IN HIGH And mid-latitudes, blizzards are some of the most widespread and hazardous of weather events. They are most common in Russia and central and northeastern Asia, northern Europe, Canada, the northern United States, and Antarctica. It is likely that climate change will give rise to changes in the number, severity, and geographical occurrence of blizzards. Although it is common for the term blizzard to be employed to refer to any disruptive winter storm, there is a more precise scientific usage. Blizzards are a winter phenomenon occurring when snow is blown along the ground surface by strong winds. In different countries, official definitions of blizzard conditions vary according to: high wind speed, high wind chill values, low visibility, the presence of falling or blowing snow, and the length of time the conditions persist. Although no particular temperature threshold is associated with these definitions, blizzard conditions may produce extreme wind-chill values through a combination of low temperatures and high wind speeds.
It is possible for blizzards to occur in conditions of clear skies when no snow is falling if conditions are conducive to the movement of existing surface snow, called ground blizzards. In many storms in continental interiors, it is not uncommon for little new snow to be associated with a blizzard, due to a lack of moisture associated with Arctic and Antarctic air masses in winter. In such circumstances, blizzard conditions are largely the result of winds blowing the existing snow cover. However, blizzards in some regions (for example, those in Western Europe and the Asian coast of the North Pacific, and "nor'easters" in the northeastern United States) are characteristically accompanied by heavy snowfall.
Blizzards are produced by strong winds: katabatic winds and those generated by steep sea-level pressure gradients associated with storms in high and mid-latitudes latitudes during winter. A single storm can occur over large areas of a continent and a severe blizzard may persist for a week or more. A blizzard that struck Saskatchewan, Canada, in 1947 lasted 10 days, burying a train in a snowdrift a kilometer long. The most severe blizzards occur in Antarctica, with winds exceeding 93.2 mi. (150 km.)/hour; at some Antarctic stations blizzard conditions occur on over half the days annually.
Historically, high death tolls have been associated with the most severe blizzards. A spring blizzard, in the United States in 1888, killed more than 400 people, and 277 died in a storm in 1993. In addition to the risks resulting from high wind chill values and exposure, blizzards generate other hazards. Whiteouts are often associated with blizzards, producing dangerous travel conditions. The blowing snow, limited visibility, absence of shadows, and lack of contrast between objects can cause a loss of depth perception and conditions in which even nearby objects may be rendered invisible. The persistent winds associated with blizzards may cause severe damage to buildings, and can block transportation links and bury struc tures in massive snowdrifts. Outdoor activities may come to a standstill. The resulting economic disruption can be extensive. Widespread deaths of domestic animals have occurred due to exposure and when sources of feed are cut off by blizzards. A reported 130,000 head of livestock died in Inner Mongolia as a result of a blizzard that began on New Year's Eve in 2000. However, very disruptive winter storms may not qualify as blizzards (for example, those involving high snowfalls, but without one or more other defining characteristics).
There are significant inter- and intra-annual variations in blizzard frequency and severity that make statistical analysis of past trends problematic. Detailed studies are limited. Nonetheless, at least in North America, historical evidence points to a decline in the frequency and severity of blizzards on the Canadian prairies and a significant decline in the frequency of
An increase in the vigor of winter storms in a warmer atmosphere may result in more frequent severe blizzards.
blowing snow conditions in the Canadian Arctic. On the other hand, there is evidence for the occurrence of stronger blizzards along Russia's Pacific coast.
It may be reasonable to assume that in a warmer world there would be fewer blizzards. However, the occurrence of a blizzard is dependent on a specific combination of physical and meteorological factors. A systematic variation in one or more of these, as influenced by future climate change (for example, storm intensity, shifting storm paths, wind velocity, ambient temperature, the amount of snowfall, and the amount and condition of snow on the ground), may affect the number, intensity, and geographical distribution of blizzards.
An increase in the vigor of winter storms in a substantially warmer and energetic atmosphere may result in severe blizzard conditions becoming more frequent. A warmer climate may also be conducive to greater weather extremes. Regional differences in temperature variations in response to climate change (such as between ocean and continents, or between polar and mid-latitudes) may reduce or enhance temperature contrasts and thereby affect the frequency and severity of storms.
There is evidence from some regions that winter storms have increased in intensity; however, some modeling and empirical studies suggest a decrease in the frequency of winter storms. There may be considerable regional variation in the response to climate change (for example, potentially fewer blizzards in North America and more frequent and more severe storms in Western Europe). Storm tracks may also shift. Forecasts are speculative, as there is still a need to verify the conclusions derived from empirical studies of past blizzard patterns and projections from climate models.
SEE ALSO: Climate Change, Effects; Ice Ages; Ice Albedo Feedback; Ice Component of Models; Snowball Earth; Weather.
BIBLIOGRAPHY. Michael Allaby, Blizzards (Facts on File, 2003); B.D. Lawson "Trends in Blizzards at Selected Locations on the Canadian Prairies," Natural Hazards (v.29/2, 2003).
MlCHAL J. BARDECKI Ryerson University
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