How snowflakes get their intricate shapes

January 13, 2025
Above: University of Utah researchers test instrumentation called Differential Emissivity Imaging Disdrometer, or DEID, which measures hydrometeor mass, size and density of snowflakes, at Red Butte Canyon. This equipment is used in groundbreaking snowflake research Utah’s mountains.

Snowflakes are like letters from the sky, each crystal a note describing the atmosphere as it falls to the ground. They float effortlessly, but their creation is one of nature’s most complicated physics feats.

From stars to needles to amorphous globs, scientists are demystifying a snowflake’s complex construction — showing how factors such as temperature can influence their shape. Some researchers have already observed how a warming world can drive structural changes, including flakes that melt quicker, fall faster and gravitate toward specific shapes.

“Snowflakes are far more varied and interesting than we had previously imagined,” said Tim Garrett, an atmospheric physicist at the University of Utah.

The science of snowflake shapes

Tim Garrett

The creation of all snowflakes begins with liquid water droplets in a cloud. As the temperature dips below freezing, some cloud droplets begin to freeze around dust particles in the sky and form hexagonal crystals. All snowflakes are six-sided because water molecules bond with one another in a hexagonal lattice.

A crystal begins to grow by absorbing water vapor from the surrounding air. Other liquid droplets evaporate, adding more water vapor that the crystals can tap into to grow larger. As the crystals get bigger and heavier, they start to fall.

In the 1930s, Japanese physicist Ukichiro Nakaya — who famously described snowflakes as “letters from heaven” — created the first artificial snowflake and found that different snowflakes form under different conditions.

But why do certain shapes appear at different temperatures? Growing snowflakes in his lab, Libbrecht uncovered processes that help explain this decades-long mystery.

For example, at different temperatures, flat, smooth surfaces — called facets — can appear around the crystal on certain sides. Imagine an even glossy surface like on a diamond face but on ice.

Water molecules have a hard time sticking to these flat surfaces because there are less available chemical bonds to connect to. As a result, these facets act like shields and prevent crystals from growing in certain directions.

If these smooth surfaces are on the top and bottom (called basal facets) of the crystal, the snowflake is more likely to grow as a column or needle. If they are set up around the sides of the hexagon (called prism facets), then the snowflake is more likely to grow as a plate.

But our warming world is also influencing how snowflakes — including the most common ones — form.

“That’s all going on at once. It takes about 100,000 droplets to make a good sized snowflake,” said Ken Libbrecht, a physics professor at the California Institute of Technology and snowflake consultant for the movie “Frozen.” The process can take about 30 to 45 minutes.

Read the full Washington Post article which features three-dimensional animations of snowflake architecture.