Pressure Belts of the Earth

Pressure Belts of the Earth:

Equatorial Low-Pressure Belt or Doldrums:

• This zone is located between latitudes 10°N and 10°S. It is the area where the trade winds converge.The trade winds are coming from subtropical high-pressure belts from both hemispheres. The zone has extremely calm movements; that is why it is called Doldrums. Because of the apparent movement of the sun, the position of the belt varies.
• This region receives the highest amount of insolation because of the equator. Air gets warmer and rises above the equator.
• The region has vertically upward movement that means the surface will be at low pressure. So, the belt along the equator is called the equatorial low-pressure belt and has calm conditions. The region is not having surface winds because winds approaching the region will rise near to the margin.
• Thus only vertical currents are found. The winds contain a huge amount of moisture because of the presence of oceans.Cyclones do not form at the equator due to the absence of the Coriolis force.

Subtropical High-Pressure Belt or Horse Latitude:

• These belts extend from near the tropic to about 35⁰ N and S. After the complete loss of moisture that is saturation at the ITCZ, the air moving away from the equatorial low-pressure belt in the upper troposphere becomes dry and cold.
• The air descends at 30°N and 30°S, creating high pressure in these regions due to the subsidence of cold, dry, and heavier air. As the air descends, it becomes warm and dry, which is why deserts are commonly found in this belt.
• This creates calm or anticyclonic conditions in the region. The descending air currents supply winds to the adjacent low-pressure belts. This subtropical high-pressure zone is known as the Horse Latitude.
• It is because earlier sailing vessels of the cargo of horses found it difficult to sail under calm conditions of this high-pressure belt. They used to throw the horse into the sea when fodder ran out, and hence it is called Horse Latitude.

Subpolar Low-Pressure Belt:

• This belt is located between 45⁰ N and S to the Arctic and Antarctic circles (66.5⁰ N and S). These belts are not very pronounced year long.The subpolar pressure belts in the northern hemisphere are divided into two types of atmospheric activities.
• One is Icelandic Low and the other is Aleutian Depression. In the southern hemisphere, these belts surround Antarctica and are not well differentiated.
• Coriolis force is produced by the rotation of the earth, and these belts are also dynamically produced. This is the convergence of westerlies and polar easterlies and a zone of ascending air is created.
• This produces polar jet streams because of the contrast between cold and warm air masses. This encircles the earth at 60⁰ latitude and is focussed in these low-pressure areas.

Polar High-Pressure Belts:

• This region surrounds the poles and covers a relatively small area, located between 80° and 90° latitudes in both the northern and southern hemispheres. The air arriving from the subpolar low-pressure belts dries out after reaching saturation.
• This cold, dry air descends toward the poles from the upper troposphere, creating a high-pressure belt at the poles as it subsides.

General Circulation of the Atmosphere:

The movement of planetary winds is called the general circulation of the atmosphere. This general-circulation also influences the ocean water.

The wind is the result of a pressure gradient, which is caused by the differential heating of the earth’s surface and the atmosphere. Wind in the atmosphere is neither unidirectional nor has the same pattern horizontally. Wind movement in the atmosphere is classified into three broad categories:

1. Primary Circulation: It includes a planetary wind system. It is related to the arrangement of pressure belts on the earth’s surface. It is the primary circulation which prepares a framework for the other circulation.
2. Secondary Circulation: It includes cyclones, anticyclones, and monsoons.
3. Tertiary Circulation: It includes all the local winds produced by local causes such as topographical features, sea influences, etc. The impact is visible only in a particular area.

Primary Circulation and Planetary Winds:

It blows from high-pressure belts to low pressure belts in the same direction throughout the year. They blow from vast areas of continents and oceans. It consists of trade winds, Westerlies, and polar easterlies. The pattern of planetary winds depends upon:

• Pressure belt emergence
• Latitudinal variation of atmospheric heating
• Rotation of Earth
• Belts are moving because of the apparent movement of the sun
• Distribution of continents and oceans

Sun’s rays are vertical at the equatorial zone, and the air becomes light and forms a low-pressure area. This is called doldrums or ITCZ. The air rises at the ITCZ because of convection, and the low-pressure area is created. The wind coming from the tropic converges at the low-pressure area and rises with the convective cell. It reaches to the top of the troposphere up to 14 km and moves towards poles. This causes accumulation of air at the 30⁰ N and S. This air sinks and creates a high-pressure area. On the surface, air flows towards the equator are called easterlies. The easterlies or trade winds on either side of the hemisphere converge at the ITCZ.

In the middle latitude, cold air sinks, which is coming from the poles and rising warm air from the subtropical high. At the surface, these winds are called westerlies.

• At polar latitudes, cold, dense air subsided near the poles and blew towards middle latitude as the polar easterlies.

One Cell Model of Troposphere Circulation:

It was given by George Hadley in 1735. It was on the assumption that earth is not a rotating and uniform surface of the earth that is either land or water.

• The Hadley cell is based on the conventional current, which is an upward movement of warm air. According to Hadley, in each hemisphere, there is a large convection cell powered by the heating of the equatorial region. By means of this convectional cell, the surplus energy is transferred polewards.
• Hadley did not consider the rotation of the earth; therefore, his model got oversimplified and, as a result, failed to describe and explain the complexities of tropospheric circulation.

Tri-cellular Meridional Circulation:

The heat in the atmosphere transferred through:

1. Horizontally: The horizontal distribution of heat is mainly because of the unequal heating of different latitudes.
2. Vertically: The vertical distribution of heat is because of ascent and descent of heated and cold air. The meridional circulation of heat transfer results in the formation of certain cells.

• Tropical Cells (Hadley Cells):

Sun rays fall vertically in the equatorial zone. Air becomes light and forms a low-pressure area at the equator. This is known as Doldrums or ITCZ. The warm ascending air releases the latent heat, and it produces the cumulus clouds.

• Cumulus clouds provide the required energy to drive the tropical cells. It rains in the equatorial region. The rising air moves polewards in the upper troposphere. The air of the Hadley cell descends at 30⁰ N and S. This cell is more pronounced in the southern hemisphere than the northern hemisphere because of less land and more water.
• Polar Front Cell (Ferrell Cell):

It develops between 30⁰ and 60⁰ in both hemispheres. In this, the wind blows from southwest to northeast in the Northern Hemisphere and northwest to southeast in the southern hemisphere. It is because of the Coriolis force that winds blow from west to east. In these latitudes, the upper troposphere has movement of winds parallel to the trade winds in both hemispheres. The westerly in this zone is influenced by migratory temperate cyclones. It will help in the mixing of temperature because the direction of the wind in temperate cyclones is variable coming from different directions. Middle latitude circulation is having a vital role in maintaining terrestrial heat balance.

• Polar or Subpolar Cells:

It is located between 60⁰ to 90⁰ N and S in both hemispheres. This is the area of high pressure and also anticyclones. The air descends downwards from the upper troposphere in this area. The air moves towards the subpolar low pressure from the polar high pressure.

• The direction of the wind in the Northern hemisphere is from northeast to southwest, and for the southern hemisphere, it is from southeast to northwest. But under the impact of Coriolis force, wind generally moves from east to west. The cold polar easterlies clash with the warmer westerlies when it moves towards the equator.
• The zone of convergence of these two airflows of different nature is known as Polar Front. The mixing of heat transfer in this cell is done by the waves in westerlies. The wind blows from 60⁰ towards the poles in the upper troposphere.