Atmospheric Pressure and Winds

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Chapter 28

• Atmospheric Pressure and Winds

BFRB Pages 111-116
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Air Pressure

• the weight of the air in a certain area
• Air pressure is measured using an instrument
  called a barometer (aneroid or mercury)
• It is measured in inches (height of the mercury
  column in the barometer), or with the metric
  units called millibars
• Use your reference tables for converting inches
  to millibars
• (ESRTs Page 13 Right Top)

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• FYI the increments are not the same on both
  sides. Figure out each sides increments and
  write them in right now

• Right here tells you which side is millibars and
  which side is inches

Inches Increment __________
Millibars Increment __________
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• Air pressure decreases with increasing altitude
  (less air above you means less pressure pushing
  down on you)
• This is an inverse (indirect) relationship
• Weather maps show sea level air pressure. It
  ranges from 960mb 1050mb
• Isobars are lines on the weather maps which
  connect areas of equal air pressure
• VIF!!!! - The same rules apply for isobars as
  isotherms and contour lines. If the isobars are
  spaced close together, the pressure gradient is
  steep and winds are fast.

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Changes in Air Pressure

• How does temperature affect air pressure?
• Warm airs molecules are further apart than cold
  airs molecules!
• It is less dense and not as many molecules are on
  top of an area to add pressure.
• Therefore, as temperature ________________ , air
  pressure _________________.

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Changes in Air Pressure

• How does humidity (the amount of water vapor held
  in a parcel of air) affect air pressure?
• Humid (moist) air contains more water vapor
  molecules (H2O)
• These H2O molecules have less atomic mass than N2
  and O2 molecules that they take the place of!!!
• Lets PROVE it!!!!!

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Winds and Air Pressure

• Winds are caused by the unequal heating of the
  atmosphere (remember that temperature causes
  changes in air pressure)
• Warm air is lighter (lower pressure) and colder
  air is heavier (higher pressure)
• Ex Sea breezes and land breezes

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Sea Breeze the surface wind is blowing in FROM
the sea.
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Land Breeze the surface wind is blowing out
FROM the land.
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Air Pressures Effect on Winds on a Larger Scale

• Winds blow out of high pressure and into low
  pressure
• The greater the difference between the high
  pressure and low pressure, the faster the wind
  speed (this is called the pressure gradient)
• This is shown on a weather map by closely spaced
  isobars (think of closely space contour lines on
  a topographic map steeper slope)

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What parts of the US and Canada have the fastest
winds? the slowest?
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Coriolis Revisited!

• The Coriolis Force (caused by the spin of the
  Earth) causes winds to bend to the right in the
  northern hemisphere
• Winds blow out of the High and in to the Low and
  bend to the right, causing a High Pressure System
  to spin clockwise
• Winds blow into the Low from the High and bend to
  the right, causing the Low Pressure System to
  spin counter-clockwise
• FYI - Hurricanes are areas of extreme low
  pressure and always flow counter-clockwise in the
  Northern Hemisphere

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In a High, the air wants to move directly out
from High to Low Pressure but is deflected to its
right by the Coriolis Force, creating a Clockwise
flow out from the High. VIF!!!!!!
H
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In a Low, the air wants to move directly in from
High to Low Pressure but is deflected to its
right by the Coriolis Force creating a
Counterclockwise flow into the Low. VIF!!!!!!
L
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MEASURING WIND

• Wind direction is determined by using a wind vane
  or a wind sock (like you see at the airport)
• Wind speed is measured using an anemometer (units
  are knots 1 knot 1.15 mph) 

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Pressure Belts and Winds

• The jet stream is a narrow zone of high speed air
  found in the upper troposphere (about 30,000
  feet)
• Wind speeds here may be over 300 knots!
• Large convection currents of air occur on the
  Earths surface due to the unequal heating of the
  atmosphere
• ESRTs Page 14 Bottom
• Link to global_circ_anim.swf

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3-D Version of the ESRTsPlease note the
deflection of the wind to ITS right in the N.H.
due to CF
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This is what the air circulation would look like
if the Earth DID NOT ROTATE NO CF!!!!!!