Atmosphere – Pressure Belts of the World

Atmospheric Pressure

  • Defined as weight of column of mercury 76 cm / 760 mm high at sea level
  • Measured by Barometer

 

Isobar           

  • An isobar is an imaginary line drawn through places having equal atmospheric pressure reduced to sea level.
  • Spacing of isobars represent pressure gradient i.e. rate of change of pressure

 


Variation in Atmospheric Pressure

  • Variation in atmospheric pressure is responsible for horizontal movement of winds
  • Some of the factors affecting atmospheric pressure are “Temp. of air, Altitude, Water Vapour, Gravitational force & Rotation of earth
Rise in temperature Pressure Decreases
Increase in Altitude Pressure Decreases
Increase in Water vapour Pressure Decreases (Weight of dry air > weight of moist air)
Gravitational force Inversely proportional to square of radius of earth
Rotation of Earth Exerts centrifugal force outwards

Pressure Belts

  • Horizontal distribution of pressure is expressed in terms of pressure belts
  • Run in E – W direction along certain specific latitudes
  • Total seven pressure belts

Pressure Belts

Polar cell, Ferrel cell, Hadley cell

 

 

Thermally induced pressure belts

  • Equatorial low & polar high

 

Dynamically induced pressure belts

  • Subtropical high & sub polar low

Equatorial Low

  • Low pressure belt extending upto 10 * N & S of equator
  • Low pressure prevails due to high temperature, high water vapour in air & max. speed of earth’s rotation at the equator
  • Surface winds are generally absent as winds rises vertically near this region, hence only vertical currents are found

 

Polar High

  • From 80 – 90* N – S of equator Polar Region
  • Caused by extremely low temp. at poles, which remains below freezing point even during summers
  • Air remains cold & dense resulting in high pressure

 

Subtropical High

  • From 23 ½ * – 35 * N – S of equator
  • A region of high pressure caused due to descent of cool air from high altitudes as shown back
  • Descending air creates calm conditions with feeble winds in this high pressure belt
  • Also known as Belt of Calm or Horse Latitude

 

Subpolar low

  • 45 – 60 * N – S of equator
  • Due to blowing away of air vertically up from the land surface mainly because of
  • Cold air from poles becomes comparatively hot on reaching circles & rises vertically up
  • Due to comparatively greater axial rotation of earth at 45 – 60* N – S than the poles

Wind

  • Horizontal movement of air caused by horizontal atmospheric pressure difference
  • Greater the pressure gradient, greater the velocity of wind
  • Deflection of winds is caused by coriolis effect i.e. Winds get deflected to their right in N – Hemisphere & to their left in S – Hemisphere → Ferrels law
  • Coriolis force changes only direction of wind not its speed
  • Coriolis force speed is 0 at the equator & max at poles
  • That means only horizontal component of coriolis force is considered
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