How does snow form?
Snow crystals form in clouds when the temperature is underneath freezing point. They are created by water droplets freezing on small ice particles. As an ice crystal drops through the cloud it bumps and knocks others and becomes a snowflake. This process of bumping others, along with a little melting and re-freezing aids the creation of their complex design. The air that the snowflake drops through has to be under freezing otherwise the snowflake will simply melt and turn into rain.