Actually Dhama you are almost right.

For a snow flake to form you need a microscopic piece of material...a particle of dust for example.

In an experimant I saw in tv a glass vial of pure water was supercooled to well below zero, but ice did not form until an small speck of some dust was added. Then the water turned to ice almost instantaniously.

Available on Amazon for the Kindle E-Reader

All the Woes of a World by Jonathan Icknield aka The Bryster

And in my final hours - I would cling rather to the tattooed hand of kindness - than the unblemished hand of hate...