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 0.8 in (20 mm) of rain could produce at least 8 in (20 cm) of snow. Two inches (5 cm) of snow 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., Dallas, Atlanta, Seattle, London, Dublin, Canberra, Vancouver and Las Vegas). In places where snowfall is common, such as Utica, Detroit, Denver, Ottawa, Montreal, Quebec City, Chicago, Syracuse, Buffalo, Columbus, Pittsburgh, Toronto and Minneapolis, such small snowfalls are rarely disruptive, because of effective snow and ice removal by municipalities, greater usage of snow tires and drivers being more accustomed to winter conditions. Snowfalls in excess of 6 inches (15 cm) are usually universally disruptive.
A massive snowstorm with strong winds and other conditions meeting certain criteria is known as a blizzard. A large number of heavy snowstorms, some of which were blizzards, occurred in the United States during 1888 and 1947 as well as the early and mid-1990s. The snowfall of 1947 exceeded two feet (0.61 m) with drifts and snow piles from plowing that reached twelve feet (3.7 m) and for months, temperatures did not rise high enough to melt the snow. The 1993 "Superstorm" was manifest as a blizzard in most of the affected area.
Large snowstorms could be quite dangerous: a 6 in (15 cm) snowstorm will make some unplowed roads impassable, and it is possible for automobiles to get stuck in the snow. Snowstorms exceeding 12 in (30 cm) especially in southern or generally warm climates will cave the roofs of some homes and cause the loss of power. Standing dead trees can also be brought down by the weight of the snow, especially if it is wet or very dense. Even a few inches of dry snow can form drifts many feet high under windy conditions.
