WINTER WEATHER

• Meteorological Winter (3 coldest months): DEC-JAN-FEB

• Biggest blizzards (1888, 1993) took place in MARCH.

• SNOW: ice crystals grow at the "expense" of neighboring supercooled water droplets through Bergeron-Findeisen process.

• Ice nuclei in a saturated environment (T <= 0 C); minuscule particles (tiny clay minerals) that have a microscopic crystal lattice structure that resembles that of ice; average 1000 active ice nuclei per cubic meter at a temp of -20 C (approx. -4 F).

• Bergeron-Findeisen is a a maximum at a temp of -12 C (10.5 F), when the difference between saturation vapor pressure of liquid water and ice are at a maximum.

• Aggregation: snow crystals collide and stick together to form a snowflake (all uniquely shaped); best at 0 C, if coated by a thin layer of water.

• Ice Fragmentation: snow crystals that break up into ice fragments; can serve as additional sites for snow crystal growth.

• Riming: snow crystals become coated with soft shell of ice during descent when they pass through environment of supercooled water droplets.

• Snow Pellets or Graupel: completely coated with thicker layer of rime (obscured).

• Snow Squall: heavy, brief burst of snow (e.g., lake effect)

• Snow Shower: moderate, but brief period of snow, with spotty, light accumulations.

• Snow Flurry: very light and brief snow shower, with little or no accumulation.

• Light Snow: horizontal visibility greater than a half-mile.

• Moderate Snow: horizontal visibility greater than a quarter mile, but less than or equal to a half-mile.

• Heavy Snow: horizontal visibility less than or equal to a quarter-mile.

• The snow squall, snow shower & snow flurry fall from convective, cumuliform clouds, whereas the steadier snow falls from stratiform clouds.

Winter Weather Types

• Snow melts to rain if temp > 0 C for at least 300 m (1000 feet) of descent.

• A 1000 station study showed that there is a 95% probability of snow at a temp of 34 F and a 95% probability of rain at 42 F; the 50/50 rain/snow "gray" zone was in the temp range of 36-37 F.

• Freezing Rain: supercooled rain drops impact a surface (T <= 0 C) and produce a glaze (clear) ice.

• Ice Pellets (Sleet): frozen rain drops (clear) or complete icing of snowflakes (translucent); not to be confused with hail.

• Forecast SNOW if T-850 is <= 0 C, the 1000-500 mb Thickness is <= 540 dam, the 1000-850 mb Thickness is <= 130 dam, and the 850-700 mb Thickness is <= 154 dam.

• Forecast RAIN if all the indicators are higher than the values listed above; forecast FREEZING RAIN if the surface temp is <= 0 C.

• Forecast RAIN if the 1000-850 mb Thickness is greater than 132 dam and/or T-850 is greater than +1 C; forecast FREEZING RAIN is the surface temp is <= 0 C.

• Forecast ICE PELLETS (Sleet) if the 1000-500 mb Thickness is greater than 540 dam and/or 850-700 mb Thickness is greater than 155 dam and the 1000-850 mb Thickness is less than 129 dam and T-850 <= 0 C.

• Forecast MIXED PRECIP is indicators fall in "gray zone", such as 1000-850 Thickness in the 129-131 dam range, the 850-700 mb Thickness in the 153-155 dam range, etc.

• Note that greater (i.e., warmer) values will still produce SNOW in mountain locations and the lower (i.e., colder) values are required to produce SNOW at coastal locations.

Storm Tracks

• The most common Storm Track in Winter is the Alberta Clipper Track; fast-moving; limited moisture; most precip falls in upper right-hand (NE) quadrant; can deepen rapidly once they move off east coast.

• The Texas & Gulf/Atlantic Track produces the big, paralyzing snowstorms; are moisture laden due to source regions.

• The Jet Stream sets up the various storm tracks.

• The jet stream can led to Cold Waves, the "definition" of which varies from city to city; Chicago a drop of 20 F in 24 h with a MIN <= 5 F; New Orleans a drop of 16 F in 24 h with a MIN <= 32 F.

• Phasing: when mid-latitude polar jet stream and sub-tropical jet stream "merge" for form one trough of low pressure.

• Jet Streak: a core of enhanced jet stream winds caused by the natural temp contrast between the continents and oceans in Winter.

• Jet Streaks enhance divergence aloft, which aids the process of cyclogenesis.

• Upper-level divergence is enhanced at the right-entrance and left-exit regions of the jet streak, with the left-exit region the stronger of the two.

• A primary storm on the Texas Storm Track will give way to a secondary storm on the coast, due to enhanced baroclinic zone (i.e., strong temp gradient) and become a Nor'easter.

• Cold Air Damming: strong arctic high pressure over NE-U.S. or SE-Canada; the high directs a flow of low-level, shallow cold, dry air down the eastern foothills of the Appalachian Mountains; evaporating precip (virga) into the dry air chills the air further, helping to maintain the reservoir of cold, dense air dammed up against the mountains.

• Conveyor Belt Model (see diagram).

• Blizzard: strong winds > 30 knots (35 mph) and blowing (or falling) snow creating a visibility of less than a quarter-mile for at least 3 hours; note that falling snow is not a requirement; can be called a "Ground Blizzard" in the Great Plains.

• Snow & ice makes up 2% of all water on earth; 80% of the fresh water.

• California relies heavily on winter snow in mountains for water supply.

Human Impacts of Winter Weather

• Freezing Rain can glaze roadways.

• A burst of snow with a ground temp of approx. 35 F can melt snow and create an instant glaze on roadways; especially prone in early Winter.

• Deep snow.

• Blowing & drifting snow.

• Aircraft icing: de-icing spray; inflatable "boots"; wing heaters.

• Salting roads: the salt reduces the melting point of ice to approx. 20 F

• Ash, dirt & gravel increase traction and absorb sunlight to promote melting.

• Avalanche: snow builds up on lee side of a mountain; a cornice is especially dangerous.

• Wind Chill Chart: original chart was created by Paul Siple (1939-40) in Antarctica; is based on the freezing of exposed water in wind; uses the wind speed for a height about 33 feet above ground (the typical height of an anemometer).

• New Wind Chill Chart: developed and first used in Winter of 2001-2002; based on heat loss in humans; uses a wind speed at a height at face level (about 5 feet); there are plans to possibly add in adjustments for solar radiation (sunshine) and sky condition (amount of cloud cover) for the Winter of 2002-2003.

• Hypothermia: caused by significant heat loss; body temp drops; danger at 80 F; if wet, the heat loss can make it worse.

• Seasonal Affective Disorder (SAD): caused by less sunlight and an increase of melatonin; "Cabin Fever"; hibernation?