Air Masses and Air Masses and FrontsFronts

Air MassAir Mass Large unit of air in which Large unit of air in which

temperature and moisture temperature and moisture conditions are uniform conditions are uniform at a given at a given altitude.altitude.

Moves separately from Moves separately from surroundingssurroundings

temperature and moisture temperature and moisture conditions determined by conditions determined by SOURCE REGIONSOURCE REGION of air mass of air mass

Properties of source Properties of source region:region:

Moisture:Moisture: continental “c”, maritime “m”continental “c”, maritime “m” determined by underlying surface determined by underlying surface

(ocean or land)(ocean or land)

Thermal:Thermal: Polar “P”, Tropical “T”, Polar “P”, Tropical “T”,

Equatorial “E”, Arctic “A”Equatorial “E”, Arctic “A” determined by latitudedetermined by latitude

Combination:Combination:

cPcP mPmP mTmT cTcT cA (or cAA)cA (or cAA) mEmE

Air Masses that affect U.S. weather:Air Masses that affect U.S. weather: cPcP: : cold dry air from cold dry air from

Canada; High pressureCanada; High pressure

cAcA: : from Arctic Ocean from Arctic Ocean region; extremely coldregion; extremely cold

mPmP, , PacificPacific: : cool, cool, moist air moist air mP, mP, AtlanticAtlantic: : cool, moist, cool, moist, but often but often no effect due to no effect due to prevailing windprevailing wind

mTmT: : from Gulf and from Gulf and CaribbeanCaribbean: : hot, wet airhot, wet air

mTmT, , PacificPacific: : slightly slightly cooler and driercooler and drier

cTcT: hot dry air; : hot dry air; persistentpersistent

Air masses…Air masses…

modify the local weather of places modify the local weather of places they invadethey invade

are modified by the places they are modified by the places they invadeinvade

FrontFront: the boundary between : the boundary between two unlike air massestwo unlike air masses unlike in temperature, moisture, or unlike in temperature, moisture, or

bothboth

named according to motion of named according to motion of the cold airthe cold air

Cold frontCold front

Cold frontCold front Cold air is advancingCold air is advancing 20 mph20 mph leading edge is steep (1:40)leading edge is steep (1:40) unstable along front: warm air unstable along front: warm air

rises over cold air rapidlyrises over cold air rapidly short duration (hours)short duration (hours) heavy precipitation in narrow band heavy precipitation in narrow band

behind frontbehind front followed by clear weatherfollowed by clear weather Symbol : Symbol :

Cold front animation

Warm FrontWarm Front

Warm frontWarm front Cold air is retreatingCold air is retreating 10 mph10 mph leading edge is not steep (1:150)leading edge is not steep (1:150) warm air gently climbs over cold air warm air gently climbs over cold air

(not as unstable as cold front)(not as unstable as cold front) duration (days)duration (days) stratus and nimbostratus cloudsstratus and nimbostratus clouds light precipitation in wide band light precipitation in wide band

ahead of frontahead of front Symbol: Symbol:

Warm front animation

Stationary frontStationary front

no movement of air massesno movement of air masses

precipitation at the front: precipitation at the front: not much on warm side not much on warm side stratus clouds and steady precipitation stratus clouds and steady precipitation

on cold side on cold side

Weather map symbol :Weather map symbol :

Occluded frontOccluded frontOccluded front occurs when a cold front Occluded front occurs when a cold front

overtakes a warm frontovertakes a warm front

In occluded fronts, warm air wedge is In occluded fronts, warm air wedge is lifted off of ground; clouds are high, lifted off of ground; clouds are high, then they lower and thicken into then they lower and thicken into middle and low clouds; precipitation middle and low clouds; precipitation occursoccurs

Symbol : Symbol :

Occluded fronts are associated with Occluded fronts are associated with storms called “wave cyclones”storms called “wave cyclones”

Cyclogenesis: the Cyclogenesis: the development of a wave development of a wave

cyclonecyclone

Wave cyclone: Wave cyclone: migrating center of low pressuremigrating center of low pressure causes storms in the midlatitudes causes storms in the midlatitudes Form along polar frontForm along polar front

Favorable Favorable conditionsconditions::

““Trough” of low Trough” of low pressure pressure between two between two Highs along Highs along polar frontpolar front

converging airconverging air unstableunstable

1.1. Early stageEarly stagesurface convergence; lifting around surface convergence; lifting around

LowLow

2. Open Stage2. Open Stage

3. Occluded Stage3. Occluded Stage

4. Dissolving Stage4. Dissolving Stagewarm air mass completely cut off from warm air mass completely cut off from

ground; no more upliftground; no more uplift