Snow crystals form when tiny supercooled cloud droplets (about 10 mm in diameter) freeze. These droplets are able to remain liquid at temperatures lower than -18C (0F), because to freeze, a few molecules in the droplet need to get together by chance to form an arrangement similar to that in an ice lattice; then the droplet freezes around this "nucleus." Experiments show that this "homogeneous" nucleation of cloud droplets only occurs at temperatures lower than -35C (-31F). In warmer clouds an...