Air Pressure and Winds II

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Air Pressure and Winds II
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RECAP

• Ideal gas law how the pressure, the temperature
  and the density of an ideal gas relay to each
  other.

• Pressure and pressure measurements
• Constant height charts isobars
• Constant pressure charts contour lines
• Note wind direction in NH
• Low P -gt Cyclones
• High P -gt Anticyclones

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Isobaric chart

• Constant pressure, P500 mb, then plot heights
  contour lines
• Pressures on surface maps lt-gt Heights on isobaric
  charts
• Low lt-gt Low
• High lt-gt High
• Pressure lt-gt Temperature
• Low lt-gt Low
• High lt-gt High
• Note wind direction here
• Different from surface winds
• Along the contour lines,
• no crossing

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Newtons Laws of Motion

• Newtons First Law an object will
• stay at rest, or
• maintain its motion at a constant velocity and in
  a straight line
• as long as
• no force is exerted on the object, or
• all forces cancel each other
• Says who? An observer at rest

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Newtons Second Law

• The force (F) experienced by an object is equal
  to its mass (m) times the resulting acceleration
  (a).
• Acceleration change in velocity (magnitude or
  direction)
• Examples of accelerated motion
• Speeding up
• Slowing down
• Making a turn
• A body with a large mass is more difficult to
  accelerate (or stop). Objects with large masses
  are more inertial.

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Forces acting in the atmosphere.

• Gravity force.
• Vertical force in a downward direction
• Atmospheric drag force (friction).
• Acts against the motion
• Proportional to velocity squared
• Pressure force.
• From high to low pressure regions
• The bigger the pressure gradient the larger the
  pressure force
• Centripetal force
• Acts perpendicular to the wind velocity,
• towards the center of the curvature.
• It changes the wind direction not the magnitude.
• Larger velocity and larger curvature (smaller
  radius) correspond to a bigger centripetal force.
  
• Coriolis force due to the Earths rotation

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Pressure gradient force

• Example two tanks
• Tank A full
• Tank B half-full
• Pressure at the base of tank A is higher than the
  pressure at the base of tank B
• The pressure gradient force causes the water to
  flow from A to B (from high pressure to low
  pressure)
• Analogous phenomenon in the atmosphere

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Pressure Gradient

• Vector It has a magnitude and direction.
• Magnitude how fast the pressure is changing in a
  given direction.
• Direction the direction
• of the fastest increase of
• the pressure.
• Two examples
• Color gradient
• Mountain slope

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What is the direction of the color gradient?
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Topographic maps

• Elevation contours, index contours (bold), slope
  angle.
• Steep slope dense contours
• Gentle slope contours are further apart
• Elevation -gt pressure slope -gt gradient

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In which direction is the pressure gradient?
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Pressure Gradient Force

• The pressure gradient force is analogous to the
  gravity force on a mountain slope
• Its magnitude is proportional to the pressure
  gradient.
• Its direction is opposite to the direction of the
  pressure gradient. It is from HIGH to LOW
  pressure.
• It is perpendicular to the lines of constant
  pressure (isobars).
• The closer to each other the isobars are the
  larger the gradient is and the larger the
  pressure force is.
• The pressure force along an isobar is ZERO!

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In which direction is the pressure gradient
force?
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Wind and Pressure Map
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Coriolis Force (Effect)

• It is an apparent force
• Due to the rotation of the coordinate system
  (Earth)
• It makes a moving object deflect from a straight
  line even in the absence of any forces acting on
  it.

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The Magnitude of the Coriolis Force

• The rotation of the Earth
• The faster the planet rotates the bigger the
  force
• The speed of the object
• Bigger V -gt bigger effect
• The latitude
• Min. at the equator
• Max. at the poles

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Coriolis force as a function of

• The speed of the object
• The latitude
• Min. at the equator
• Max. at the poles

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The Direction of the Coriolis Force

• In the Northern hemisphere the deflection is to
  the right of the direction of motion.
• In the Southern hemisphere the deflection is to
  the left of the direction of motion.
• The winds in the Northern hemisphere will be
  deflected to the right and in the Southern
  hemisphere they will be deflected to the left.
• Hurricanes spin differently in the Northern and
  Southern hemisphere

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The Coriolis Force and the Earth

• The Coriolis effect is important when moving over
  LARGE distances (air plane travel), with large
  velocities, away from the equator.