Atmospheric Pressure

Atmospheric Pressure

Atmospheric pressure is the weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere.

  • Therefore, at a place, if the air is dense for instance near the Earth’s surface (due to Gravity), the atmospheric pressure will be more.
  • It is expressed in atm (Atmosphere), mb (millibar) and Pa (Pascal).
  • It is measured with the help of a mercury barometer or the aneroid barometer.
  • At sea level, the average atmospheric pressure is 1 atm or 1,013.2 mb or 1,013.2 h Pa (kilo Pascal).
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VARIATIONS OF ATMOSPHERIC PRESSURE
  • As the Pressure depends on the number of air molecules present at any place, it varies both vertically as well as horizontally.
  • This variation of the Atmospheric Pressure has been playing a very important role in Weather and Climate.
  • Its variation is the main cause of air motion/ wind.
 
Atmospheric Pressure
 
VERTICAL VARIATION
  • The pressure decreases with height because air gets thinner.
  • The average decrease is about 1 mb per each 10m increase in elevation, subject to other factors such as Temperature, local topography, closeness to the sea, etc.
  • Therefore, if the surface Atmospheric Pressure at any place is 1,000 mb, then the Pressure at 1 km above the surface will be (1000 – 100) mb i.e. 900mb.
  • Despite high vertical pressure gradient, there is weak upward wind because the pressure gradient gets weakened by the Gravitational force.
 
HORIZONTAL VARIATION
  • The horizontal variation of the Pressure depends on the differential heating (insolation) of the surface which causes the differential air volumes.
  • These variations are highly significant in terms of wind direction and speed. Though the direction and speed depend also on Frictional force and Coriolis force.
  • Horizontal distribution of pressure is studied by drawing isobars (lines connecting places having equal pressure) at constant levels.

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