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What would cause a snow day in Greece?

Sometimes a snowstorm strikes a location that's usually snow-free. When that happens, for some reason air masses are not behaving normally. Usually an atypical snow is fun for the people who live there, especially since everything usually gets shut down — including schools!

Blizzards

A blizzard is distinguished by certain conditions:

  • Temperatures below –7°C (20°F); –12°C (10°F) for a severe blizzard.
  • Winds greater than 56 kmh (35 mph); 72 kmh (45 mph) for a severe blizzard.
  • Snow so heavy that visibility is 2/5 km (1/4 mile) or less for at least three hours; near zero visibility for a severe blizzard.

Blizzard in Washington D.C.

A blizzard obscures the Capitol in Washington, DC.

Formation

Blizzards happen across the middle latitudes and toward the poles, usually as part of a mid-latitude cyclone. Blizzards are most common in winter, when the jet stream has traveled south and a cold, northern air mass comes into contact with a warmer, semitropical air mass ( Figure below ). The very strong winds develop because of the pressure gradient between the low-pressure storm and the higher pressure west of the storm. Snow produced by the storm gets caught in the winds and blows nearly horizontally. Blizzards can also produce sleet or freezing rain.

Satellite image of snow and blizzard over the East Coast in 2010

Blizzard snows blanket the East Coast of the United States in February 2010.

Lake-Effect Snow

In winter, a continental polar air mass travels down from Canada. As the frigid air travels across one of the Great Lakes, it warms and absorbs moisture. When the air mass reaches the leeward side of the lake, it is very unstable and it drops tremendous amounts of snow. This lake-effect snow falls on the snowiest metropolitan areas in the United States: Buffalo and Rochester, New York ( Figure below ).

Satellite image of lake-effect snow

Frigid air travels across the Great Lakes and dumps lake-effect snow on the leeward side.

Summary

  • Blizzards are often part of a mid-latitude cyclone where the jet stream brings cold air into contact with warm moist air.
  • The difference in pressure between the air masses brings about strong winds.
  • Cold polar air absorbs moisture as it travels over the Great Lakes and then dumps it as snow downwind to create lake-effect snow.

Explore More

Use this resource to answer the questions that follow.

https://www.youtube.com/watch?v=KFK7kXBLwy0

  1. How are blizzards the same as snowstorms?
  2. What types of air masses meet and what do they do to form a snowstorm?
  3. When does a snowstorm become a blizzard?
  4. When do blizzards usually happen?
  5. Where do blizzards usually occur in the U.S.?
  6. How can you survive a blizzard?

Review

  1. Under what circumstances does a blizzard form?
  2. What causes lake-effect snow?
  3. What is a blizzard?

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