test 6

• Atmospheric pressure

  The weight of the air above a point
• Pressure decreases with height


  Because there is less air above, so less weight
• Pressure does not decrease at the same rate with elevation
• Reason pressure does not decrease at same rate


  The atmosphere is compressible and the density of air molecules is greater near the surface
• Standard atmospheric pressure


  1013.25 mb = 1013.25 hPa = 29.92 in.Hg
• Types of barometers


  • Mercury barometer
  • Aneroid barometer
  • Altimeter
  • Barograph
• Aneroid barometer


  No mercury, pressure changes the thickness of an airtight chamber/diaphragm
• Barograph


  Another form of aneroid barometer
• Altimeter


  Aneroid barometer inside
• Automated weather station


  Solid state pressure sensor
• Isobars


  Lines drawn on a weather map connecting points of equal pressure (MSLP)
• Global Telecommunication System


  Surface weather chart
• Pressure used instead of height in meteorology

  Pressure decreases with height, 1013 hPa ~ 0 ft, 850 hPa ~ 5000 ft, 500 hPa ~ 18,000 ft, 250 hPa ~ 34,000 ft
• Station pressure


  After corrections of temperature, gravity, and instrument error
• Sea-level pressure


  After corrections of altitude (1mb/10m)
• Standard pressure levels


  • Isobaric maps
  • Contour lines
  • Ridges
  • Troughs
• Atmospheric forces and causes of air movement

  • Pressure Gradient Force
  • Coriolis Force
  • Centrifugal Force
  • Frictional Force
• Geostrophic balance/wind


  When the Pressure Gradient Force and Coriolis force are in balance, the wind flows parallel to the height contours at a constant speed with no change of direction
• In the Northern Hemisphere


  The geostrophic wind blows with lower pressure to its left and higher pressure to its right
• In the Southern Hemisphere


  The geostrophic wind blows with lower pressure to its right and higher pressure to its left
• Gradient balance


  When isobars are curved, there is a mismatch between the pressure gradient and Coriolis forces causing a direction change, so the flow cannot be in geostrophic balance
• Gradient wind


  The balance between pressure gradient force, Coriolis force and centrifugal force along curved isobars
• Near surface winds in low pressure systems

  Counterclockwise flow in Northern Hemisphere, clockwise flow in Southern Hemisphere
• Convergence of air into low pressure systems

  Rising motion creating favourable conditions for rain clouds
• Divergence out of high pressure systems

  Sinking motion leading to clear and dry conditions
• Sea breezes


  Caused by differential heating
• Land breezes


  Local wind systems
• Winds around ridges in Northern Hemisphere


  Turn clockwise, follow isobars (gradient balance)
• Winds around troughs in Northern Hemisphere

  Turn anticlockwise, follow isobars (gradient balance)