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Question Is it true that no two snow crystals are alike?

Answer

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

Winter scenes: Snowflakes.  ca. 1920-1950. Theodor Horydczak, photographer. Prints & Photographs Division, Library of Congress.

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!

Pileated woodpecker in the great December 2009 snowstorm of the Mid-Atlantic states.   Dr. Igor Smolyar, photographer.  NOAA Photo Library.

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.

Studies among the Snow Crystals. Wilson Bentley, “The Snowflake Man,” 1902.  NOAA Photo Library.

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.

High-resolution images show snowflake complexity. Tim Garrett, University of Utah, photographer. National Science Foundation Multimedia Gallery.

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.

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