Pressure Belts and Planetary Winds

Pressure varies from one place to the other place and also decreases with altitude resulting in world 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 exists 7 alternating high and low pressure belts of Earth. In the 7 pressure belts, there exists 4 types of pressure belts namely,
  
  (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 )
  
  (iv) 2 Polar High Pressure Belts (PHP)( around 90⁰ North and South )
  
  

  

• Of the 7 world pressure belts, Equatorial Low Pressure belt and Polar High Pressure belts of the Earth 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.
  
  Subtropical High Pressure belt is formed due to massive subsidence of winds which results in high pressure. Sub Polar Low Pressure belt, also called Circum-Polar Low Pressure belt, is formed due to massive ascent of wind, resulting in low pressure.
  
  
• These pressure belts of the world 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 Sub-Tropical High Pressure belt to Equatorial Low Pressure belt, the winds are deflected moderately and are called Trade Winds. From Subtropical High Pressure belt to Sub Polar Low Pressure belt, the deflection is high and appears as if blowing from west to east and hence are called Westerlies.
  
  From Polar High Pressure belt to Sub Polar Low Pressure belt, the deflection is high and appears as if 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 Winds. Out of the three types of winds that exist in the world, these winds are of first kind.

Main Features of Planetary Winds

• These 3 types of Planetary winds are permanent winds. They flow throughout the year and also flow throughout the globe. They are responsible for the weather in a given region.
  
  
• Equatorial Low Pressure Belt 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. Equatorial Low Pressure belt is called Doldrums Zone. It is also known to be Intertropical Convergence Zone (ITCZ) as winds from Tropics converge here.
  
  
• Sub-Tropical High Pressure Belt 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 world pressure belts and planetary winds are very strong in nature and are consistent in their direction.