Discuss in detail the Tri-cellular Model of Atmospheric Circulation.
(UPSC geography optional paper 1 2019, 20 Marks)
Answer.
Due to the spherical shape of Earth, rotation of the earth in inclined axis, and revolution; the distribution of insolation is not equal all over the earth's surface and unequal heating creates pressure gradients; As a result, three low-pressure belts and four high-pressure belts have developed permanently on Earth. The location of the pressure belts is shown in the figure below.
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| World Pressure Belt |
As we know that winds always flow from high pressure to low pressure, and when heated the air rises, and when it cools it sinks.
The following three types of wind movements make atmospheric circulation circular:
- Rising of air at the low atmospheric belt.
- The sinking of air at the high atmospheric belts.
- Movement of surface winds from high atmospheric pressure to low atmospheric pressure.
There are three circulation motions of air over larger areas in each hemisphere, and these are also called cells:
- Hadley cell
- Ferrel Cell
- Polar cell
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| Tri cellular Model of atmospheric circulation |
Hadley cell:
- The ITCZ has a low atmospheric pressure belt because the region receives high insolation and is also the meeting point of two trade winds from both hemispheres. In this region, the wind rises up to the tropopause and cools down due to adiabatic loss. Cold air does not settle down in this region (the low atmospheric pressure belt) because here the wind is rising, it moves towards the north and south poles and settles down in latitudes of about 30 to 35 degrees in both hemispheres.
- At the boundary of the upper troposphere, the wind does not move beyond 30 degrees north and south latitudes as the wind cool and settles down near 30 to 35 degrees latitude in both hemispheres.
- At the surface, part of the wind from 30 degrees north and south of the hemisphere moves toward the equator as trade winds [high pressure to low-pressure wind movement].
- The movement of winds between 0-degree latitude to 30 degrees north and south latitude is called the Hadley cell.
Ferrel cells:
- The movement of winds in both the hemispheres from 35 to 60-degree latitude is called Ferrell cell.
- On surfaces, cold air from 30 degrees north and south of the hemispheres moving toward the poles is called westerly winds. As it moves across the surface, it heats up and rises at latitudes 60 to 65 north and south.
- In the upper troposphere, some of the air raised above latitudes 60 to 65 north and south moves toward the poles and settles down on polar latitudes. A portion of the air moves towards the equator and cools down and settles down at 30 to 35 degrees latitude.
Polar cell:
- The movement of air from 65 to 90 degrees latitude in both hemispheres is called Polar cells.
- At the surface, since the poles have low temperatures and high pressures, the wind thus blows from the poles toward the equator as easterly winds. Moving on the surface, the air gets heat and it rises at 60 to 65 degrees latitude in both the hemispheres.
- The air rising above 60 to 65 degrees latitude in the upper layer moves towards the poles and after cooling settles down at the poles.
- The movement of air from 65 to 90 degrees latitude in both hemispheres is called Polar cells.
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1 Comments:
Click here for CommentsSubtropical high and subpolar lows are dynamically induced, they don't rise due to heat near the surface as there is no heat to do so
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