You've likely been told no two snowflakes are alike -- that each is as individual as a human fingerprint. Yet, if you've had the chance to closely examine snowflakes, some snow crystals do look like others. What's the truth? It depends how closely you look. To understand why there's dispute about snowflake similarity, start by understanding how snowflakes work.
How Snowflakes Form
Snowflakes are crystals of water, which has the chemical formula H2O. There are multiple ways water molecules can bond and stack with each other, depending on the temperature, air pressure, and concentration of water in the atmosphere (humidity). Generally the chemical bonds in the water molecule dictate the traditional 6-sided snowflake shape. One a crystal starts forming, it uses the initial structure as the basis to form branches. The branches may continue to grow or they can melt and reform depending on conditions.
Why Two Snowflakes Can Look the Same
Since a group of snowflakes falling at the same time form under similar conditions, there's a decent chance if you look at enough snowflakes, two or more will look the same to the naked eye or under a light microscope. If you compare snow crystals at the early stages or formation, before they have had a chance to branch out much, the odds that two of them might look alike is high. Snow scientist Jon Nelson at Ritsumeikan University in Kyoto, Japan, says snowflakes kept between 8.6ºF and 12.2ºF (-13ºC and -11ºC) maintain these simple structures for a long time and can fall to Earth, where it would be hard to tell them apart just looking at them.
Although many snowflakes are six-sided branched structures (dendrites) or hexagonal plates, other snow crystals form needles, which basically look much like each other. Needles form between 21°F and 25°F and sometimes reach the ground intact. If you consider snow needles and columns to be snow "flakes", you have examples of crystals that look alike.
Why No Two Snowflakes Are Alike
While snowflakes might appear the same, at a molecular level, it's very nearly impossible for two to be the same. There are multiple reasons for this:
- Water is made from a mixture of hydrogen and oxygen isotopes. These isotopes have slightly different properties from each other, altering the crystal structure formed using them. While the three natural isotopes of oxygen don't significantly affect crystal structure, the three isotopes of hydrogen are distinctly different. About 1 in 3,000 water molecules contains the hydrogen isotope deuterium. Even if one snowflake contains the same number of deuterium atoms as another snowflake, they won't occur in the exact same places in the crystals.
- Snowflakes are made up of so many molecules, it's unlikely any two snowflakes are exactly the same size. Snow scientist Charles Knight with the National Center for Atmospheric Research in Boulder, Colorado estimates each snow crystal contains around 10,000,000,000,000,000,000 water molecules. The number of ways these molecules can arrange themselves is nearly infinite.
- Each snowflake is exposed to slightly different conditions, so even if you started with two identical crystals, they wouldn't be the same as each by the time they reached the surface. It's like comparing identical twins. They might share the same DNA, but they are different from each other, especially as time passes and they have unique experiences.
- Each snowflake forms around a tiny particle, like a dust mote or pollen particle. Since the shape and size of the starting material isn't the same, snowflakes don't even start out alike.
To summarize, it's fair to say sometimes two snowflakes look alike, especially if they are simple shapes, but if you examine any two snowflakes closely enough, each will be unique.