winter storm
A winter storm is an event 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 necessarily restricted to the winter season, but may occur in the late autumn and early spring as well. Very rarely, they may 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.
Snow
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, Canberra, Vancouver). In places where snowfall is common, such as Utica, Detroit, Montreal, Quebec City, Toronto and Minneapolis, such small snowfalls are rarely disruptive, though snowfalls in excess of 15 cm (6 in.) usually are.
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 the early and mid-1990s, and the 1993 "Superstorm" was manifest as a blizzard in most of the affected area.
Large snowstorms could be quite dangerous: a 15 cm (6 in.) snowstorm will make some unplowed roads impassible, and it is possible for automobiles to get stuck in the snow. Snowstorms exceeding 30 cm (12 in.) 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.
Snowstorms are usually considered less dangerous than ice storms. However, the snow brings secondary dangers. Mountain snowstorms can produce cornices and avalanches. An additional danger, following a snowy winter, is spring flooding if the snow melts suddenly due to a dramatic rise in air temperature. Deaths can occur from hypothermia, infections brought on by frostbite, car accidents due to slippery roads, fires or carbon monoxide poisoning due to alternative heating methods after a storm causes a power outage, or heart attacks caused by overexertion while shoveling heavy wet snow.
Wintry showers or wintry mixes
Many factors influence the form precipitation will take, and atmospheric temperatures are influential as well as ground conditions. Sometimes, near the rain/snow interface a region of sleet or freezing rain will occur. It is difficult to predict what form this precipitation will take, and it may alternate between rain and snow. Therefore, weather forecasters just predict a "wintry mix". Usually, this type of precipitation occurs at temperatures between -2 °C and 2° C (28°F and 36°F).
Freezing rain storms and ice storms
Heavy showers of freezing rain are one of the most dangerous types of winter storm. They typically occur when a layer of warm air hovers over a region, but the ambient temperature is near 0 °C (32 °F), and the ground temperature is sub-freezing. A storm in which only roads freeze is called a freezing rain storm; one resulting in widespread icing of plants and infrastructure is called ice storm.
While a 10 cm (4 in.) snowstorm is somewhat manageable by the standards of the northern United States and Canada, a comparable 1 cm (0.4 in.) ice storm will paralyze a region: driving becomes extremely hazardous, telephone and power lines are damaged, and crops may be ruined. Because they do not require extreme cold, ice storms often occur in warm temperature climates (such as the southern United States) and cooler ones. Ice storms in Florida will often destroy entire orange crops.
Notable ice storms include an El Niño-related North American ice storm of 1998 that affected much of eastern Canada, including Montreal and Ottawa, as well as upstate New York and part of New England. Three million people lost power, some for as long as six weeks. One-third of the trees in Montreal's Mount Royal park were damaged, as well as a large proportion of the sugar-producing maple trees. The amount of economic damage caused by the storm has been estimated at $3 billion Canadian.
The Ice Storm of December 2002 in North Carolina resulted in massive power loss throughout much of the state, and property damage due to falling trees. Except in the mountainous western part of the state, heavy snow and icy conditions are rare in North Carolina.
The Ice Storm of December 2005 was another severe winter storm producing extensive ice damage across a large portion of the Southern United States on December 14 to 16. It led to widespread power outages and at least 7 deaths.
In January 2005 Kansas had been declared a major disaster zone by President George W. Bush after an ice storm caused nearly $39 million in damages to 32 counties. Federal funds were provided to the counties during January 4-6, 2005 to aid the recovery process. [Presidential Disaster Declaration authorized for Kansas counties hit by January ice storm]
See also
 Comprehensive discussion of weather related terms, such as Winter Storm Warning :
* Severe weather terminology (United States)
* Severe weather terminology (Canada)
Winter storms of 2006-07
Winter storms of 2007-08
Winter Storm Warning
Winter Storm Watch
Winter Weather Advisory
Heavy snow warning
Ice Storm Warning
Snow Advisory
Cold wave
Siberian Express
References
Social & Economic Costs of Snow & Ice Storms from "NOAA Socioeconomics" website initiative
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