Winter Weather

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Winter Weather

• October 17, 2007

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Winter Weather

• A winter storm is a low-pressure system that
  covers a large area and contains weather fronts.
• In the Northern Hemisphere, circulation is
  counterclockwise
• Fronts can produce rain, sleet, freezing rain, or
  snow

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Winter Cyclones

• Winter cyclones are produced by (or spin off of)
  the Aleutian and Icelandic Lows (both
  semi-permanent pressure cells)
• They can impact weather between September and May
• Most intense in January and February
• The severity of the storm can be determined by
  checking the pressure at the storms center
  anything below 1000mb is severe

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Winter Lows
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• The severity of winter cyclones depends on
• Latitude
• Distance from the ocean
• Source region
• Continentality
• Southern Italy and Nebraska are at the same
  latitude
• Topography
• Mountains alter wind patterns
• Altitude
• Influence temperature distribution

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Jet Stream

• Parks over the US during the winter
• Can change position and contour often, or stay in
  the same position for months
• Persistent winter ridges produce a warm, dry
  winter
• Persistent winter troughs produce a cold, wet
  winter and spring floods

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Sleet and Freezing Rain

• The most dangerous winter weather because no one
  takes them seriously
• Ice pellets or snowflakes that melt as they fall,
  but refreeze before hitting the surface
• Freezing rain freezes on the first thing it hits
  power lines, roads, trees, geese
• If more than 1 in of ice forms at the surface, it
  is termed an ice storm

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Freezing Rain
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Sleet
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Hail

• Hail consists of ice pellets formed in roughly
  concentric layers
• Formed when water is frozen in the atmosphere.
  The ice pellet falls and encounters water, which
  freezes to the ice pellet forming a second layer
• The size of hail is determined by the strength of
  the updraft
• Hail has more water volume, snowflakes have more
  air volume

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Hailstones
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Snow

• Snow is precipitation in the form of frozen water
  with a crystalline structure
• For powdery snow, 8-12 inches of snow will melt
  into 1 inch of water
• For wet snow, 4-6 inches of snow will melt into 1
  inch of water

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Winter Precipitation
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Drifting Snow

• Drift control used to prevent roads from becoming
  impassible
• Temporary snow fences erected to capture snow
  blowing from the west and northwest
• Rows of conifers also used

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Blizzards

• Blizzards are characterized by blowing and
  drifting snow, limited visibility, and cold
  temperatures
• Occur on the polar sides of fronts behind cold
  fronts
• Temperatures below 25F and winds above 35mph
• Blizzards are strongest when reaching the
  farthest point south in the jet stream

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• Rain into snow
• Precipitation changes from rain into snow as a
  cold front passes
• Wind changes from SSW to WNW as the front passes
• Snow into rain
• When a center of low pressure passes over an area
  before the warm front
• Snow is melted into rain
• Causes freezing rain and ice storms

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Noreasters

• Aka Northeasterns
• When trough sets up over Atlantic seaboard
• Atlantic Ocean feeds extreme snow storms along
  the coast
• Winds out of the NE
• Snow 1-4 feet deep, coastal flooding, beach
  erosion

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Cold Air Damming

• When high pressure sets up over the NE US or
  eastern Canada, it forces cold air into the
  Appalachian Mountains
• The air is too dense to rise over the mountains,
  so it becomes trapped
• This produces extreme temperatures, ice storms,
  freezing rain, and/or blizzards
• Example, Blizzard of 1993

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Cold Air Damming
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FYI

• Alcohol increases the danger of hypothermia
  (which is extremely low body temperature)
• It impairs judgment
• It causes small blood vessels in the skin to
  dilate, increasing the rate of heat loss from the
  body

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Folklore

• The wooly boogers!
• Their coloring predicts winter weather
• Hornets
• Placement of nests predicts winter weather
• Punxsutawney Phil

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