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Air massesAir masses
An air mass is a relatively homogenous large mass of air in terms of temperature and moisture characteristics.
Four air masses commonly affect our region:
cP: continental polar
cT: continental tropical
mP: maritime polar
mT: maritime tropical
What do these regions have in common? Why?
Air mass modificationAir mass modification
Continental polar (cP)Continental polar (cP)
Only occurs in the northern hemisphere Only occurs in the northern hemisphere at middle and high latitudesat middle and high latitudes
More dominant in the winterMore dominant in the winter Can be extremely cold and dryCan be extremely cold and dry Generally stable (cold days, bright blue Generally stable (cold days, bright blue
skies)skies)– However, where cP air masses meet moist However, where cP air masses meet moist
air masses, the weather conditions can air masses, the weather conditions can become severebecome severe
Maritime polar (mP)Maritime polar (mP)
Source areas are over the Pacific Source areas are over the Pacific and Atlantic Oceans in the mid and Atlantic Oceans in the mid and high latitudesand high latitudes
Generally produce cool and Generally produce cool and unstable conditionsunstable conditions
Maritime tropical (mT)Maritime tropical (mT)
Originates in the tropics in the Originates in the tropics in the Atlantic and Pacific OceansAtlantic and Pacific Oceans
Pacific Ocean mT air mass Pacific Ocean mT air mass contains less moisture than the contains less moisture than the Atlantic mT air massAtlantic mT air mass– Also more stableAlso more stable
Air mass modificationAir mass modification
Since air masses move, they can Since air masses move, they can migrate over areas and inherit migrate over areas and inherit characteristics from those areascharacteristics from those areas– e.g. the Great Lakese.g. the Great Lakes
In winter cold cP or cA air can move In winter cold cP or cA air can move over the lakes and pick up moistureover the lakes and pick up moisture
The moisture is precipitated in the lee The moisture is precipitated in the lee of the lakes as snowof the lakes as snow– Lake effect snowLake effect snow
Types of frontsTypes of fronts
A front separates two different air masses.
WARM COLD
STATIONARY OCCLUDED
Cold frontsCold fronts Occurs between advancing cP or mP Occurs between advancing cP or mP
air and warmer air massesair and warmer air masses Warm air is forced to rise upwards Warm air is forced to rise upwards
against the colder denser air, against the colder denser air, forming cumulus and cumulonimbus forming cumulus and cumulonimbus cloudsclouds Cold front
symbol
Cloud types with a Cloud types with a cold frontcold front
These cloud types usually occur ahead of a cold front.
In which order would you expect them to occur as a front approaches?
Cirrus
CumulonimbusAltocumulus
Stratocumulus
BeforeBefore DuringDuring After After
TemperaturTemperaturee
WarmWarm Sharp fallSharp fall Slight fallSlight fall
Dew PointDew Point HighHigh Sharp fallSharp fall Slight fallSlight fall
WindsWinds SSWSSW ShiftShift W to NW to N
PressurePressure FallFall LowestLowest RiseRise
Identifying a cold front’s passage
Precipitation generally falls in the vicinity of the cold front.
Warm frontsWarm fronts
Occurs when warm air advances Occurs when warm air advances over colder airover colder air
Warm front symbol
Cloud types with a Cloud types with a warm frontwarm front
Cirrus
Nimbostratus
AltostratusCirrostratus
These cloud types usually occur ahead of a warm front.
In which order would you expect them to occur as a front approaches?
BeforeBefore DuringDuring After After
TemperaturTemperaturee
RisingRising RisingRising WarmWarm
Dew PointDew Point RisingRising RisingRising HighHigh
WindsWinds E to SEE to SE ShiftShift S to SWS to SW
PressurePressure FallFall LowestLowest RiseRise
Identifying a warm front’s passage
Precipitation generally falls ahead of the warm front.
Cold Fronts and Warm Cold Fronts and Warm FrontsFronts
Dry LineDry Line
Mid-latitude cyclonesMid-latitude cyclones
Migrating low-pressure cells Migrating low-pressure cells Contain converging, rising air that Contain converging, rising air that
spirals towards the center of the spirals towards the center of the system, in a counterclockwise system, in a counterclockwise directiondirection
Also called wave cyclonesAlso called wave cyclonesL
Life cycle of a mid-Life cycle of a mid-latitude cyclonelatitude cyclone
Takes 3-10 daysTakes 3-10 days Goes through 4 stagesGoes through 4 stages
– CyclogenesisCyclogenesis– Open stageOpen stage– Occluded stageOccluded stage– Dissolving stageDissolving stage
CyclogenesisCyclogenesis
Occurs where cold Occurs where cold air masses and air masses and warmer air masses warmer air masses collidecollide
Where air converges Where air converges at ground level it at ground level it must diverge aloftmust diverge aloft
Common at the Common at the polar front, the gulf polar front, the gulf coast, eastern areas coast, eastern areas of mountain barriersof mountain barriers
Open stageOpen stage
Air to the south of Air to the south of the cold air forces the cold air forces it’s way northward it’s way northward along a warm frontalong a warm front
Air from the Air from the north/west forces north/west forces its way southward/ its way southward/ eastward along a eastward along a cold frontcold front
Occluded stageOccluded stage
Cold air “catches Cold air “catches up on itself”up on itself”
When this When this happens it forces happens it forces warm air aloft with warm air aloft with only cold air belowonly cold air below
Most occluded Most occluded fronts run from the fronts run from the north to the southnorth to the south
Dissolving stageDissolving stage
Usually all warm Usually all warm air is now aloft air is now aloft and all cold air is and all cold air is at ground levelat ground level
This will This will eventually eventually dissipate causing dissipate causing air to mixair to mix
cyclogenesis
Open stage
Occluded stage