A winter storm is a type of precipitation in which the dominant varieties of precipitation are forms that only occur at cold temperatures, such as snow or sleet, or a rainstorm where ground temperatures are cold enough to allow ice to form (i.e. freezing rain). In temperate continental climates, these storms are not restricted to the winter season, but may occur in the late autumn and early spring. Also, there are very rare occasions that they form in summer, though it would have to be an abnormally cold summer, such as the summer of 1816 in the Northeast United States of America. In many locations in the Northern Hemisphere, the most powerful winter storms usually occur in March and, in regions where temperatures are cold enough, April.
Snowstorms are storms where large amounts of snow fall. Snow is less dense than liquid water, by a factor of approximately 10 at temperatures slightly below freezing, and even more at much colder temperatures. Therefore, an amount of water that would produce 2 cm (0.8 in.) of rain could produce as much as 20 cm (8 in.) of snow. Five centimeters of snow (2 in.) is enough to create serious disruptions to traffic and school transport (because of the difficulty to drive and maneuver the school buses on slick roads). This is particularly true in places where snowfall is uncommon but heavy accumulating snowfalls can happen (e.g., Atlanta, Seattle, London, Memphis, Canberra). In places where snowfall is common, such as Buffalo, New York, and Minneapolis, such small snowfalls are rarely disruptive, though snowfalls in excess of 15 cm (6 in.) usually are.
A massive snowstorm with heavy winds is known as a blizzard. A large number of heavy snowstorms, some of which were blizzards, occurred in the United States during the early and mid-1990s, and the 1993 "Superstorm" was manifest as a blizzard in most of the affected area.