Chapter 13 Surface Water by yurtgc548


									 Chapter 19: The Atmosphere
          in Motion
19.1 Air Pressure and Wind
• What is Air Pressure?
 –The weight of the atmosphere as it
  pushes down upon Earth’s surface
 –Pressure is a force per unit area
 –Units: lbs/in2, millibars,
  hectopascals, inches of mercury
 –pressure decreases with height
• What is Air Pressure?
• air pressure is measured with a
 –mercury barometers use mercury in a
  glass tube; the mercury rises and
–aneroid barometers use a metal
 capsule filled with air that expands
 and contracts with air pressure
• Why Does Air Pressure Change?
 –As temperature increases, pressure
   • Molecules are farther apart in warmer
     air; closer together in cooler air
 –As humidity increases, pressure
   • Water vapor weighs less than oxygen or
     nitrogen gas; pushes out these gases
     and exerts less force
• Why Does Air Pressure Change?
 –Isobars: lines of equal pressure on a
  weather map
 –pressure is reduced to sea-level
  (removes elevation differences) to
  track high and low pressure systems
 –High pressure area = general good
  weather (clear, cool)
 –Low pressure area = general bad
  weather (cloudy or precipitation,
• Why Does Air Pressure Change?
 –Pressure Gradient: pressure change
  over distance
   • isobars spaced closer together = windy
   • isobars spaced far apart = little wind

        Weather Map
• What Makes the Wind Blow?
 –High pressure always moves toward
  low pressure to achieve balance
 –The greater the pressure difference,
  the stronger the winds
 –Ultimately caused by unequal heating
  of Earth’s surface
• Measuring Wind
  –Wind vane: shows wind direction (a
   south wind comes from the south
   and moves toward the north)
  –Anemometer: measures wind speed
• Review Questions
 1. What is air pressure? How does air
    pressure vary with elevation?
 2. Explain why humid air is lighter than
    dry air.
 3. How would you be able to tell where
    the windiest areas are on a weather
    map that shows isobars?

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