21.2 FRONTS Chapter 21 – Weather Cooler air is denser and often does not mix with warmer air… Thus a FRONT FORMS !

VOCABULARY  Front – Boundaries between air masses of different densities  Cold Front – When cold air overtakes warm air (lifts it like a wedge)  Squall Line – line of heavy thunderstorms; form ahead of fast cold fronts  Warm Front – warm air overtakes cold air (warm air rises as a gentle slope  Stationary Front – forms when 2 air masses move (parallel to each other) very slow or not at all  Occluded Front – fast moving cold front that completely overtakes a warm front, lifting it from the ground  Polar Front – The boundary near mid latitudes where cold polar air meets warm tropical air (esp. over water)

WAVES…  Large waves are bends that develop along the polar front  Waves are the beginnings of low-pressure storm centers called midlatitude cyclones. ( area of low pressure and rotating wind that moves toward the rising air of the central low pressure system.) counterclockwise  They last several days and move easterly as they spin counterclockwise from the Pac. to Atl. coast.

FOUR STAGES OF FRONTS… 1. Stationary Front (cold/warm air masses moving in opposite directions), 3. Occluded Front (cold air lifts the warm air off the ground-most intense part of storm) 2. Wave (bulge of cold air “jumps ahead” of the front) 4. Dissipating (eventually storm loses energy)

ANTICYCLONES  Anticyclones: clockwise due to air sinking and flowing outward from high pressure center…brings dry air because sinking air does not cause cloud formation and can cause drought.

SEVERE WEATHER

THUNDERSTORMS Thunderstorms: rain, thunder, lightning, strong winds  Happens in three stages: 1. Cumulus – warm, moist air rises and forms cumulus cloud (updraft). 2. Mature - cumulonimbus forms and can produce torrential rain and hailstones. Air dragged downward by precipitation (downdraft) 3. Dissipating – strong downdrafts stop rising currents, supply of water vapor decreases.

LIGHTNING Lightning: (seconds ÷ 3 = how many kilometers away the lightning is)  Discharge of electricity (spark) from clouds in an attempt to equalize electrical charges. Upper cloud (+), charge and Lower cloud (-) charge.  Air is heated, expands rapidly, and produces thunder as a result.

HURRICANE  Most destructive storm that develops over tropical oceans, has winds >120 km/h that spiral in toward the intensely low pressure center…causing huge storm surges.  Most hurricane deaths are caused by drowning.  Warm, moist air over ocean rises rapidly, condenses, latent heat released in large amounts which strengthens the rising air.  Thick cumulonimbus clouds spiral upward around a very low pressure center and cause the center winds to increase speeds. The center is clear, sinking air.  How do we rate them? The Saffir-Simpson scale (uses several factors: pressure, wind speed, storm surge) Categories are from 1-5

HURRICANE

TORNADOS  Are destructive, rotating columns of air  VERY high wind speeds up to 400 km/h.  If it touches the ground, it follows a haphazard (wandering) path.  Forms when a thunderstorm meets high horizontal winds and causes a narrow, funnel- shaped extension that may or may not touch the ground.

MOST COMMON IN TORNADO ALLEY. MOST DEATHS ARE FROM BEING TRAPPED IN COLLAPSING BUILDINGS AND FLYING DEBRIS.