The science of snow: How do flakes form, and why are they all unique?

Universally known to indicate that the winter season has arrived and known in the Inland Northwest to indicate that it is time to don warm boots and gloves to scrape your car before work, snow is familiar to many around the world.

But aside from being cold, white specks of ice that fall from the sky, what really is snow, and how does it form?

At the most basic level, snow is a collection of snowflakes, which is a collection of individual snow crystals.

Kenneth Libbrecht, a physics professor at California Institute of Technology who has studied snowflakes for more than two decades, said snow crystals include the “star-shaped” hexagonal kind called dendrites that people might imagine when they think of an individual snowflake. In comparison, what he calls the “puff ball” snowflakes seen falling from the sky are made up of many such snow crystals, which have partially melted and stuck together.

“A snow crystal is also called a snowflake, but a puff ball is not called a snow crystal,” Libbrecht said. “Snowflakes are kind of anything that falls out of the sky that is snowflakelike.”

The presence of tiny particles in the air is the key to snow crystal formation, which typically occurs around 10,000 feet up.

“You have to have something in the atmosphere such as pollen or a piece of dust,” said Steve Bodnar, National Weather Service meteorologist. Liquid water in the atmosphere is supercooled to below freezing, and then it will “freeze onto that piece of dust or pollen.”

Those little frozen droplets then act as the seed for a snow crystal to build on. Libbrecht, who grows snow crystals in his lab and holds the world record for making the largest snow crystal (10mm in diameter), said that snowflakes’ distinctive geometric shape begins forming at this point because of the way water molecules organize themselves when frozen.

“It soon forms a faceted prism. Material does not stick well to (the prism’s) flat surfaces, and that comes from the lattice structure of the ice crystal,” Libbrecht said. “And so, the slowest-growing surfaces sort of define the shape, and so you get a little prism – a little hexagonal prism.”

As the prism gets heavier and begins to fall, changes in the atmosphere to temperature, wind and humidity lead to unique “arms” forming on each of the hexagon’s six sides. Because the arms are all exposed to the same environment, they typically grow synchronously and symmetrically to one another.

“The final shape you see, when the thing lands on your sleeve and you can take a look at it, really just depends on the path that crystal took through the clouds,” Libbrecht said. “Each crystal is taking a different path through the clouds, and the shape of the crystal depends on the whole history of that path, and so each one ends up looking a little different.”

The largest, most photogenic dendritic snow crystals grow best at around 5 degrees Fahrenheit, said Libbrecht, while needle and column shapes are more common around 21 degrees Fahrenheit. Each crystal takes anywhere from 15 to 45 minutes to form.

Bodnar said dendritic snowflakes tend to lead to faster snow accumulations on the ground because of the increased space between them. The consistency of the snow depends on the surface temperature rather than flake shape, though.

“You can have dendrites falling when it’s 33 degrees out. You can also have them falling when it’s 15 degrees out, because it’s all about what’s going on in the atmosphere up above in those clouds more than at the surface,” Bodnar said. “So then once they fall, if it’s 33 then you’re gonna get essentially some sort of melting going on, (allowing) you to pack that snow into a better snowball, where if it’s 20 degrees out, it’s gonna stay light and fluffy – very powdery – and remain in the ice form.”

Due to cooler atmospheric temperatures, Bodnar said that almost all precipitation starts as snow, requiring small pieces of ice to form. Freezing rain and sleet are both cases where snowflakes might melt and then refreeze at different elevations as they fall.

Though humans have observed snow for centuries, Libbrecht said there are still mysteries surrounding ice crystal formation. The surface of ice has influential properties scientists don’t quite understand, and computer models have not been able to fully mimic the process of growing snow crystals, he said.

“I was surprised when I first started reading about this that people just hadn’t figured this out yet,” Libbrecht said. “You know, this stuff falls out of the sky, I just assumed everything was known. But not really.”

Libbrecht said that he always tells people “to just go outside and look and see,” and that they might find a rare snowflake shape if they are lucky.

“The more you look, the more you see,” he said. “It’s kind of fun to think about – all this stuff happening at once. All these little works of art just forming all around you, and then you just trample them and, you know, curse that you have to shovel them all.”