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