A perfect snowflake. Credit: Courtesy of John Holyoke

No two snowflakes are exactly alike, but some certainly stand out more than others.

Snowy days are upon us in Maine, and social media feeds are sure to flood with snapshots of picture-perfect snowflakes. Such snowflakes might seem magical, but they are a result of a careful balance of weather conditions.

The snowflake forming process is more complicated than most people might expect, said Donald Dumont, warning coordination meteorologist at National Weather Service in Gray. Meteorologists keep a close eye out for perfect snowflakes, too — but not for the reasons that you might think.

“This is actually something we look at when we’re trying to figure out the fluff factor,” Dumont said. “Those perfect snowflakes people really like, they’re fluffier and they add up a lot faster. Your nice, fluffy snow is typically from your perfect snowflakes.”

The perfect snowflake starts forming the same way that all less photogenic snowflakes do. Snow forms when water vapor up in the atmosphere comes in contact with particles in the air, like dust, air pollution or salt from the ocean. That contact causes the water vapor to freeze into a crystal structure, the exact pattern of which depends on the temperature of the air and the humidity high up in the clouds.

The intricate shape of a single “arm” of the snowflake is determined by the atmospheric conditions experienced by the entire crystal as it falls, James Sinko, meteorologist at the National Weather Service in Caribou, said.

“A crystal might begin to grow arms in one manner, and then minutes or even seconds later, slight changes in the surrounding temperature or humidity causes the crystal to grow in another way,” Sinko said. “You will never see two snowflakes exactly the same because each snowflake experiences different atmospheric conditions on its way down resulting in different shapes or looks.”

All snowflakes have six sides because of the way that water molecules are shaped and join together, but there are some 35 different types of crystals that can form depending on the exact atmospheric conditions, from slender six-sided needles to chunky hexagonal plates. The picture perfect snowflakes that look like they were cut out of paper generally fall into a category of crystals known as dendrites.

Scientists are still parsing out exactly why the crystals form the way they do with certain conditions, Dumont said. They do know that there is a “sweet spot” for dendrite formation between -12 and -18 degrees Celsius up in the clouds, coupled with high humidity and low wind.

“Those perfect dendrites come down but if it’s windy they crash into each other and break apart,” Dumont said. “You need a relatively light wind, you need it to be very moist and you need the cloud to be in the sweet spot for the temperature.”

There is no surefire way to judge whether the atmospheric conditions are just right when you’re standing on the ground waiting for the snow to fall, Dumont said.

“In the end if you want to see beautiful snow crystals, you have to look up,” he said. “What’s going on above you is what drives it, not what’s going on where you’re standing.”