Chapter 8Chapter 8Global Weather SystemsGlobal Weather Systems

Global Weather SystemsGlobal Weather Systems

LowLow--latitudes latitudes –– Hadley Cell CirculationHadley Cell CirculationWet near the equatorWet near the equatorDry near 20Dry near 20--30°N and 2030°N and 20--30°S30°SPeriods of wet and dry in betweenPeriods of wet and dry in betweenEasterly Winds (NE & SE Trades)Easterly Winds (NE & SE Trades)

HighHigh--latitudes latitudes –– Polar HighsPolar HighsDry conditions Dry conditions –– polar desertspolar desertsEasterly Winds (Polar Easterlies)Easterly Winds (Polar Easterlies)

MidMid--Latitude Weather SystemsLatitude Weather SystemsCycloneCyclone –– LowLow--pressure systempressure system

Rising air, precipitationRising air, precipitationRelatively small, compact weather systemRelatively small, compact weather system

AnticycloneAnticyclone –– HighHigh--pressure systempressure systemDescending air, clear skiesDescending air, clear skiesRelatively large, expansive weather systemRelatively large, expansive weather system

“weather” caused by the interaction of cyclone and “weather” caused by the interaction of cyclone and anticycloneanticyclone

Usually see interactions of cyclones and anticyclonesUsually see interactions of cyclones and anticyclones

Air MassAir Mass –– large body of air with fairly uniform large body of air with fairly uniform temperature and moisture characteristics temperature and moisture characteristics –– 1000s of 1000s of km acrosskm across

Air masses are labeled by their source region and also Air masses are labeled by their source region and also based on thermal and moisture characteristics:based on thermal and moisture characteristics:

1. Thermal1. Thermal•• Tropical (T) Air Mass Tropical (T) Air Mass –– warmwarm•• Polar (P) Air Mass Polar (P) Air Mass –– coldcold

2. Moisture2. Moisture•• Continental (c) Air Mass Continental (c) Air Mass –– drydry•• Maritime (m) Air Mass Maritime (m) Air Mass -- moistmoist

Air masses are labeled by their source region:Air masses are labeled by their source region:

1. Polar Air Masses1. Polar Air Masses (Polar Highs)(Polar Highs)•• Arctic (Arctic (AA) Air Mass) Air Mass•• Antarctic (Antarctic (AAAA) Air Mass) Air Mass

2. Equatorial Air Mass2. Equatorial Air Mass (Equatorial Oceans)(Equatorial Oceans)•• Equatorial (Equatorial (EE) Air Mass) Air Mass

Seldom affect midSeldom affect mid--latitude weatherlatitude weather

Air Mass NameAir Mass Name ThermalThermal Moisture.Moisture.Continental Arctic (Continental Arctic (cAcA)) Very ColdVery Cold Very DryVery Dry

Regions near North and South Poles (Regions near North and South Poles (cAAcAA as well)as well)Continental Polar (Continental Polar (cPcP)) ColdCold DryDry

Northern Continental InteriorsNorthern Continental InteriorsMaritime Polar (Maritime Polar (mPmP)) CoolCool MoistMoist

MidlatitudeMidlatitude OceansOceans

Continental Tropical (Continental Tropical (cTcT)) WarmWarm DryDrySubtropical DesertsSubtropical Deserts

Maritime Tropical (Maritime Tropical (mTmT)) WarmWarm MoistMoistWarm Oceans near 30°N and 30°SWarm Oceans near 30°N and 30°S

Maritime Equatorial (Maritime Equatorial (mEmE)) Very WarmVery Warm Very MoistVery MoistWarm Oceans near the EquatorWarm Oceans near the Equator

What happens when air masses collide?What happens when air masses collide?

When air masses collide…..When air masses collide…..

FrontFront –– boundary between air masses having different boundary between air masses having different thermal and/or moisture characteristicsthermal and/or moisture characteristics

Cold Front:Cold Front: when a cold air mass collides with a warm air when a cold air mass collides with a warm air mass a the surfacemass a the surface

Cold FrontCold Front

Cold air is more dense, so the warm air is forced to rise Cold air is more dense, so the warm air is forced to rise along the frontalong the front

Lifting is strong Lifting is strong –– cumuloformcumuloform cloudsclouds

Thunderstorms are common along a cold frontThunderstorms are common along a cold front

Cold FrontCold Front

When air masses collide…..When air masses collide…..

FrontFront –– boundary between air masses having different boundary between air masses having different thermal and/or moisture characteristicsthermal and/or moisture characteristics

Cold Front:Cold Front: when a cold air mass collides with a warm air when a cold air mass collides with a warm air mass a the surfacemass a the surface

Warm Front:Warm Front: when a warm air mass collides with a cold air when a warm air mass collides with a cold air mass at the surfacemass at the surface

Warm frontWarm front

Warm Front

Cold air is more dense, so it is difficult to force it to riseCold air is more dense, so it is difficult to force it to rise

Lifting is weak Lifting is weak –– stratiformstratiform cloudsclouds

Light, continuous rains are common along a warm frontLight, continuous rains are common along a warm front

Warm FrontWarm Front

Stationary FrontStationary Front –– boundary between warm air and cold air boundary between warm air and cold air where there is no movementwhere there is no movement

Lifting is extremely weakLifting is extremely weak

Light, intermittent rains are common along a stationary Light, intermittent rains are common along a stationary frontfront

It will become a warm/cold front if there is movement (or it It will become a warm/cold front if there is movement (or it may dissipate)may dissipate)

Stationary FrontStationary Front

Occluded frontOccluded front –– occurs when a cold front overtakes a occurs when a cold front overtakes a warm frontwarm front

Warm fronts move more slowly than cold frontsWarm fronts move more slowly than cold fronts

Lifting is mixed as warm air is pushed aloftLifting is mixed as warm air is pushed aloft

A combination of rainfall types occur along an occluded A combination of rainfall types occur along an occluded frontfront

Will dissipate rapidly when the warm air is liftedWill dissipate rapidly when the warm air is lifted

A sharp cold front boundary can be seen on A sharp cold front boundary can be seen on both satellite pictures and radar compositesboth satellite pictures and radar composites

CyclonesCyclones

Three types of cyclones:Three types of cyclones:

1. Wave cyclones:1. Wave cyclones: cyclone/lowcyclone/low

2. Tropical cyclone:2. Tropical cyclone: hurricane/typhoonhurricane/typhoon

3. Tornado 3. Tornado

1) Wave cyclones1) Wave cyclones

Occur in the midOccur in the mid--latitudes and polar regionslatitudes and polar regions

Formed when air masses of different temperature and Formed when air masses of different temperature and moisture characteristics meet along the polar frontmoisture characteristics meet along the polar front

http://www.physicalgeography.net/fundamentals/7s.htmlhttp://www.physicalgeography.net/fundamentals/7s.html

Formation of a wave cycloneFormation of a wave cyclone

Form between the two high pressure cells in a Form between the two high pressure cells in a low low pressure troughpressure trough

when a cold, dry polar air mass meets a warm, moist, when a cold, dry polar air mass meets a warm, moist, maritime air mass….. Airflow converges setting up an maritime air mass….. Airflow converges setting up an unstable situationunstable situation

Mid-latitude Cyclone

• Cold and warm fronts meet at the center of low pressure

• Cold air dominates larger area

Characteristics of a wave cycloneCharacteristics of a wave cyclone

Cyclones normally move eastward, propelled by prevailing Cyclones normally move eastward, propelled by prevailing westerlieswesterlies aloftaloft

Cold front pushes south and east, supported by a flow of Cold front pushes south and east, supported by a flow of cold, dry polar aircold, dry polar air

The wind direction and temperature change abruptly as the The wind direction and temperature change abruptly as the cold front passescold front passes

Warm front is moving northeast, with warm moist maritime Warm front is moving northeast, with warm moist maritime tropical air followingtropical air following

Precipitation patterns:Precipitation patterns:A broad zone near the warm front A broad zone near the warm front A central core of the cyclone, andA central core of the cyclone, andA thin band extending down the length of the cold frontA thin band extending down the length of the cold front

Cloudiness generally prevails over much of the cyclone but Cloudiness generally prevails over much of the cyclone but may be more limited over the cold frontmay be more limited over the cold front

Example of an OcclusionExample of an Occlusion

Cold Front Approaches a Warm FrontCold Front Approaches a Warm Front

Example of an OcclusionExample of an Occlusion

• Cold front meets the warm front ahead of itCold front meets the warm front ahead of it

•• Warm air is lifted upward, away from the surfaceWarm air is lifted upward, away from the surface

•• Occluded front becomes longer Occluded front becomes longer more of the cold front more of the cold front converges converges with the warm frontwith the warm front

Example of an OcclusionExample of an Occlusion

• Eventually, the cold front completely overtakes the Eventually, the cold front completely overtakes the warm front warm front

•• Entire system is occludedEntire system is occluded

Cold frontCold front cold air moves cold air moves toward the warm air mass aheadtoward the warm air mass ahead

Warm frontWarm front warm air mass warm air mass moving toward a cold air massmoving toward a cold air mass

Stationary frontStationary front two unlike air two unlike air masses remain side by side, butmasses remain side by side, butneither air mass has recently neither air mass has recently undergone substantial movementundergone substantial movement

Occluded frontOccluded front cold front meets cold front meets warm front, warm air gets lifted warm front, warm air gets lifted upward, away from the surfaceupward, away from the surface

When Air Masses Collide…When Air Masses Collide…

Dry LineDry Line –– a front between warm, moist air and a front between warm, moist air and warm, dry airwarm, dry air

Lifting is extremely strongLifting is extremely strongHeavy thundershowers, hail, and possible tornadoes Heavy thundershowers, hail, and possible tornadoes are characteristic of a dry lineare characteristic of a dry lineSeen almost exclusively in the southern states west of Seen almost exclusively in the southern states west of the Mississippithe MississippicTcT air versus air versus mTmT air massesair masses

DrylineDryline Over TexasOver Texas

Notice the differences in the Notice the differences in the dew pointsdew points on both sides of the on both sides of the drylinedryline..

Cyclone TracksCyclone Tracks

Wave cyclones tend to form in certain areas:Wave cyclones tend to form in certain areas:North Pacific and AlaskaNorth Pacific and AlaskaCentral U.S. east of the Rocky MountainsCentral U.S. east of the Rocky MountainsGulf StreamGulf Stream

Wave Cyclones travel common paths until they dissolve:Wave Cyclones travel common paths until they dissolve:North Pacific Ocean pathNorth Pacific Ocean pathConvergence of paths over Northeast U.S. that head toward Convergence of paths over Northeast U.S. that head toward EuropeEurope

Cyclone TracksCyclone Tracks

In the Southern Hemisphere storm tracks are more nearly In the Southern Hemisphere storm tracks are more nearly along a single lane, following the parallels of latitudealong a single lane, following the parallels of latitude

This track is more uniform because of the uniform pattern of This track is more uniform because of the uniform pattern of ocean surface circling the globeocean surface circling the globe