to create his own ice crystals in the lab, or go to more frigid climes, like Michigan or Alaska or Ontario, to make his high-resolution microscope images of snowflakes. (You can see movies of lab-based snow crystals forming here.)

Even then it's a tricky business. He has to use a small paintbrush to transfer the delicate structures to a glass slide, taking the picture with a digital camera mounted on a high-resolution microscope. All of this is done outside to keep the crystals from melting too quickly. The final images are quite striking -- so much so that in 2007, they were featured on a new 39-cent commemorative postage stamp, courtesy of the US Postal Service.

Not surprisingly, the shapes of snowflakes and snow crystals have long fascinated scientists, like Johannes Kepler, who took some time away from his star-gazing in 1611 to publish a short paper entitled "On the Six-Cornered Snowflake." He was intrigued by the fact that snow crystals always seem to exhibit a six-fold symmetry.

Some 20 years later, Rene Descartes waxed poetical after observing much rarer 12-sided snowflakes, "so perfectly formed in hexagons and of which the six sides were so straight, and the six angles so equal, that it is impossible for men to make anything so exact." He pondered how such a perfectly symmetrical shape might have been created, and eventually arrived at a reasonably accurate description of the water cycle, adding that "they were obliged to arrange themselves in such a way that each was surrounded by six others in the same plane, following the ordinary order of nature."

(The lack of a detailed explanation can be excused: it took the development of x-ray crystallography for scientists to really be able to study the shape and structure of snow crystals/flakes in any great detail.)

Libbrecht has an historical predecessor in Robert Hooke. Hooke's Micrographia, published in 1665, contained a few sketches of snowflakes he observed under his microscope -- sketched rapidly, one assumes, since the flakes no doubt melted soon after being placed under the lens, even working outdoors. If only he'd had access to Libbrecht's equipment, he wouldn't have had to do everything by hand -- and he would have appreciated the far more intricate details observable under orders-of-magnitude increases in resolution.

But nobody performed a truly systematic study of snow crystals until the 1950s, when a Japanese nuclear physicist named Ukichiro Nakaya identified and cataloged all the major types of snow crystals. (Nakaya had the bad luck to be appointed to a professorship in Hokkaido, with no available facilities for his nuclear research, so he applied his considerable skills to what was readily available: snow crystals. Now that's taking lemons and making lemonade.)

Nakaya also proved Descartes wrong in the Frenchman's assertion that no man could make anything so perfect. Nakaya was the first person to grow artificial snow crystals in the laboratory. In 1954 he published a book on his findings: Snow Crystals: Natural and Artificial. Here's what Libbrecht's Website has to say about it: "Nakaya's book offers a superb look at a scientific investigation which begins with almost nothing, and proceeds through systematic observation toward an accurate description of a fascinating natural phenomenon."

Thanks to Nakaya's pioneering work, we now know that certain atmospheric conditions, like temperature and humidity, can influence a snowflake's shape. For instance, those shapes tend to be simpler in low humidity. The higher the humidity, the more complex the shape, and if the humidity is especially high, they can even form into long needles or large thin plates.

Scientists aren't entirely sure why, but they suspect it has to do with the complex underlying physics of how water vapor molecules are slowly incorporated into the growing ice crystal -- what Descartes termed the "ordinary order of Nature." There's still a lot of mystery in that ordinariness.

That's why NASA launched the Global Snowflake Network a few years ago, a massive project that aims to involve the general public to "collect and classify" falling snowflakes. The data is being compiled into a massive database, along with satellite images, that will help climatologists and others who study climate-related phenomena gain a better understanding of wintry meteorology as they track various snowstorms around the globe. Participating students, teachers, and other interested parties now have the chance to take part in real science, and learn more about how climate, temperature and other atmospheric features combine to produce weather phenomena.

So next time snow falls in your area this winter, take a few moments from building snowmen and lobbing snowy missiles at the annoying kid down the street, and look more closely at each individual flake. You might even consider signing up with the GSN, thereby recording your observations for scientific posterity.

NOTE: This post adapted from an older post in the archives.