- The concept of meridional circulation was first put forward by Sir George Hadley in 1735.
- He postulated that the air circulates in giant cells in both the hemispheres where cold air falls at the pole and moves along the surface towards the equator.
- Where the air coming from the poles collides and rises up and travels towards the pole in upper atmosphere completing the convectional loop.
- But later it was found that instead of one giant loop there are three different loops forming in the atmosphere which causes global pressure belts and global winds.
- This tri cellular model was given by Palmer in 1951.
- The three different loops are called Hadley cell, ferrel cell and polar cell.
Tropical or Hadley cell:
- There are two Hadley cells found in both hemispheres between 30oN and S latitudes.
- The heated air rises near the equator in Inter Tropical Convergence Zone (ITCZ), flowing pole ward aloft, it descends at the latitude 30o-40o.
- This descend causes a high pressure belt known as sub-tropical high pressure belt.
- This descending then flows either toward the pole or towards the equator.
- The winds flowing towards the equator turns towards their right in northern hemisphere and towards their left in southern hemisphere due to coriolis.
- These are called trade winds or tropical easterlies.
- Similarly, the winds flowing from tropical high pressure belt towards the poles appears to move west to east. They are called Westerlies.
- The mid latitude circulation cells are known as Ferrel cells.
- The winds blowing from the tropical high pressure belt towards the poles encounters high density cold air coming from the poles and rises near 600N and S latitude and move towards tropics in upper air circulation and descends at tropical high pressure belt.
- The air that has grown cold over the polar surface starts flowing towards the equator.
- It encounters the warm air coming from the tropics.
- As there is high density difference the air does not easily intermix but ascends and moves towards the poles completing the third loop.
- The surface winds coming from poles towards the subpolar low pressure belt are swift and because of high coriolis effect near the poles, turns towards their right in northern hemisphere and left in Southern Hemisphere.
- Since they appear to come from east, they are called polar easterlies.
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