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World Geography / Climatology

Pressure Belts and Planetary Winds

    Pressure varies from one place to the other place and also decreases with altitude resulting in pressure belts and planetary winds. Atmospheric pressure is nothing but the force exerted by the atmospheric column on a unit area of earth's surface. It is measured in the units of kg/cm2 or bars or millibars or length of Mercury ( Hg ) column in cm or mm. At Mean Sea Level, the pressure is 76 cm of Hg column or 1013 millibars.

    Pressure Belts of the World

    • There are 7 alternating high and low pressure belts on the Earth. The belts are, (i) 1 Equatorial Low Pressure belt ( ELP ) ( 100 North – 50 South), (ii) 2 Sub Tropical High Pressure Belts ( STHP )( 300 – 350 North and South ), (iii) 2 Sub Polar Low Pressure Belts ( SPLP )( 600 – 66 1/20 North and South ) and (iv) 2 Polar High Pressure Belts ( PHP ) ( around 900 North and South ).

    • Of the 7 belts, ELP belt and PHP belts are formed due to thermal factors. High temperature in equatorial region leads to low pressure in ELP belt and low temperature in polar regions leads to high pressure in PHP belts of the world. STHP belts are formed due to massive subsidence of winds which results in high pressure. SPLP belts are formed due to massive ascent of wind, resulting in low pressure.

    • These pressure belts are called Global pressure belts. Since pressure gradient is there from one belt to the other, wind blows from high pressure region to low pressure region. But these winds are deflected by the Coriolis force ( Centrifugal forces ) arising due to earth's rotation.

    • According to Ferrel's Law, in the northern hemisphere, the Coriolis force deflects the winds towards the right and in the southern hemisphere, they are deflected towards the left. The deflection is minimum near the Equator and maximum at the Poles.

    • From STHP belts to ELP belt, the winds are deflected moderately and are called Trade Winds. From STHP belts to SPLP belts, the deflection is high and appear as blowing from west to east and hence are called Westerlies. From PHP belts to SPLP belts, the deflection is high and appears as blowing from east to west and hence are called Polar Easterlies.

    • All these winds formed by the pressure belts are called Global winds or Planetary winds or Primary Wind System.

    Main Features of Planetary Winds

    • These are permanent winds. They flow throughout the year and also flow throughout the globe. They are responsible for the weather in a given region.

    • ELP is zone of wind convergence ( having strong convective activity ). Trade winds generally do not reach the Equator ( up to 100N – 50 S ).

    • Heated winds in ELP ( convective winds ) ascend vertically and even Trade Winds start ascending. Within the Equatorial region, there are no horizontal winds. ELP zone is known as Doldrums Zone. It is also known to be Intertropical Convergence Zone ( ITC ) as winds from Tropics converge here.

    • STHP is a zone of wind divergence. Here no strong ( feeble ) horizontal winds are present and hence called "Zone of Calm".

    • The Latitudes in 30 – 350 S are called Horse Latitudes because, as there are no horizontal winds here, the boats carrying horses through these latitudes, used to come to halt and hence the horses were thrown into the seas in order to reduce the weight of boats.

    • Towards the south of 350 S, there are very strong Westerlies, which are strong in the Southern hemisphere than in Northern hemisphere and boats used to overturn here.

    • 400 South Latitudes are called Roaring Forties. 500 South Latitudes are called Furious Fifties. 600 South Latitudes are called Screaming Sixties or Shrieking Sixties.

    • The winds formed due to pressure belts and planetary winds are very strong in nature and are consistent in their direction.