Pressure Belts of Earth, Types, Diagram, Shifting of Pressure Belts

Pressure Belts

Pressure belts are areas on the earth’s surface where the same pressure is distributed differently depending on latitude. These are caused by High or Low-Pressure cells. These high- and low-pressure cells generate high- and low-pressure belts, respectively.

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Pressure Belts Across the Latitudes

There are majorly seven Pressure Belts on the surface of Earth i.e., the Equatorial Low, two Subtropical Highs, two Subpolar Lows, and two Polar Highs. With the exception of the Equatorial low-Pressure Belts, others form matching pairs in the Northern and Southern Hemispheres.

1. Equatorial Low-Pressure Belt

This belt extends from the equator to 10 degrees N and 10 degrees S latitudes. This belt is thermally produced due to heating by the Sun. As the sun shines almost vertically on the equator throughout the year, it heats the air and the warm air rises over the equatorial region and produces equatorial low pressure.

Due to excessive heating, the horizontal movement of air is absent here, and only vertical conventional currents of air blow in this belt. This belt is called the doldrums (the zone of calm) because of the virtual absence of surface winds. This belt is also called an Inter Tropical Convergence Zone (ITCZ) because the trade winds or the winds flowing from subtropical high-pressure belts converge here.

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2. Sub-Tropical High-Pressure Belts

These belts extend roughly between 25 degrees and 35 degrees latitudes in both Hemispheres. These belts are formed due to the rising air of the equatorial region, which is deflected towards the poles because of the earth’s rotation. After becoming cold and heavy, it descends in these regions and piles up. This results in the formation of a high-pressure zone.

Calm conditions with feeble, variable winds are found here. In the southern hemisphere, this belt is broken by small low-pressure areas in summer over Australia and South Africa. In the northern hemisphere, the belt is more discontinuous by the presence of landmasses, and high pressure occurs only over the ocean areas as discrete cells.  These are termed the Azores and Hawaiian cells in the Atlantic and Pacific areas, respectively. These belts are also called Horse latitudes.

In older days, vessels with a cargo of horses passing through these belts found it difficult to sail under these conditions. They used to throw the horses into the sea to make the vessels light. These are the regions of divergence as the winds from these areas blow towards equatorial and subpolar low-pressure belts.  In these belts’ upper atmosphere, the upper-level westerlies and anti-trade winds converge. This sets up descending currents in the atmosphere.

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3. Sub-Polar Low-Pressure Belt

It extends along 60 degrees latitudes (55-65 degrees ) in both hemispheres. These belts are not thermally induced like equatorial low-pressure belts, rather are formed by the rotation of the Earth. The winds from the subtropics and the polar regions converge in this belt and rise upward. The great temperature contrast between the subtropical and the polar regions gives rise to cyclonic storms in this belt.

In the Southern hemisphere, this low-pressure belt is more pronounced due to the vast ocean presence and is also referred to as the sub-antarctic low. But in the northern hemisphere, there are large land masses along 60 degrees latitudes which are very cold. Therefore, the pressure over these landmasses increases. Thus, the continuity of the belt breaks.

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4. Polar High-Pressure Belt

Air compresses, and their density increases due to low temperatures. Hence, high pressure is found at the pole throughout the year. This is more marked over the land area of the Antarctic continent than over the northern oceans. In the northern hemisphere, high pressure is not centred at the pole, but it extends from Greenland to Islands situated in the northern part of Canada.

This system of pressure belts that we studied is a generalized picture. In reality, the placement or location of these pressure belts is not permanent. They shift northward in the month of July and southward in the month of January, following the changing position of the Sun’s direct rays, they migrate between the Tropic of Cancer and the Tropic of Capricorn.

Apart from the pressure varying from place to place, the pressure varies from season to season also. This change in pressure with the change in season affects the weather and climate of a region.

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Pressure Belts Diagram

High-pressure areas known as subtropical highs can be found along 30° N and 30° S. Subpolar lows are low-pressure belts that extend further north and south along 60° N and 60° S. The polar high is characterised by high pressure near the poles.

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Shifting of Pressure Belts

The pressure belts as stated above would not exist if the earth were not inclined toward the sun. Because of the earth’s 23 1/2° inclination towards the sun, therefore, this is not the case. This tendency causes significant changes in the temperature of the continents, oceans, and pressure conditions in the month of January and July.

Pressure Belts January Condition

In the month of January, the Sun drifts in the southward direction of the equator. This results in the shift of the equatorial belt south of the mean equatorial position. South America, South Africa, and Australia become areas of low pressure as the land heats up more rapidly than the water. The subtropical high-pressure cells are centred over the ocean in the southern hemisphere.

The belt of high pressure is interrupted by the continental landmasses where the temperature is significantly high. The subtropical high-pressure cells are well developed in the eastern part of the ocean, where the cold ocean currents dominate. While in Asia, high pressure is developed. This is due to the fact that land cools more rapidly than oceans.

Pressure Belts July Conditions

The equatorial low-pressure belt shifts little north of the mean equatorial position because of the northward movement of the Sun in the month of July. In Asia, low pressure is developed as the lands are rapidly heated up than the ocean. The subtropical highs are more developed over the Pacific and the Atlantic Ocean in the northern hemisphere.

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Pressure Belts UPSC

There are seven such pressure belts on the planet. Subtropical high-pressure belts can be found in both the northern and southern hemispheres at approximately 30 degrees latitude. This article will discuss an important phenomenon known as Pressure Belts in the context of the UPSC IAS Exam.

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