Atmospheric Circulation and Weather Systems

Take-Away Points1. Weather is driven by unequal solar heating and

cooling2. Air motions are affected by the Coriolis Effect and

“centrifugal” force3. High and Low Pressure Systems4. Air flows parallel to pressure contours (Geostrophic

winds)5. Air masses meet along sharp boundaries or fronts6. Weather is inherently chaotic and that limits our

ability to forecast it

The Seasons

Heat Lag

1. Weather is driven by unequal solar heating and cooling

Heat Lag

Atmospheric Circulation

1. Weather is driven by unequal solar heating and cooling

Atmospheric Circulation

1. Weather is driven by unequal solar heating and cooling

Asymmetric Earth

1. Weather is driven by unequal solar heating and cooling

Asymmetric Earth

1. Weather is driven by unequal solar heating and cooling

Atmospheric Circulation

1. Weather is driven by unequal solar heating and cooling

Zonal and Meridional Flow

1. Weather is driven by unequal solar heating and cooling

Semi-Permanent Features, January

1. Weather is driven by unequal solar heating and cooling

Semi-Permanent Features, July

1. Weather is driven by unequal solar heating and cooling

Rotation Effects

2. Air motions are affected by the Coriolis Effect and “centrifugal” force

The Coriolis Effect

• Due to moving on a rotating earth

• Things on equator are moving faster than points near poles

• Affects:–Winds–Ocean Currents– Tides

2. Air motions are affected by the Coriolis Effect and “centrifugal” force

The Coriolis Effect

• Things moving toward the equator are deflected west

• Things moving poleward are deflected east

• Deflected to Right in Northern Hemisphere

• Deflected to Left in Southern Hemisphere

2. Air motions are affected by the Coriolis Effect and “centrifugal” force

The Coriolis Effect

2. Air motions are affected by the Coriolis Effect and “centrifugal” force

The Coriolis Effect

2. Air motions are affected by the Coriolis Effect and “centrifugal” force

The Coriolis Effect

2. Air motions are affected by the Coriolis Effect and “centrifugal” force

Inertial Circles

2. Air motions are affected by the Coriolis Effect and “centrifugal” force

“Centrifugal” Force ?

2. Air motions are affected by the Coriolis Effect and “centrifugal” force

Or No Force?

2. Air motions are affected by the Coriolis Effect and “centrifugal” force

“Centrifugal” Force Does Not Exist• When anything turns, the only forces that

act are in the direction of the turn• These forces are called centripetal (center-

seeking) force• “Centrifugal” force is an illusion• “Centrifugal” force is due to inertia and

centripetal force opposing each other

2. Air motions are affected by the Coriolis Effect and “centrifugal” force

High Pressure Systems

3. High and Low Pressure Systems

High Pressure Systems• Air flows out from center• Spin clockwise in Northern Hemisphere• No air mixing• Stable, fair weather• Sinking Air, few clouds• Long duration can result in inversions,

pollution• Winter: often extreme cold– Cold Air is Dense– Clear Skies and Radiational Cooling

3. High and Low Pressure Systems

Low Pressure Systems

3. High and Low Pressure Systems

Why Counterclockwise?

3. High and Low Pressure Systems

Low Pressure Systems• Air flows in toward center• Spin counter-clockwise in Northern

Hemisphere• Mixes air of different properties• Associated with fronts• Stormy, sometimes violent weather• Passage often results in sharp change in

weather conditions

3. High and Low Pressure Systems

Geostrophic Winds• As air flows in or out of pressure cells, Coriolis

Effect deflects it• At surface, friction limits the deflection.– Winds blow about 45 degree angles to isobars

• Aloft, friction not a factor– Deflection continues until limited by pressure

gradient (winds can’t go against pressure)– Winds blow parallel to contours– This is called geostrophic flow

4. Air flows parallel to pressure contours (Geostrophic winds)

Geostrophic Flow

Geostrophic Flow

Geostrophic Flow

4. Air flows parallel to pressure contours (Geostrophic winds)

1905 Weather Map of US

First U.S. Weather Map With Fronts

Fronts and Low Pressure Systems

5. Air masses meet along sharp boundaries or fronts

Fronts

5. Air masses meet along sharp boundaries or fronts

Warm Fronts

5. Air masses meet along sharp boundaries or fronts

Warm Fronts

• Gradual Onset• Warm Air over Cool Air• Little Turbulence• Weather Rarely Violent

5. Air masses meet along sharp boundaries or fronts

Cold Fronts

5. Air masses meet along sharp boundaries or fronts

Cold Fronts

• Abrupt Onset• Cold Air Lifting Warm Air• Considerable Turbulence• Weather Sometimes Violent• Thunderstorms Common• Can Spawn Tornadoes

5. Air masses meet along sharp boundaries or fronts

Old Low Pressure Systems

5. Air masses meet along sharp boundaries or fronts

Occluded Fronts

5. Air masses meet along sharp boundaries or fronts

Occluded Fronts• Two fronts merge• Any two types of front can occlude• Most common: Cold Front overtakes Warm

Front• Starts off like a warm front, finishes like a

cold front

5. Air masses meet along sharp boundaries or fronts

Weather Prediction

5. Air masses meet along sharp boundaries or fronts

Weather Prediction

5. Air masses meet along sharp boundaries or fronts

Weather Prediction

5. Air masses meet along sharp boundaries or fronts

Chaos

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Chaos Theory

Does Not Mean:• Cloned Dinosaurs will run amok• Systems do not follow physical laws• Systems behave with wild unpredictability• Systems do not have limits• Phenomena cannot be predicted

6. Weather is inherently chaotic and that limits our ability to forecast it

Chaos Theory

Does Mean:• Small differences compound over time• There are limits to how accurately

phenomena can be predicted• Examples:–Weather– The Planets– Traffic

6. Weather is inherently chaotic and that limits our ability to forecast it