Pressure Belts Of Earth Explained In Detail

Nilesh Kodag | Updated: Aug 5, 2021 14:42 IST

This post is also available in: हिन्दी (Hindi)

Pressure Belts of Earth refer to the regions that are dominated either by high-pressure cells or low-pressure cells. The high and low-pressure belts are arranged alternatively on the Earth's surface.

  • In general, the atmospheric pressure varies indirectly with the temperature. Due to unequal heating of the Earth, low pressure is developed over the equatorial region and high pressure is developed near the poles.
  • Due to other factors such as different albedo of the earth's surface and Coriolis force, there is a huge variation in the horizontal distribution of the air pressure in the World. They are represented by seven pressure belts – One Equatorial low-pressure belt, two Subtropical high-pressure belts, two Sub-polar low-pressure belts and two Polar high-pressure belts.

In this article on pressure belts of the Earth, we shall discuss the air pressure, the 7 pressure belts and also the variations caused in these pressure belts. This is a very significant topic under the climatology part of Geography and will be helpful for the UPSC IAS aspirants in their upcoming preliminary as well as mains examinations.

Aspirants can also check the NCERT Notes on Koppen's group C climate from the linked article.

Pressure Belts Of Earth (UPSC Geography): Download PDF Here!

About Air Pressure

  • The weight of a column of air contained in a unit area, from the mean sea to the top of the atmosphere is known as atmospheric or air pressure. In layman's terms, it is the pressure exerted by the weight of air on the earth's surface.
  • The unit of atmospheric pressure is millibar and they are measured using a mercury barometer or aneroid barometer.
  • The variations in atmospheric pressure are resultant of the air which gets expanded at higher temperatures and compressed at lower temperatures. Thus, low pressure is developed in the regions where the temperature is high and high pressure is developed in the regions where the temperature is low.
  • The air always moves from high-pressure areas to low-pressure areas.
  • The atmospheric pressure also varies seasonally, diurnally and spatially.

Vertical Distribution of Air Pressure: The air pressure is maximum at sea level and it decreases gradually with an increase in height. This is because the air is denser at the surface. Though the vertical pressure gradient force is comparatively more than the horizontal pressure gradient force, they are balanced by the gravitational force which is equal and opposite.

Horizontal Distribution of Air Pressure: It is determined on the basis of isobars drawn at constant levels. Isobars are the lines which connect the places having equal pressure.

Earth's Pressure Belts

  • On the basis of the distribution pattern of surface pressure on Earth, there are seven alternate low and high-pressure belts on the earth's surface:
    1. Equatorial low-pressure belt
    2. Sub-Tropical high-pressure belt – Northern hemisphere
    3. Sub-Tropical high-pressure belt – Southern hemisphere
    4. Sub-polar low-pressure belt – Northern hemisphere
    5. Sub-polar low-pressure belt – Southern hemisphere
    6. Polar high-pressure belt – Northern hemisphere
    7. Polar high-pressure belt – Southern hemisphere
  • Out of the seven pressure belts, six pressure belts form matching pairs in the Northern and Southern hemispheres. An exception is the Equatorial low-pressure belt which remains at the equator.
  • The pressure belts in the Southern hemisphere are more or less continuous, whereas the pressure belts in the Northern hemisphere are discontinuous.
  • The distribution of pressure from the equator towards the poles does not follow a definite pattern. This is because atmospheric pressure is determined not only by the temperature but also by other factors such as Earth's rotation and the presence of water vapour.
  • Based on the mode of genesis, the seven pressure belts of the Earth are broadly divided into two types:
    1. Thermally Induced pressure Belts, and,
    2. Dynamically Induced pressure Belts.
Block diagram of Classification of Pressure Belts Of Earth
Classification of Earth's Pressure Belt

Let us discuss the pressure belts of Earth in detail.

Thermally Induced Pressure Belts

The Thermally Induced Pressure Belts are further divided into two types:

  1. Equatorial Low-Pressure Belts
  2. Polar High-Pressure Belts

We will explain these two types in detail below.

Equatorial Low-Pressure Belts

  • The equatorial low-pressure belt lies along the equator and between 5°N and 5°s latitudes.
  • Due to vertical sun rays, the equatorial belt/region experiences a high temperature throughout the year. This in turn develops low pressure over the belt and it is referred to as the equatorial low-pressure belt. Thus it is a thermally induced pressure belt.
  • This zone is not stationary because the position of the belt varies with the movement of the sun.
  • This belt is a zone of convergence of northeast and southeast trade winds.
  • Since the equatorial low-pressure belt experiences calm conditions i.e. without any air movement, it is also known as doldrums.

