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Everyday Mysteries: Fun Science Facts from the Library of Congress

Is it true that no two snow crystals are alike?

The scientific consensus states that the likelihood of two large snow crystals being identical is zero.

The probability that two snow crystals (a single ice crystal) or flakes (a snow crystal or multiple snow crystals stuck together) will be exactly alike in molecular structure and in appearance, is very minute. And to prove otherwise would not be easy. Each winter there are about 1 septillion (1, 000, 000, 000, 000, 000, 000, 000, 000 or a trillion trillion) snow crystals that drop from the sky!

To go through all of the snow crystals produced every winter would be a daunting task. So, we rely on cloud physicists, crystallographers, and meteorologists to study snow crystals and explain to us why there are no two snow crystals alike.

First, we need to understand that not all water molecules are exactly alike. Generally speaking, water molecules have two hydrogen molecules with one ¹⁶O atom. However, not all water molecules will have this arrangement. Some water molecules will have an atom of deuterium in place of one of the hydrogen atoms and some water molecules will have an atom of ¹⁸O. Since the molecular makeup of snow crystals varies greatly from one to another, it follows that each snow crystal will be slightly different.

Furthermore, the unique and complex features of snow crystals are very much affected by unstable atmospheric conditions. Snow crystals are sensitive to temperature and will change in shape and design as they fall from the cloud and are exposed to fluctuating temperatures. To have two snow crystals or flakes with the same history of development is virtually impossible.

Back in 2007, new stories flourished that the old adage “No two snowflakes are alike,” might not be true. What these stories were highlighting is that smaller crystals with simple shapes (e.g. hexagonal prisms) may look similar in appearance. The stories also reported that it is possible for snow crystals that have a small number (e.g. 10) of water molecules to be alike (a typical snow crystal contains 1018 water molecules !). As you can tell, depending upon how you define alike or snow crystal you might find two snow crystals that are alike. However, scientific consensus still believes that it is very unlikely for two larger complex snow crystals to be identical in molecular structure and appearance.

Related Web Sites

Bentley Snow Crystal Collection of the Buffalo Museum of Science - Digital library of Bentley’s original glass slide snow crystal negatives. Passages from Bentley’s notebook, a bibliography of Bentley’s papers, and notes on the photographic process are included.
Cryosphere - The cryosphere is that portion of the Earth's surface where water is a solid form, usually as snow or ice, including sea ice, freshwater ice, snow, glaciers, and frozen ground (or permafrost). The National Snow and Ice Data Center sponsors this Web site which has features such as “Cold Facts: Earth's Snow, Ice, and Frozen Soils” and “State of the Cryosphere.”
Electron Microscope Unit Snow Page: Snow Crystal Site - This Web site contains snow crystal images that were obtained by using a low temperature scanning electronic microscope at the Beltsville Agricultural Research Center. The site includes links to other sites as well as a publications list.
Snowcrystals.com - “This site is all about snow crystals and snowflakes -- what they are, where they come from, and just how these remarkably complex and beautiful structures are created, quite literally, out of thin air.”
Wilson A. Bentley: The Snowflake Man. Jericho Historical Society - See Museum link http://bentley.sciencebuff.org/collection.asp to view a selection of Bentley’s snow crystal images and take a virtual tour of the museum. Also of interest are full text articles about or by Bentley under the Resources Link.