CH. 19: Air Pressure

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CH. 19 Air Pressure Wind

• Air Pressurethe pressure exerted by the weight
  of air above.
• Air pressure is very important factor in
  predicting the weather!
• Air pressure at sea level is 14.7 lbs per square
  inch.

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Barometerinstrument used to measure air pressure.

• 2 TYPES
• 1. Aneroidcanister like device, pressure
  changes on the spring move the arrow
• 2. Mercuryliquid device, pressure changes make
  the liquid go up(high) go down(low)

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What causes wind?

• Wind is the horizontal movement of air.
• Wind is caused by the horizontal differences in
  air pressure.
• The differences in air pressure are in turn
  caused by differences in temperature(solar
  radiation)

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3 Factors Affecting Wind

• 1. Pressure Differencesgreater the difference
  the greater the wind.
• Air moves from high to low pressures
• Isobarslines of equal pressure
• Closely packed windy
• Widely spaced not as windy
• Pressure Gradientsteep means windy
• Pressure Gradientweak/not steep means Not windy

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• 2. Coriolis EffectEarths rotation affects
  moving objects
• --Free moving objects move to the
  right in the northern hemisphere
• to the left in the southern.
• Frictiononly important near Earth.
• Friction slows wind can change wind direction.
• JET STREAMShigh up fast moving!

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19.2 Pressure Centers Wind

• Pressure Centershelp 2 predict weather (2)
• Low (Cyclones) air is unstable rises, cools,
  condenses, clouds stormy, cloudy, air moves ccw
  in N. hemisphere, winds move inward (?) Pressure
  decreases toward center
• High (Anti-Cyclones) air is stable descends,
  warms, evaporates, clear/fair, air moves cw in N.
  hemisphere, winds move outward (?)
• Pressure increases toward center

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GLOBAL WINDS (read pgs 540-542)

• 0 degrees equatorial low
• 30 degrees subtropical high
• 60 degrees subpolar low
• 90 degrees polar high

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• Trade Winds belt between 0 30
• Prevailing Westerliesbelt between 30 60
• Polar Easterliesbelt between 60 90
• What is the polar front? Page 541
• What are monsoons? Page 542

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19.3 Regional Wind Systems

• Local Windssmall-scale winds are produced by a
  locally generated pressure gradient.
• Local Windsare caused by 2 things
• 1. by topographic effects (mtns. Etc.)
• 2. by variations in surface composition
• (land or water, etc.)

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4 Types of Local Winds

• 1. Sea Breezehappens during the day, land heats
  up quicker causes air to rise allowing cooler
  air to come from the sea.
• 2. Land Breezehappens during the night, land
  cools of quicker causes the warmer air over the
  sea to rise allowing cooler air to come from the
  land.

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• 3. Valley Breezeoccurs during the day, air
  rises from valley floors.
• 4. Mountain Breezeoccurs at night, air sinks
  from the mountain tops toward the valley floor.

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MEASURING WIND

• Wind Directionwind vane, wind direction is given
  by the direction the wind is coming FROM,
  (N,S,E,or W) or sometimes given in degrees!
• Wind Speedanemometer.
• Wind Sockssee them at airports measure wind
  speed direction!

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El Nino La Nina

• Read about these phenomena carefully!
• When do they occur?
• Why do they occur?
• What global/local affects occur?
• Global ppt. Distribution?
• Read page 548Why does it rain in some places
  more than others?