Jennifer Ouellette examines our understanding of the snowflake:

The higher the humidity, the more complex the shape, and if the humidity is especially high, they can even form into long needles or large thin plates. Scientists aren’t entirely sure why, but they suspect it has to do with the complex underlying physics of how water vapor molecules are slowly incorporated into the growing ice crystal — what Descartes termed the "ordinary order of Nature." There's still a lot of mystery in that ordinariness.

Cheryl Murphy explains why freshly fallen snow looks white: