Air Masses, Fronts and Global Wind Patterns

Meteorology

Air MassesAir Masses

Definition:Definition: Air mass - a large dome of air

which has similar horizontal temperature and moisture characteristics throughout.

Very similar to a balloon.

Continental Arctic (cA): Continental Arctic (cA): • Frigid – record low temperatures• Dry - very low dew points• Dense - very high barometric pressure• Usually originate north of the Arctic Circle

Siberian Express • Usually once or twice a winter• very rarely form during the summer

because the sun warms the Arctic.

Continental polar (cP): Continental polar (cP): • Cold and dry - stable• Usually originates in NW Territory of Canada• Influences mainly the northern USA• Responsible for clear and pleasant weather

during the summer • Usually in winter• Creates troughs in the polar jet stream• Lake effect snow in Great Lakes areas

Maritime polar (mP):Maritime polar (mP):• Cool and moist - unstable • Originate over N. Atlantic and N. Pacific• Main Influence - the Pacific Northwest

and the Northeast. • can form any time of the year • Generally not as cold as cPcP air masses

Maritime tropical (mT): Maritime tropical (mT): • Warm and very moist – unstable• Originate in the Gulf of Mexico and

the Southern Atlantic Ocean• Influences the eastern USA • Most prevalent during summer • Responsible for hot, humid summer

days across the South and the East.

Continental Tropical (cT):Continental Tropical (cT): • Very Hot and very dry – stable aloft • Originates in Desert Southwest and

northern Mexico • Occurs in the summer, rarely in winter• Usually keeps the Desert Southwest

scorching above 100oF during summer• Generally clear skies, hot, low humidity

Source RegionsSource Regions

Reasoning for TornadoesReasoning for Tornadoes

Orographic Perfection

Meeting of• Moist - mT• Hot - cT• Cool – cP

Roc

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tn.

Tornado AlleyTornado Alley http://www.britannica.com/thunderstorms

_tornadoes/video/ocliwea124v4.mov

Fujita ScaleFujita Scale

(NationalAtlas.com)

Fronts and their symbolsFronts and their symbols

Fronts:Fronts: Boundary between two air masses Characterized by shift in weather

Cold Warm Stationary Occluded

5 Characteristics of a 5 Characteristics of a FrontFront

Sharp temperature changes over a relatively short distance.

Changes in air moisture content Shifts in wind direction Pressure changes Clouds and precipitation

Cold FrontsCold Fronts• Temperature – drops rapidly• Pressure – rises steadily• Clouds – Vertical building• Precipitation – Heavy along front• Winds – Strong and shifting

• Typically move faster than warm front

Cold FrontCold Front

(Fozzy)

Cold FrontCold Front

Cold FrontCold Front

In the summer, cold In the summer, cold fronts can trigger:fronts can trigger:

thunderstorms large hail dangerous winds tornadoes

Graphic Depiction!

Warm FrontsWarm Fronts• Temperature – rises slowly• Pressure – slight rise, then fall• Clouds – strato- and cirro-• Precipitation – long, steady• Winds – variable and light

• Typically will have affect for days

Warm FrontWarm Front

Warm FrontWarm Front

Warm FrontWarm Front

Effects of warm frontsEffects of warm fronts Slow-moving warm front can mean

days of wet weather before warm air Sometimes water vapor in warm fronts

condense to producerainsnowsleet freezing rain

Stationary FrontStationary Front

Stationary FrontsStationary Fronts• Temperature – stagnent• Pressure – slightly fluctuates• Clouds – altocumulus• Precipitation – none• Winds – variable and light

• Can last for days weeks

Occluded FrontOccluded Front

Occluded FrontsOccluded Fronts• Temperature –

•Warm – gets milder•Cold – gets colder

• Pressure – •Warm - slight drop•Cold – slight rise

• Clouds – cumulus• Precipitation – steady and light• Winds – variable and light

Occluded FrontOccluded Front

Different Temperatures - Different Temperatures - Different Pressures Different Pressures

CoolAir

WarmAir

Denser

MorePressure

Less Dense

LessPressure

Pressure and Air MovementPressure and Air Movement

Pressure Gradient ForcePressure Gradient ForceDifference in pressure over a given

distance---between isobarsClose together = step pressure gradient

STRONG windsFar apart = gentle pressure gradient

Light winds• Just like contour lines

Pressure Gradient ForcePressure Gradient Force

Isobaric MapsIsobaric Maps

Coriolis EffectCoriolis EffectApparent force due to the

rotation of the Earth (Think Merry-go-round)

N. Hemisphere wind turns right S. Hemisphere wind turns left

Strength depends on latitude and wind speed

Coriolis EffectCoriolis Effect

Coriolis EffectCoriolis Effect

Centripetal ForceCentripetal ForceIn-ward directed forceAllows an object to remain in

circular motionWinds moving around high and

low pressure areasClockwise around Highs.Counter-clockwise around Lows.

Friction Friction (What a Drag)(What a Drag) The resistance to movement Surface winds are affected by friction Why? Ground resistance:

trees, mountains, houses, buildings, etc. This drag causes winds to blow across

pressure gradient at the surface.

Pressures All TogetherPressures All Together

General Planetary CirculationGeneral Planetary Circulation

Winds Winds Horizontal movements at

surfaceNames from WHEREWHERE it came

from…not where it is going!!!

Around Pressure CellsAround Pressure Cells

Pressure CellsPressure Cells High – In and Up

Converge at surfaceAscend in centerDiverge Aloft

Low – Down and OutConverge aloftDescend in centerDiverge at surface

X-section of Planetary CirculationX-section of Planetary Circulation

January Global Pressure Map

January

July Global Pressure Map

July

Summer HighsSummer Highs