Snowstorms occur when the moisture in clouds freezes and falls to the ground, causing precipitation. The temperature at ground level must also be below freezing.
Every snowstorm begins with a precipitation cloud. These clouds are clusters of moisture that develop from evaporation or when the air blows across large bodies of water. When warm air combines with cold air or climbs the side of a mountain, the moist air inside the clouds rises. When the air temperature is below the freezing point, this rising moist air freezes in crystallized forms called snowflakes. When a lot of moisture turns into a large mass of snowflakes, the result is a snowstorm.