NCERT Notes: Atmospheric Pressure [Geography Notes For UPSC]

NCERT notes on important topics for the UPSC Civil Services Exam. These notes will also be useful for other competitive exams like banking PO, SSC, state civil services exams and so on. This article talks about the Atmospheric pressure.

Atmospheric pressure
  • The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called the atmospheric pressure.
  • It is measured in force per unit area.
  • It is expressed in ‘milibar’ or mb unit.
  • In application level, the atmospheric pressure is stated in kilo-pascals.
  • It is measured by the aneroid barometer or mercury barometer.
  • In lower atmosphere, pressure declines rapidly with height.
  • The vertical pressure gradient force is much larger than that of the horizontal pressure gradient and is commonly balanced by an almost equal but opposite gravitational force.
  • Low-pressure system is encircled by one or more isobars with the lowest pressure at centre.
  • High pressure system is also encircled by one or more isobars with highest pressure in centre.
  • Isobars are lines connecting places having equal pressure.

Pressure Gradient
  • The rate of change of pressure in regard to distance is the pressure gradient.

Pressure belts
  • There is a pattern of alternate high and low-pressure belts over the earth.
  • There are seven pressure belts.
  • Except the Equatorial low, there are two Sub-Tropical highs (in North and South), the two Sub-polar lows (in North and South), and the two Polar highs (in North and South).
  • The above-given pressure belts oscillate with the movement of the sun.
  • In the northern hemisphere, they move southwards in winter, and in summers they move northwards.
  • The Equatorial region gets abundant heat and warm air being light, the air at the Equator rises, generating a low pressure.
  • Equatorial low
    • It is found near the equator.
    • The sea level pressure is low.
  • Subtropical high
    • The region in 30 degrees North and 30 degrees South, which are high-pressure areas.
  • Sub-polar Lows
    • The region in 60 degrees North and 60 degrees South, which are low-pressure belts.
  • Polar Highs
    • These occur near poles which have high pressure.

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