How snowflakes get their distinct and various shapes

Large, star-shaped snowflake with many symmetrical branches on each point, on blue background. — John Entwistle caught this snowflake on January 13, 2019, and wrote: “The beauty of an individual snowflake that fell on the Jersey Shore, New Jersey.” Read more about snowflakes below.

No doubt, exquisite crystalline snowflakes capture the imagination. But what is the science behind their formation, and is it true there are no two snowflakes alike?

First of all, the temperature and humidity of the atmosphere determine the shapes of snowflakes. And snowflakes form in the atmosphere when cold water droplets freeze onto dust particles. Finally, depending on the temperature and humidity of the air where the snowflakes form, the resulting ice crystals will grow into a myriad of different shapes.

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Graph: temperature on X axis, humidity on Y axis, with different kinds of snowflakes. — In fact, it’s the effects of temperature and humidity that determine snowflake formation. Image via National Weather Service/ NOAA.

Wilson Bentley

Wilson Bentley (1865–1931) from Jericho, Vermont, was the first person to capture photographs of snowflakes through a microscope attached to a camera. His collection of over 5,000 images – no two alike – introduced many people to the astounding diversity of snow crystals.

A man in a winter coat and a hat standing in front of a large, old-fashioned camera setup on a table outside. — Wilson “Snowflake” Bentley, who photographed over 5,000 snowflakes between 1885 and 1931. Image via Wikimedia Commons (public domain).

Nine snowflakes in shapes ranging from a flat hexagon to complex 6-branched flakes. — Snowflakes by Wilson Bentley showing a range of shapes. Image via Wikimedia Commons (public domain).

Classification systems

In 1951, scientists from an organization now called the International Association of Cryospheric Sciences (IACS) devised a classification system that characterized snowflakes into 10 basic shapes. Kenneth Libbrecht, professor of physics at the California Institute of Technology, in his guide to snowflakes at SnowCrystals.com, provides this chart of 35 types of snow crystals. They range from ice fragments to rods to the much more complex fern-like dendritic flakes we are all familiar with.

Chart with 35 drawings of different shapes from flat to rods to six-sided fernlike flakes. — View larger. | Here’s a chart from Kenneth Libbrecht’s SnowCrystals.com website that shows 35 types of snowflakes. Image via Kenneth Libbrecht. Used with permission.

How snowflakes form from water vapor

Libbrecht has made extensive observations of how water molecules get incorporated into snow crystals. In his research, he observed that the most intricate snowflake patterns form when there is moisture in the air. Snowflakes produced in drier conditions tend to have simpler shapes.

According to Libbrecht’s research, temperature also has a large effect on the formation of snowflakes. Snowflakes formed in temperatures below -7.6 degrees Fahrenheit (-22 degrees C) consist primarily of simple crystal plates and columns. Meanwhile, snowflakes with extensive branching patterns are formed in warmer temperatures.

Be sure to visit Libbrecht’s website, SnowCrystals.com. It has a wealth of information about snowflakes and includes short videos of lab-grown snowflakes forming.