Science Puzzle
Why Do Snowflakes Have Six Sides?
Catch snowflakes on a dark sleeve and look closely. Some are flat hexagons, some are lacy stars, some are tiny columns. Every one is different, yet the flat ones all have six sides or six arms, never five or eight.
Why do snowflakes have six sides?
The Answer
It's because of the way water molecules fit together when they freeze. In ice, the molecules link up into a framework made of rings of six, so every ice crystal starts out as a tiny six-sided prism and grows with six-fold symmetry.
A water molecule is bent, like a shallow V, and each one can grip four neighbours through weak links called hydrogen bonds. Freezing locks the molecules into the arrangement that satisfies all those links at once: a honeycomb-like framework of six-sided rings. A snowflake is one crystal of that framework, grown large enough to see.
High in a cloud, a tiny six-sided crystal grows as water vapour freezes onto it. Its six corners stick out furthest into the damp air, so they grow fastest and sprout arms. As the crystal falls, it passes through changing temperatures and dampness. All six arms meet the same conditions at the same moment, so they grow alike, but no two flakes take exactly the same path, so no two look the same. Temperature decides the style: flat plates, feathery stars, needles or columns.
The astronomer Johannes Kepler wondered about this in 1611, in a little essay he gave a friend as a New Year's present, centuries before anyone knew about molecules. In Vermont, Wilson Bentley photographed more than 5,000 snowflakes from the 1880s onwards.
The principle: Crystal structure. Water molecules freeze into a framework of six-sided rings, so ice crystals grow with six-fold symmetry, and the conditions each crystal meets decide the details.