A blizzard (or winter hurricane) is a severe winter storm condition characterized by low temperatures, strong winds, and heavy blowing snow. Blizzards are formed when a high pressure system, also known as a ridge, interacts with a low pressure system; this results in the advection of air from the high pressure zone into the low pressure area.

[edit] Definition

According to Environment Canada, a winter storm must have winds of 40 km/h (25 mph) or more, have snow or blowing snow, visibility less than 500 ft (about ¹⁄₁₀ mile), a wind chill of less than −25 °C (−15 °F), and that all of these conditions must last for 3 hours or more before the storm can be properly called a blizzard.

In the United States, the National Weather Service defines a blizzard as sustained 35mph (56 km/h) winds which lead to blowing snow and cause visibilities of 500ft or less, lasting for at least 3 hours. Temperature is not taken into consideration when issuing a blizzard warning, but the nature of these storms is such that cold air is often present when the other criteria are met.^[1] . Temperatures are generally below 0 °C (32 °F).

Many European countries, such as the UK, have a lower threshold: the Met Office defines a blizzard as "moderate or heavy snow" combined with a mean wind speed of 30 mph (48 km/h) and visibility below 650 feet (200 m).

When there are blizzard conditions but no snow falling, meteorologists call this a ground blizzard because all the snow is already present at the surface of the earth and is simply being blown by high winds. Ground blizzards require large expanses of open and relatively flat land with a sufficient amount of accumulated and loosely packed, powdery snow to be blown around.

[edit] Whiteouts

Although the word is commonly used to describe heavy snow and high winds, this is not a true "whiteout". Real "whiteouts" occur mostly in the Arctic and Antarctic during the spring, when snow is still deep on the ground and there is lots of daylight and surprisingly calm weather and excellent visibility. Whiteouts occur when rays of sunlight are bounced in all directions between bright white clouds, especially a thin layer of overcast and bright snow or ice. Clean snow and ice reflects nearly 85% of incoming light. Falling snowflakes, suspended fog droplets or ice particles in the air would make conditions even worse. In a true whiteout, neither shadows, nearby objects, landmarks, nor are clouds discernible. All sense of direction, depth perception and even of balance may be lost. Land and sky seem to blend, and the horizon disappears into a white nothingness. Whiteouts trick pilots and travelers into believing down is up and thinking far is near.

[edit] Economic impact of blizzards

Like all severe weather events such as hurricanes, droughts and floods, blizzards can be disruptive to local economies. This is especially the case when blizzards hit in localities in generally warmer climates where snowfall is infrequent. In cities that do not have snow removal equipment, traffic and commerce can be brought to a stand still for days and in some cases weeks. The economic impact ranges across industries, from lost productivity in companies because people cannot get to work, parents must stay home with children due to school closings, airport closures, product delivery delays and the actual cost of snow removal.^[2]

[edit] See also

• Great Blizzard of 1888
• Halloween Blizzard
• Lake effect blizzard
• Saskatchewan blizzard of 2007
• Schoolhouse Blizzard

[edit] References

1. ^ NOAA - National Weather Service
2. ^ [1]

[edit] External links

Wikimedia Commons has media related to: Blizzard

• Dr Richard Wild Website dedicated to the history, news and facts about heavy snow and blizzards.
• Digital Snow Museum Photos of historic blizzards and snowstorms.
• Blizzards Photo Gallery
• Environment Canada's definition of Blizzard
• Farmers Almanac List of Worst Blizzards
• Severe Winter Weather Events Excerpts from The Canadian Weather Trivia Calendar
• http://www.ussartf.org/blizzards.htm
• http://content.scholastic.com/browse/article.jsp?id=4912