Pressure Belts of the 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. The pressure belts are formed on the earth’s surface because Warm air is light, and the air at the Equator rises, creating low pressure.

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

Also, check out the article on the Composition of the Atmosphere here.

What are Pressure Belts of the Earth?

Pressure belts of the Earth refer to the distinct zones of atmospheric pressure that encircle the planet. These belts are also known as atmospheric circulation cells. They are created by the unequal heating of the Earth's surface by the sun. These pressure belts play a crucial role in shaping global weather patterns.

• Based on the distribution pattern of surface pressure on Earth, there are seven alternate low and high-pressure belts on the Earth’s surface:
  • Equatorial low pressure belt
  • Sub-Tropical high-pressure belt – Northern hemisphere
  • Sub-Tropical high-pressure belt – Southern hemisphere
  • Sub-polar low-pressure belt – Northern hemisphere
  • Sub-polar low-pressure belt – Southern hemisphere
  • Polar high pressure belt – Northern hemisphere
  • 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 those in the Northern Hemisphere are discontinuous.
• The pressure distribution from the equator toward the poles does not follow a definite pattern. This is because atmospheric pressure is determined by the temperature and 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:
  • Thermally Induced pressure Belts and,
  • Dynamically Induced Pressure Belts.

Check out notes on the Heating And Cooling Of The Atmosphere here.

Classification of Earth’s Pressure Belt

Thermally Induced Pressure Belts

Thermally-induced pressure belts are regions of the Earth's atmosphere where variations influence the air pressure in temperature. These belts are formed due to solar radiation's uneven heating of the Earth's surface.

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

• Equatorial Low-Pressure Belts
• 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 develops low pressure over the belt and is referred to as the equatorial low pressure belt. Thus it is a thermally induced pressure belt.
• This zone is not stationary because the belt's position varies with the sun's movement.
• 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 air movement, it is also known as the doldrums.

Also, check out the Bonn Climate Change Conference here.

Polar High-Pressure Belts

• The polar high pressure belts are 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 forms gales (very strong winds).
• This is a thermally induced pressure belt.
• Owing to the earth's diurnal rotation, the air in this belt experiences thinning of its layers.

Dynamically Induced Pressure Belts

Dynamically induced pressure belts are regions in the Earth's atmosphere where air pressure is influenced by dynamic factors, particularly the movement and interaction of air masses and the Earth's rotation. Unlike thermally induced pressure belts, primarily driven by temperature variations, dynamically induced pressure belts are shaped by atmospheric circulation patterns and the Coriolis effect.

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

• Sub-Tropical High-Pressure Belts
• 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 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 earth's rotation, 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 earth's rotation.
• This is called 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.

Also, check out the National Action Plan on Climate Change here.

Sub-Polar Low-Pressure Belts

• The sub-polar low-pressure belts are 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 polar high pressure belts converge at this zone. This, in turn, produces cyclonic storms.
• It is a dynamically induced pressure belt which is evident from the low pressure over this zone, 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.

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

Seasonal Shifting of Pressure Belts

The pressure belts vary in January and July. Let’s learn the variations in detail below:

Pressure Belt Variations in January

• During January, the equatorial low pressure belt shifts a little south of the mean equatorial position due to the apparent southward 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. In contrast, 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.

Also, study the Air (Prevention and Control of Pollution) Act, 1981 here.

Pressure Belt Variations in July

• During July, the equatorial low pressure belt shifts a little north of the mean equatorial position due to the apparent northward 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; 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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