Atmospheric Circulation and Weather Systems

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Atmospheric Circulation and Weather Systems
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Take-Away Points

• Weather is driven by unequal solar heating and
  cooling
• Air motions are affected by the Coriolis Effect
  and centrifugal force
• High and Low Pressure Systems
• Air flows parallel to pressure contours
  (Geostrophic winds)
• Air masses meet along sharp boundaries or fronts
• Weather is inherently chaotic and that limits our
  ability to forecast it

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The Seasons
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Heat Lag
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Heat Lag
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Atmospheric Circulation
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Atmospheric Circulation
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Asymmetric Earth
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Asymmetric Earth
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Atmospheric Circulation
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Zonal and Meridional Flow
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Semi-Permanent Features, January
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Semi-Permanent Features, July
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Rotation Effects
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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

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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

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The Coriolis Effect
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The Coriolis Effect
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The Coriolis Effect
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Inertial Circles
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Centrifugal Force ?
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Or No Force?
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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

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High Pressure Systems
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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

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Low Pressure Systems
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Why Counterclockwise?
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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

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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 cant go against pressure)
• Winds blow parallel to contours
• This is called geostrophic flow

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Geostrophic Flow
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Geostrophic Flow
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Geostrophic Flow
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1905 Weather Map of US
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First U.S. Weather Map With Fronts
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Fronts and Low Pressure Systems
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Fronts
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Warm Fronts
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Warm Fronts

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

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Cold Fronts
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Cold Fronts

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

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Old Low Pressure Systems
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Occluded Fronts
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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

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Weather Prediction
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Weather Prediction
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Weather Prediction
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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

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Chaos Theory

• Does Mean
• Small differences compound over time
• There are limits to how accurately phenomena can
  be predicted
• Examples
• Weather
• The Planets
• Traffic