The Science Behind How A Snowflake Forms

From a distance, snow may seem simple. But the journey of a single snowflake is anything but ordinary — and it involves chemistry, clouds and just the right atmospheric conditions.

To understand how snowflakes form, weather.com digital meteorologist Sara Tonks says it all starts inside clouds.

“Clouds are made up of water droplets and tiny little ice crystals,” she explains. “They’re actual little drops of water that are just kind of hanging out up there.”

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When temperatures drop low enough, those droplets become liquid water that’s below freezing but hasn’t turned to ice yet. But those droplets don’t freeze on their own.

“If it’s just a little water droplet hanging out up there, it’s got nothing to freeze onto,” Tonks says. What the droplets need is a tiny, jagged particle, like dust or pollen, to cling to. “Those are the nuclei of snowflakes,” she explains.

(MORE: Why It Feels So Quiet After Fresh Snow)

Once freezing begins, water vapor (the gas form of water) molecules start to freeze onto that starting bit of ice. “It starts to get bigger and bigger and bigger until it is eventually heavy enough to fall out of the cloud,” adds Tonks.

Despite how they look piled up on the ground, individual snowflakes aren’t white. “They’re actually clear,” Tonks says. Snow appears white because of how light interacts with the ice. Each flake is “so multifaceted and rough” that it reflects all visible light in every direction. “So your eyes perceive the color white,” she adds.

Temperature and humidity play a huge role in determining what a snowflake looks like. In very cold air, snowflakes tend to be tiny and powdery. When temperatures hover closer to freezing, flakes can partially melt and stick together.

(MORE: What’s The Difference Between Sleet And Freezing Rain?)

“Snowflakes can come in a variety of shapes and sizes,” Tonks explains, noting that some don’t resemble the classic snowflake at all. “They kind of look like almost prisms of a sort,” depending on how moist the air is and how cold it gets.

The reason snowflakes famously have six sides goes back to chemistry.

“Snowflakes are made up of water molecules, H2O,” Tonks says. As those molecules freeze, they bond together in a specific way. Because of how oxygen and hydrogen atoms connect, the bonds naturally form at angles that create a six-sided structure.

But not all snow is created equally.

"Man-made snow is a different shape than natural snow," she says. It’s rounder and less jagged, which is why skiers and snowboarders often prefer the real thing. "I am not a talented skier. The one time I tried it, I wiped out pretty bad, but from what I've heard, natural is the best snow out there."

While the biggest single snow crystal on record measured about four-tenths of an inch, groups of snowflakes stuck together (what Tonks jokingly calls "fellowships of the flake") can grow to the size of dinner plates under the right conditions.

So the next time snow falls, remember: Each flake carries the story of the cloud it came from, shaped by temperature, moisture, chemistry and a long, delicate journey through the atmosphere.