ABE 57

• Pressure


  Force per unit area
• Atmospheric Pressure


  • Force exerted by the air molecules in a given area
  • Exists in all directions
• Atmospheric Pressure at sea level

  • 101.325 kPa
  • 1013.25 hPa
  • 1013.25 mbar
  • 760 mmHg
  • 29.92 inHg
  • 14.7 psi
• Mercury Barometer


  • The standard instrument for measuring atmospheric pressure
  • Correction for temperature, gravity, and instrument error, is applied to obtain Station Pressure (P)
• Isobaric Map Analysis


  1. Observation - With varying elevations above sea level

  2. Adjust to Mean Sea Level Pressure (MSLP) - Add 10 mbar per 100 m distance from sea level

  3. Isobars are drawn on the chart (dark solid lines)
• Wind


  • Horizontal component of air movement
  • In the absence of other forces, nature will push mass from high to low pressure
• Types and Direction of Wind

  • Onshore wind
  • Offshore wind
  • Upslope wind
  • Downslope wind
• Naming winds


  • Easterly wind – blowing from the east
  • Eastward – blowing to the east; thus also a westerly
• Wind Measurement


  • Anemometer – wind speed
  • Wind vane – wind direction
• Principal Forces


  • Pressure gradient force
  • Coriolis force
  • Friction
  • Centripetal/ centrifugal force
• Pressure Gradient Force (PGF)

  • The closer the isobars, the greater the pressure gradient
  • The greater the pressure gradient, the stronger PGF
  • The stronger the PGF, the greater the wind speed
• Gradient Wind


  Air movement caused by pressure difference alone
• Coriolis Force (CF)


  • An apparent force that is due to the rotation of the earth
  • CF = 2VΩsin Φ (V = wind speed, Ω = angular acceleration, Φ = latitude)
• Geostrophic Wind


  • The resultant of the interaction of the PGF and CF
  • Constant wind speed at straight, evenly placed isobars
• Friction


  • Acts in opposite direction with the wind direction
  • Slows down the wind
  • Strongest near the surface
• Surface Wind


  Near the ground, the wind crosses the isobars and blows toward lower pressure
• Centripetal/ Centrifugal Force


  • Force that makes a body follow a curved path
  • Spin creates low pressure
  • Results to cyclostrophic winds
• Combined Forces


  1. PGF creates gradient winds

  2. PGF and CF create geostrophic winds

  3. Friction creates imbalances resulting to surface winds

  4. Centripetal/ centrifugal forces create cyclostrophic winds
• Microscale


  Eddies of small diameters (a few meters or less), disperse smoke, sway branches & swirl dust & papers into the air, are usually short-lived (last only a few minutes at best)
• Mesoscale


  Range from a few kilometers to about a hundred kilometers in diameter, last longer (often many minutes, hours, or in some cases as long as a day), include local winds along shorelines and mountains, thunderstorms, tornadoes, & small tropical storms
• Macroscale


  Synoptic scale: dominate regions of hundreds to even thousands of square kilometers, typically last for days & sometimes weeks
• Macroscale


  Planetary (Global) scale: largest wind patterns ranging over the entire earth
• General Atmospheric Circulation


  The average flow of air in the entire globe, based on wind observations (20 or more years), complex pattern of winds & pressure systems, the underlying cause is the unequal heating of the earth's surface
• Single-Cell Model


  • Assumptions: The earth's surface is uniformly covered with water, differential heating between land and water does not come into play, The sun is always directly over the equator, winds will not shift seasonally, The earth does not rotate, only pressure gradient force is present
• Three-Cell Model


  The tropical regions still receive an excess energy and the poles a deficit, In each hemisphere, 3 cells instead of 1 take the task of energy redistribution
• Features of Three-Cell Model

  • Polar Cell
  • Ferrel Cell
  • Hadley Cell
• Features of Three-Cell Model with land and water distribution

  • Equatorial Low
  • ITCZ
  • Doldrums
  • Subtropical high
  • Horse latitude
  • Subpolar low
  • Stormy latitude
  • Polar high
  • NE Trade winds
  • SE Trade winds
  • Westerlies
  • Polar easterlies
• Identify the three cells in order