From Technology Review:
Snowflakes ought to be hexagonal. So why are triangular ones so often observed?
The beautiful six-fold symmetry of snowflakes is the result of the hydrogen bonds that water molecules form when they freeze.
But snowflakes can form other shapes too when the growth of the crystal is perturbed on one side. In theory, diamonds, trapezoids and other irregular shapes can all occur. And yet the one most commonly observed (after hexagons) is the triangle. The puzzle for is why? What process causes deformed snowflakes to become triangles rather than say squares or rectangles?
Today, Kenneth Libbrecht at the California Institute of Technology in Pasadena provides an answer. Libbrecht, you may remember, has built an amazing snowflake machine to study the formation of these remarkable tiny crystals.
Their growth and shape, he says, is governed by two processes: the diffusion of water molecules through the air and the molecular dynamics on the surface of the crystal and it is the former that explains triangular crystal formation…
[continues at Technology Review]