Pressure Belts (Basics of Geography) – UPSC Notes/PDF

People often make mistakes by not understanding the basics. They start reading and understanding important phenomena like cyclones, tornados, etc. But you will remember those concepts only for a short duration if you don’t know the basics associated with it. So, please try to give more time to the basics. But people find it difficult to understand basics and consider it as a complex. Here, I have made it simple for you in an amazing way.

“The winds blow from High Pressure to Low Pressure areas”

There are two types of pressure i.e. Low Pressure and High Pressure. We have high temperatures at the equator and low temperatures at the poles. At high temperature (near the equator) air raise hit the tropopause (first layer of the atmosphere). What happens to these air after hitting tropopause? We will discuss it later.
The air from the low-temperature region (near poles) occupies the space at the equator. And the air which hit the tropopause earlier, diverge in two branches and moves towards poles. At poles, they descend (go down, because after moving parallel to tropopause for so much time, their temperature decreases and they cool down). And again this air moves towards the equator occupying the space left by hot air.

The High-Temperature Region is called the Low-Pressure area (because air rises) and the Low-Temperature Region is called High-Pressure Area (because air descends).

This is the broader picture of Pressure System on our planet earth. It was initially believed. However, it is PARTLY TRUE.

In reality, we do not have one low pressure and two high-pressure areas but we have three low pressure and four high-pressure areas. Let’s understand about Coriolis Force in brief to understand this phenomenon. Coriolis Force is a force generated by the rotation of the earth. It leads the winds to move clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere. Do not bother about Coriolis’s force much. We will have an elaborative discussion about it later.

Tricellular Meridional Circulation

Source: Vector Stock

In the last paragraph, we observed that there is only one pressure belt, that is associated with temperature alone i.e. High-temperature region (equator) experience low-pressure area and Low-temperature region (poles) experience high-pressure area.
But in reality, High Pressure and Low Pressure are not associate with temperature alone. They are also dynamically induced.

Now, observe the above picture after reading each points below.

  • At 0° latitude, we experience high temperatures hence low-pressure area. The air raises here and hits the tropopause and diverges in two branches going towards poles.
  • Because of Coriolis effect/force, they descend (go down) earlier before reaching poles at 30° latitude. And break into two branches, one going towards poles and another going towards the equator.
  • The one going towards equator again raises at equator forming a cell called ‘Hadley Cell’.
  • Now, come to the poles. At poles, we experience low temperatures hence high-pressure area. The air of the poles moves towards the relatively low-pressure area. Hence they move towards equator.
  • But at 60° latitude, before reaching equator, they come in contact with the surface air (one branch of the Hadley Cell, which descended at 30° latitude). As they converge, they raise and hit the tropopause. After hitting tropopause, they cut off into two branches, one moving towards the equator and one moving towards poles.
  • The poleward moving air reach the poles and again descends forming a cell called ‘Polar Cell’
  • The equatorward moving air reaches at 30° latitude and because of Coriolis force and coming in contact with one branch of Hadley Cell they also descend and form a cell called ‘Ferrel Cell’.

So, there are 7 Pressure belts. Three Low Pressure (two at 0° and one at 60°) and four High Pressure(two at 90° and two at 30°). Further, out of 07 pressure belts, 03 are thermally induced (one at 0° & two at 90°) and 04 are dynamically induced (two at 30° & two at 60°).

Do not bother about what you are seeing between the latitudes. What are they curvy arrows 😮? The wind moves from High Pressure to Low Pressure. It must be in straightway. Why do they have bend 🙄?
As of now, just remember and understand two things:
(i) where are the high pressure and low-pressure areas on earth and
(ii) the wind blows from high pressure to low pressure areas.
It is an interesting concept and we will read that in Planetary winds.

What is the implication of the High Pressure and Low-Pressure system?
What is the implication of these cells i.e. Hadley cell, Ferrel cell & Polar cell?

Our earth is all covered by winds, The winds between 0° latitude and 30° latitude are called Trade Winds.
The winds between 30° latitude and 60° latitude are called Westerlies.
And the winds between 60° latitude and 90° latitude are called Polar Winds/ Easterlies.

The winds move from High Pressure to Low Pressure.

  • At 30°, we have high pressure and at 0° we have low pressure. So, the air moves from 30° latitude to 0° latitude.
  • Again at 60° latitude, we have low pressure and at 30° latitude, we have high pressure. Therefore, winds move from 30° latitude to 60° latitude.
  • At 90°, we have high pressure and at 60° we have low pressure. Hence, winds move from 90° latitude to 60° latitude.

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Planetary Winds (Basics of Geography)