It starts very high in the sky, where temperatures are always well below freezing. One molecule of water adheres to a tiny ice crystal, and then another. They pile on in the shape of hexagons until, eventually, the snowflake is so heavy it falls to the ground.

The air is often purer in winter, largely free of pollen, dust and volatile organic compounds, the tiny particulates that trigger air pollution warnings in the summer. Without these free-floating particles, supercooled water vapor has nothing to latch onto or freeze upon.

Wintertime air masses aren’t only supercooled — they’re also supersaturated, meaning there’s more water in the air than normally possible. The humidity is greater than 100 percent, a physical state that doesn’t last long because Mother Nature likes balance. It’s in that unstable, supersaturated state that water crystallizes into snowflakes.

The water droplets freeze onto tiny ice crystals that make up winter clouds. Other supercooled water droplets pile on. Most droplets are less than a five hundredth of an inch across.

Water is two hydrogen atoms and one oxygen. The three atoms combine nicely into a Mickey Mouse-shaped molecule. When six molecules latch together, it creates a hexagon. Eventually, the accumulating hexagons create a fully fledged snowflake. Each hexagon contains a lot of empty space, which makes ice less dense than water. Frozen water molecules take up 9 percent more space than their liquid counterparts. It’s why ice floats.

A typical snowflake might contain 1,000,000,000,000,000,000, or a quintillion, water molecules. That, coupled with the seemingly infinite number of ways in which they can combine, makes it safe to say no two snowflakes are exactly alike.

Flakes fall at one to four mph, or walking pace. With a storm cloud a few miles up, most flurries drift for 30 minutes to an hour before they accumulate on the ground. Snowflakes are generally the size of a dime or less in diameter but once in a while can reach a few inches across. That’s generally when the air is closest to 32 degrees with high humidity. A number of factors — including temperature, dew point, wind speed and even the atmosphere’s electrical structure — ultimately determine just how big a flake can get. On Jan. 28, 1887, a rancher in Fort Keogh, Mont., supposedly saw snowflakes “larger than milk pans,” or 15 inches in diameter.

Much of our early knowledge of snowflakes came from Wilson Alwyn “Snowflake” Bentley. Bentley became the first to photograph a snowflake in 1885 when he affixed a microscope to a camera. Since then, scientists have been replicating his work, documenting the shapes of ice crystals in myriad conditions. Whether needles, plates, dendrites or prisms, each snowflake is beautiful and special.