Air Pressure and Winds I

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Air Pressure and Winds I
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Review precipitation types
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Sample weather map (Fig. 13.11)
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Fig. 11.18
Snow
Drizzle
Sleet
Freezing rain
Fog
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Atmospheric pressure P
Atmospheric pressure and density decrease with
altitude exponentially!!!
Units 1 bar1000 mbar
1 Standard atmosphere 1013 mbar
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Ideal Gas Law

• A relationship between the pressure, the
  temperature, and the density of an ideal gas.
• Ideal gas a simplified physical model for a gas.
  It neglects
• the volume of the individual molecules
• the interaction between the molecules
• The ideal gas model is a very good approximation
  for the air at room temperature.

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Ideal Gas Law

• The pressure P of an ideal gas is proportional to
  its temperature T and density r. C is a constant
  of proportionality air gas constant.
• Examples
• T increases, r constant -gt P increases (tea
  kettle)
• r increases, T constant -gt P increases (blow a
  balloon)
• T decreases, r decreases -gt P decreases (climb a
  mountain)
• P constant -gt T increases, r decreases (example
  in the book Fig. 8.2 (a) and (b))

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Simple model of atmospheric pressure

• Column of air molecules
• Assumptions
• Constant density
• Constant width
• Atmospheric pressure P is simply due to the
  weight of the column.
• P decreases with height because there are less
  molecules remaining above.

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From high to low pressure

• Equal surface pressures in cities 1 and 2 result
  from
• Cold dense air in city 1
• Warm, less dense air in city 2
• At higher altitudes the pressures are different
  (L vs H)
• The air flow (due to the pressure gradient force)
  is from High to Low -gt expect to see the pressure
  dropping as the air temperature increases

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Daily pressure variations
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How do we measure pressure?

• Mercury (Hg) barometer.
• The weight of the Hg column is balanced
• by the weight of the atmosphere above
• the open air surface.
• 1 atmosphere 76 cm.Hg 29.92 in.Hg
• Can we measure the atmospheric pressure with a
  water barometer?

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Altitude Corrections

• Pressure decreases with height.
• Altitude adjustment
• Why to compare pressure readings from stations
  at different altitudes.
• Convert all P readings to the pressure at the
  Mean Sea Level sea-level pressure.
• For every 100 m add 10 mbar
• This is a rough correction.
• Sea-level pressure chart
• Height surface surface of constant height
• Pressure maps on constant height surfaces show
  the horizontal variation of the pressure -gt
  isobars

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Sea-level pressure chart

• Elements isobars, high (H) and low (L) pressure
  regions
• It is an example of a constant height chart
  (sea-level)

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Constant height charts

• Pressure variations are plotted at a fixed
  altitude
• At higher altitudes, no need for altitude
  correction what you measure is what you plot
• Typical values for the atmospheric pressure at
  various altitudes
• Sea-level 1000 mb
• 3 km 700 mb
• 5.6 km 500 mb

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Isobaric charts

• Constant height chart we fix the altitude and
  plot the pressure the map shows lines of
  constant pressure (isobars).
• Isobaric chart we fix the pressure and plot the
  altitude where it is found the map shows lines
  of constant height (contour lines).
• High pressure lt-gt High height on the isobaric
  chart
• Low pressure lt-gt Low height on the isobaric chart

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The two types of pressure charts

• Surface map (constant height chart)
• Anticyclones (H) centers of high pressure
• Cyclones (L) centers of low pressure
• Upper-air chart (isobaric chart)
• Pressure contour lines are parallel to the
  isotherms
• Winds flow parallel to the pressure contour lines

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Flying on a constant pressure surface

• Airplanes measure altitude based on pressure
  readings
• They move on constant pressure surfaces

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High to Low, Look Out Below

• This is a problem when T changes. The altimeter
  needs to be calibrated often with actual altitude
  measurements.