Download - Air Masses and Fronts
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