Download SNOW

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
http://www.nws.noaa.gov/om/windchill/index.shtml
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
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)
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
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
Winter Precipitation: Snow vs Rain
Images below from Lyndon State College: http://apollo.lsc.vsc.edu
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: Sleet vs Freezing Rain
Images below from Lyndon State College: http://apollo.lsc.vsc.edu
SLEET (not Hail)
FREEZING RAIN
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)
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
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)