Blizzards

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 structures [Page 128]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).

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