Polar High-Pressure Belts

  • The polar high-pressure belts are located at 90° N and 90° S latitudes. This zone is commonly referred to as the polar highs.
  • Since the polar regions receive only slanting rays of the sun, the temperature remains low throughout the year. Thus high pressure is developed over this zone.
  • Also, the subsidence of cold and dry air in this zone increases the pressure. Due to the Coriolis effect, the subsiding air gets converted into anticyclones and leads to the formation of gales (very strong winds).
  • This is a thermally induced pressure belt.
  • Owing to the diurnal rotation of the earth, the air in this belt experiences thinning of its layers.

Dynamically Induced Pressure Belts

The Dynamically Induced Pressure Belts are further divided into two types:

  1. Sub-Tropical High-Pressure Belts
  2. Sub-Polar Low-Pressure Belts

We will explain these two types in detail below.

Sub-Tropical High-Pressure Belts

  • The Sub-tropical high-pressure belts lie between 25° to 35° latitudes in both hemispheres. It is also known as the horse latitude.
  • This belt is characterized by high temperature and high pressure. The air in this region is comparatively dry.
  • It is known as the North sub-tropical high-pressure belt in the northern hemisphere and the South sub-tropical high-pressure belt in the southern hemisphere.
  • Due to the rotation of the earth, the air from the equatorial region, which gets deflected towards the poles, descends into this region after becoming cold and heavy. This creates high pressure in the region and thereby forms subtropical high-pressure belts.
  • It is classified as a dynamically induced pressure belt because it is formed due to the rotation of the earth.
  • This is referred to as the zone of wind divergence, as the winds from this belt blow towards the equatorial low-pressure belt and the sub-polar low-pressure belt. It is also the zone of anticyclones.

Sub-Polar Low-Pressure Belts

  • The sub-polar low-pressure belts are located between 60° to 65° latitudes in both hemispheres.
  • This belt is known as the temperate convergence zone because the warm and cold air masses from the sub-tropical and the polar high-pressure belts respectively, converge at this zone. This in turn produces cyclonic storms.
  • It is a dynamically induced pressure belt which is evident from the fact that the pressure over this zone is low, despite the temperature being low.
  • In the northern hemisphere, the sub-polar low-pressure belts become discontinuous during the summer and continuous during the winter.

Seasonal Variations In Pressure Belts

The pressure belts vary in the month of January and July, let's learn the variations in detail below:

Pressure Belt Variations in January

  • During the month of January, the equatorial low-pressure belt shifts a little south of the mean equatorial position due to the southward apparent movement of the sun. As a result, all the other pressure belts shift southward.
  • The sub-polar low-pressure belt in the southern hemisphere does not develop any cells, whereas the sub-polar low-pressure belt in the northern hemisphere develops two cells called  Iceland low and Aleutian low over the North Atlantic and North Pacific oceans respectively.

Pressure Belt Variations in July

  • During the month of July, the equatorial low-pressure belt shifts a little north of the mean equatorial position due to the northward apparent movement of the sun. As a result, all the other pressure belts shift northward.
  • The low cells (Iceland low and Aleutian low) developed over the oceans disappear and now, the landmasses develop low cells.
  • The sub-tropical high-pressure belt in the southern hemisphere becomes continuous whereas the one in the northern hemisphere becomes discontinuous.

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Pressure Belts Of Earth FAQs

Q.1 What is the meaning of pressure belts of the Earth?

Ans.1 The pressure belts of the earth are the equatorial low-pressure belt at the centre and the sub-tropical high-pressure belts, sub-polar low-pressure belts and polar high-pressure belts in both the northern and southern hemispheres.

Q.2 Where is the equatorial low-pressure belt located?

Ans.2 The equatorial low-pressure belt is located along the equator and between 5°N and 5°s latitudes. Due to the seasonal variations, the width of the equatorial low-pressure belt may vary between 5°N and 5°S and 20°N and 20°S.

Q.3 What causes high pressure in the sub-tropical belt?

Ans.3 The high pressure in the sub-tropical belt is caused due to the descending of air from the equatorial region into this belt. Since the air descending into this region is cold and heavy, high pressure is developed.

Q.4 Where is the polar high-pressure belt located?

Ans.4 The polar high-pressure belts are located at 90° N and 90° S latitudes which are referred to as the polar highs.

Q.5 What are the three factors that affect the air pressure?

Ans.5 The temperature of the atmosphere, rotation of the earth and the amount of water vapour present are the three factors that influence the air pressure.

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