A cloud containing a sufficient amount of water vapor at a temperature at or below freezing and an updraft to keep the ice crystals forming are generally required for a snowy day. If the atmosphere and ground temperature remain at or below freezing, the snow can reach the ground.
Snow forms in clouds from water vapor that freezes into ice crystals and grow as they collide with one another. An updraft in the cloud provides particles for the water vapor to bond with and freeze. As the frozen particles collide, they combine and grow larger until they become heavy enough to fall from the cloud. Larger amounts of water vapor in the cloud and strong upstreams produce larger accumulations.