Chapter+13-The+Atmosphere+in+Motion

Earth & Space Science Survey

Chapter 13: The Atmosphere in Motion

Air Pressure and Weather
  • Air Pressure: The pressure exerted by the weight of air above.
  • Average Pressure:
    • At sea level: 1 kg/cm² or 14.7 lbs/in².
    • Equivalent to the pressure produced by a 1-square inch column of water 33 feet high.
    • This pressure is equal in all directions; without this equality, objects would collapse.
  • Importance: Changes in air pressure are crucial in producing weather conditions; they relate to hurricane strength.
Measurement of Air Pressure
  • Standard Measurement Units:
    • Today, meteorologists use millibars.
    • Sea-level pressure is defined as 1,013.2 millibars.
  • Historical Context of the Millibar:
    • Has been the standard unit in the US since January 1940.
    • Corresponds to 29.92 inches of mercury.
    • Coined by Torricelli in 1643 with the invention of the mercury barometer to evaluate atmospheric pressure.
  • Mercury Barometer:
    • A glass tube closed at one end, filled with mercury, inverted in a dish of mercury.
    • The weight of the mercury column balances against atmospheric pressure.
Types of Barometers
  • Aneroid Barometer:
    • Developed to create smaller, portable instruments to measure air pressure.
    • Uses a partially evacuated metal chamber that alters shape with pressure changes.
    • Can connect to recording instruments for continuous pressure data (termed barograph).
Horizontal Air Movement and Wind
  • Horizontal Air Movement:
    • The majority of air movement is horizontal (wind), which balances air pressure differences.
    • Wind flows from high-pressure to low-pressure areas due to horizontal pressure differences caused by unequal solar heating.
  • Factors Controlling Wind:
    1. Pressure-gradient force: The driving force for wind.
    2. Coriolis effect: The deflection of wind due to Earth’s rotation.
    3. Friction: The resistance air faces when moving along the Earth's surface.
Pressure Gradients and Wind Speed
  • Unbalanced Forces:
    • If an object experiences an unbalanced force, it will accelerate, generating wind due to horizontal pressure gradients.
  • Weather Stations and Isobars:
    • Pressure variations collected at weather stations are compiled into maps using isobars (lines connecting equal air pressures).
    • Isobars’ spacing indicates the pressure gradient:
    • Closely spaced indicates steep gradients and strong winds.
    • Widely spaced indicates weak gradients and light winds.
Overview of Weather Maps
  • Isobar Location:
    • Low pressure centers (L in red) and high pressure centers (H in blue).
Coriolis Effect
  • Definition:
    • Wind does not cross isobars at right angles due to the Coriolis effect caused by Earth’s rotation.
    • In the Northern Hemisphere: winds deflected to the right (clockwise).
    • In the Southern Hemisphere: winds deflected to the left (counterclockwise).
  • Impact:
    • Only affects wind direction, not speed, and is stronger at the poles, diminishing towards the equator.
Friction and Wind Movement
  • Effect of Friction:
    • Influences winds within a few kilometers of Earth’s surface.
    • Slows air movement, alters wind direction by causing flow at an angle across isobars towards low pressure.
  • Upper Air Winds:
    • Above the friction zone, pressure-gradient force and Coriolis effect work together, leading to geostrophic winds that flow parallel to isobars.
    • Upper air winds include jet streams, fast-moving currents at 75-150 miles per hour.
Atmospheric Pressure Systems
  • Cyclones and Anticyclones:
    • Cyclones = low pressure centers.
    • Anticyclones = high pressure centers.
  • Surface Wind Patterns:
    • In the Northern Hemisphere, winds flow inward and counterclockwise around low pressure; outward and clockwise around high pressure.
    • The opposite is true for the Southern Hemisphere.
Low Pressure System Dynamics
  • Characteristics:
    • Surface air flows inward, causing horizontal convergence, and forming clouds/precipitation due to rising unstable air.
  • Pressure Changes:
    • Home barometers indicate “stormy” conditions at low pressure ends.
High Pressure System Dynamics
  • Characteristics:
    • Associated with stable air that sinks, causing clear conditions.
  • Pressure Changes:
    • Words like “fair” are printed on high-pressure ends of barometers.
Atmospheric Circulation and Solar Energy
  • Global Circulation:
    • Controlled by uneven solar heating and Earth’s rotation affecting airflow patterns.
  • Atmospheric Circulation Cells:
    • Three primary types in each hemisphere:
    1. Hadley Cell: 0° to 30°
    2. Ferrel Cell: 30° to 60°
    3. Polar Cell: 60° to 90°
  • Zones of Vertical Air Movement:
    • Doldrums: Rising air leads to abundant precipitation.
    • Horse Latitudes: Sinking air that causes minimal precipitation.
Winds within Atmospheric Cells
  • Trade Winds: Found in the Hadley cell.
  • Westerlies: Found in the Ferrel cell.
  • Polar Easterlies: Found in the Polar cell.
Monsoon Winds
  • Definition:
    • Seasonal wind patterns resulting in wet summers and dry winters, influenced by land and water specific heat differentials.
Local Winds
  • Sea Breeze:
    • Cool air from sea moves inland, generated due to land heating faster than water.
  • Land Breeze:
    • Occurs at night when land loses heat quickly, causing offshore flow as air moves from the land to sea.
  • Valley Breeze:
    • Warm air rises from valley slopes during the day.
  • Mountain Breeze:
    • Cool air flows downhill post-sunset.

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