Blizzard (meteorology)

See also: Blizzard

The blizzard comes from the German language and was used in first in a daily newspaper of a small town of the Minnesota to the the United States. It indicates two conditions different from very low visibility in snow:

  • a powerful and prolonged Snowstorm which combines low TempĂ©rature S with very strong Vent S charged with flakes of Neige reducing the visibility considerably.

  • powder mill of very fine snow fallen lately, but not at the time of the phenomenon, cold temperatures. This type of blizzard can last of the days in the very exposed places (in this case the english-speaking speak about )

This phenomenon is particularly fears with the Canada and in the north of the the United States. The blizzards largely reduce the visibility sometimes until a few meters. The people who are not with the shelters can easily lose themselves in the storm.

Blizzard with storm

When a depression in intensification moves towards a more or less stationary Anticyclone, the Gradient of Atmospheric pressure becomes very tight between these two systems. The winds which are proportional to this gradient, increase on the area where the depression moves. If the whole occurs in winter during a Snowstorm, the strong winds will raise snow which fall to give powder mill. The latter reduces the visibility and forms Congère S. Moreover, wind cooling is important, increasing the risk of Engelure S. These three elements combined (snow, powder mill, wind cooling) give the first definition of a blizzard.

This type of extreme snowstorm is common in the North-East of the United States, is Canada and the Canadian Prairies.

Blizzard of surface

When it does not fall from snow but that accumulated on the ground is very powdery, it can be easily puffed up by the wind. Similar conditions with that of a snowstorm can then occur. This kind of situation will occur when a strong anticyclone intensifies behind a depression. Still there the gradient of pressure is important and the strong winds. Snow can be left by the storm or it can be accumulated before. In general, there is no cloud cover.

The configuration of the ground is crucial in their formation or their dissipation. One needs a relatively flat ground and without obstacles. The presence of trees, in particular conifers, from shrubs or another vegetation the moindrement dense one will block the displacement of snow and will attenuate the powder mill. Moreover, it is necessary that the temperature is very cold without to have been above the freezing point so that snow is not hardened and very fine crystal of ice. This is why this type of blizzard is frequent in winter in the Arctique, the the Antarctic, the Canadian Prairies, the Grandes American Plains, in Siberia and in the north of the China.

Types

Three behaviors of snow are possible in such a blizzard:
  1. horizontal Advection: strong winds moving the snow from one place to another at long distances in flat ground. In this case, the visibility can be null on the ground but be excellent with a few meters above this one.
  2. vertical advection: a certain vertical movement with the synoptic scale exists in the atmosphere which makes that snow is raised higher by the wind. One then sees the formation waves of powder mill being able to reach 500  meters thickness. Visibility varying between these waves.
  3. convectif-mechanics mixture: if the atmosphere is unstable, one with the formation of rollers of convection similar to those being formed at the end of the wings of a plane. Snow then follows the owner of the rollers and the winds can raise of them enormous quantities in little time. In these situations the snow-drifts can bury houses and make breathing impossible.

Criteria of warnings

It is considered that the visibility must be of less 0,5  km with winds of 60  km/h lasting more than three hours so that a storm is classified like a blizzard in the United States according to the National Weather Service
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