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Winter Weather

Winter Weather. Cold, a major killer. Kills about 700 in U.S. annually Wind Chill: an apparent temp: stronger winds = lower WC values Applies to exposed skin/cooling rate Uses a human face model Skin tissue resistance/heat loss model Heat removed from skin surface

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Winter Weather

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  1. Winter Weather • Cold, a major killer. Kills about 700 in U.S. annually • Wind Chill: an apparent temp: stronger winds = lower WC values • Applies to exposed skin/cooling rate • Uses a human face model • Skin tissue resistance/heat loss model • Heat removed from skin surface • Sweat evaporates faster • Does not mean that objects will freeze if the actual temperature is above freezing • Updated in 2001

  2. http://www.nws.noaa.gov/om/windchill/index.shtml

  3. Lake-Effect Snow • A Fair weather event • Occurs after the passage of a mid-latitude cyclone • Areas downwind of the relatively warm water bodies can receive incredibly heavy snow while other areas have improving weather

  4. Lake-Effect Snow • Location is determined by wind speed and direction • Intensity is determined by several factors • Stability: lake temperature vs air temperature • The larger the difference, generally the heavier the snow • Depth of lake determines how fast and whether lake freezes over- deeper lake will produce more snow (overturning- density of water)

  5. Lake-Effect Snow • Intensity (continued) • Cloud Depth: height of subsidence inversion • The deeper the cloud (higher inversion) the heavier the snow • Fetch: distance (and time) air is over the lake • Longer distance/time means more moisture and heavier snow • Wind Shear: change of wind direction with height • Little or no directional shear means heavier snow • Topography • Frictional convergence (wind slows over land) • Orographic lifting, if hilly

  6. Winter Precipitation • Overrunning: relatively warm, moist air is lifted over cold air at and just above the surface • Much of the cold-season precipitation at our latitude is produced in this way • Precipitation type in large part depends upon the depth of the cold air near the surface • Four primary types • Snow • Sleet • Freezing Rain • Rain

  7. SNOW - entire layer from cloud to ground below freezing, or only a shallow layer slightly above freezing RAIN - thick layer at and above the ground above freezing (snow from B-F process melts as it falls) Winter Precipitation: Snow vs RainImages below from Lyndon State College: http://apollo.lsc.vsc.edu

  8. SLEET (not Hail) a) Relatively thick warm (above freezing) layer melts falling snow b) Relatively thick cold (freezing) layer re-freezes rain into small balls of ice (sleet) FREEZING RAIN a) Relatively thick warm (above freezing) layer melts falling snow b) Relatively thin cold (freezing) layer ensures liquid rain reaches ground, freezing on contact- very dangerous Winter Precipitation: Sleet vs Freezing RainImages below from Lyndon State College: http://apollo.lsc.vsc.edu

  9. Long-Range Winter Forecasting, Key Figures • Winter Weather Forecasting/Teleconnections • Analogs, ENSO • NAO • Key Figures • 16.4, 16.6, 16.19, 16.24, 16.30 • Ch. 17 Teleconnections (links above)

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