Ch-4, Part-3: Pressure Gradient Force; Mapping Pressure
 
Rev 10-02-2001
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Wind driving forces within the atmosphere
  • Three forces combine to set wind direction and wind speed:
    • The pressure gradient force.
    • Coriolis force
    • Friction force
Pressure gradient force:
  • A pressure gradient is the difference between high and low atmospheric pressure compared to the distance between the low and high pressure locations.
  • High and low pressure areas are caused by differences in surface heating and cooling.
    • Land versus water.
    • Different albedos and energy absorbing abilities of land, vegetation and human built surfaces
  • Hence we can expect breezes and/or winds to occur such as lake to shore, valley to mountain, etc.

                  4-8 (a): Pressure gradient force, alone
Views of the pressure gradient force [4-8 (a), above]:
  • Zone of High pressure
    • Looking down (Top View) on the High:  Air moves outwards from the center of the High, in straight paths (not curving).
    • Sideways view:  Air descends and diverges outwards from the center of the High.
  • Zone of Low pressure
    • Looking down on the Low:  Air moves inwards towards the center of the Low.
    • Sideways view of a Low:  Air converges towards the center of the Low and ascends (rises).
  • Converging and diverging air movements, above, may be "winds."
  • And so why do you suppose the weather reporter frequently forecasts winds if a "Low pressure system" is approaching???
  • Remember, though, the above is just "part of the story," a scientific way of building an explanation for the movement of winds and breezes .... more, below ...

4-7 (a): Pressure Gradients and Isobar Spacing
Isobars are lines of equal atmospheric pressure drawn on a map.
  • Widely spaced isobars display a gradual pressure gradient (Fig. 4-7 (a), above, and 4-7 (b), below).
    • Gradual pressure gradients imply light winds.
  • Closely spaced isobars display a steep pressure gradient.
    • Steep pressure gradients imply strong winds.

4-7 (b): Isobars, pressure gradients and wind strength
Low and high pressure cells
  • Closed loops of isobars with lowest pressure in the center comprise a "Low," usually marked with a capital "L" on a weather map (as above).
  • Closed loops of isobars enclosing a high point of atmospheric pressure comprise a "High" marked with an "H" on a weather map.
  • Elongated areas of high pressure are called ridges
  • Elongated areas of low pressure are called troughs

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