Snowflakes evoke rapturous descriptions from the poetic among us.
Under a microscope, snowflakes reveal themselves as crystalline structures in the shape of six-sided stars, interlocking prisms and delicate, branching ferns.
“I found that snowflakes were miracles of beauty; and it seemed a shame that this beauty should not be seen and appreciated by others,” said Wilson Bentley, a Vermont farmer who took some of the first known photographs of snowflakes, and whose 1931 book, “Snow Crystals,” is still a classic. “Every crystal was a masterpiece of design, and no one design was ever repeated.”
The beauty Bentley admired is the result of hydrogen bonds.
Snowflakes form after tiny water droplets freeze in clouds. The droplets become ice particles surrounded by water vapor, which condenses on the surface of the ice. When the water vapor freezes, crystals form and snowflake patterns emerge.
For Aurora Clark, however, the story of snowflakes begins even earlier with the molecular structure of water – two hydrogen atoms and one oxygen atom.
“Individual water molecules are really interesting,” said Clark, an associate chemistry professor at Washington State University. “They’re what we call polar, which means they have a partially negative end (oxygen) and a partially positively charged end (hydrogen).”
The charges attract other water molecules. Water vapor in the clouds form “what’s called a hydrogen bond,” Clark said. “Because of the geometry of water, the 3-D structure … they form little hexagons, which are the basis of snow crystals.” Micro-climates inside the clouds cause the crystals to grow at different rates, giving snowflakes their endless variations. But the six-sided symmetry in the patterns results from the underlying hexagonal structure of water.
Kenneth Libbrecht, a physics professor at the California Institute of Technology, has photographed more than 10,000 snowflakes and maintains SnowCrystals.com, a website that discusses the science of snowflakes.
He grew up in “snowflake country” on a farm outside of Fargo, N.D., but it wasn’t until Libbrecht started studying crystal growth that he got interested in how snow crystals form.
He uses the term “snow crystal” to refer to a single crystal of ice. Snowflake is a more general reference. It can mean an individual snow crystal, several stuck together or masses of snow crystals that drift down like puff-balls, Libbrecht said.
“The physics of it is kind of fascinating,” he said. “Why crystals grow into the shapes they do and why it changes so much with temperature. … It’s very complicated. It really depends on the molecular surface of the ice and how the molecules are moving around.”
When snow crystals are very small, they’re mostly in the form of hexagonal prisms. But as they grow, they sprout branches and develop complexity. Both the temperature and available water vapor influence the growth of snow crystals, Libbrecht said.
The intricate, flower- and fern-like “stellar” snow crystals that he enjoys photographing fall in temperatures between 0 and 10 degrees Fahrenheit. “You just don’t see those at other temperatures,” he said.
Humidity also affects shape. Under drier conditions, snow crystals form simpler shapes – plates and prisms. Higher humidity produces branching snow crystals.
The same dynamics come into play for frost. High humidity and very cold temperatures created the hoarfrost that Clark admired when she was a graduate student in Indiana. It hung from the tree branches like falling lace.
Midwestern winters are both humid and cold, so the hoarfrost grows slowly but can extend for several inches, Clark said. “You get these long, needlelike structures, which I think are beautiful.”
Libbrecht recommends keeping a magnifying glass handy for field studies of snow crystals.
“You can sit on the chairlift and just sit there,” he said. “Or, you can sit on the chairlift and take a look at what’s falling on your sleeve.”
The most common snowflake varieties look like sand and needles. But, “if the temperature is really cold, and you get a nice, light snowfall, and conditions are just right, you can see some really beautiful crystals,” he said.