21.2 FRONTSChapter 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.)

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.

LIGHTNINGLightning: (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.