# Introduction

• General circulation of the atmosphere refers to the shifting or movement of air across a large area.
• It is the distribution of heat on the earth’s surface. The general circulation of the atmosphere varies from year to year.
• The difference in temperature between the lower latitudes and the higher latitudes causes the general circulation of the atmosphere.
• Flow of the air carries the heat of the lower latitudes’ empty zone to the upper latitudes. Temperatures towards the polar region are mainly carried out as sensible heat and partly as latent heat.

The angular momentum of the earth is the second regulator of the general circulation of the atmosphere. As the earth rotates, the atmosphere and the earth’s axis revolve around from west to east. So the general circulation of the atmosphere plays an important role on the earth’s surface (1).

## What is the general circulation of the atmosphere?

Earth is surrounded by a multilayer of atmosphere. The movement of planetary air by which heat is distributed all over the earth’s surface is known as the general circulation of the atmosphere.

The atmospheric circulation of the earth is important for mass and energy transfer mechanisms (3).

Atmospheric circulation is regulated by various forces. These are

• Gravitational forces of the earth
• Coriolis force
• Friction force

The difference in air pressure per unit distance, which is perpendicular to the equator, between high pressure and low pressure, is called the horizontal pressure gradient.

Pressure gradient force is the force that acts at right angles with the line of pressure from high pressure to low pressure when there is a difference in air pressure.

Air pressure varies from place to place. These differences occur at different distances. The slope of the air pressure depends on how far or near the two places where there is a difference in air pressure. When the isobars are located close, the air pressure slope is higher and when they are far away, the air pressure slope is lower.

If the slope of the air pressure is high then the pressure gradient force will be higher. And if the pressure gradient force is high then the wind velocity will be higher. The horizontal change in the slope of the air pressure causes the horizontal flow of air. This pressure gradient force adds energy to the flow of air pressure here (3) & (2).

### 2. Gravitational forces of the earth

The force with which the earth pulls everything towards its center is called the gravitational force. Every particle is attracted to the earth by the gravitational force. The amount of energy is fixed in all places. And it only works in the vertical direction. Due to the effect of this force, the velocity of upward wind decreases slightly and the velocity of downward wind is accelerated. However, it has no effect on horizontal airflow (2).

### 3. Coriolis forces

The direction of airflow changes due to the rotation of the earth. This is why the wind does not flow straight from high pressure to low pressure but bends to the right in the northern hemisphere and to the left in the southern hemisphere.

The effect of the earth’s rotation on the equator alone is not seen in airflow. This force of rotation of the earth is called Coriolis force. The amount of this energy depends on two regulators

• Wind velocity
• Latitude position

The amount of Coriolis force at the equator is zero. This force continues to increase at both poles (2) & (1).

### 4. Friction forces

The friction force is the effect of friction on the surface of the lower layers during airflow. Amount of the air that is supposed to be dispersed as it flows over the earth’s surface is less than the effect of friction force. The friction effect is that the surface air flows at a slight angle as it flows from the high-pressure area to the low-pressure area (3).

The large-scale movement of planetary air around high to low-pressure areas and the thermal energy of the ocean distributed over the entire earth’s surface is known as the general circulation of the atmosphere. It is driven by the rotation of the earth and incoming energy from the sun. The atmosphere carries heat and mass in two ways

• Vertical or Latitudinal atmospheric circulation
• Horizontal or Longitudinal atmospheric circulation

Three cells are seen in the general circulation of the atmosphere.

The conduction cell that exists between the subtropical high-pressure region and the equatorial low-pressure region is known as the Hadley cell.

Ferrel cell is located between the subtropical high-pressure zone and subpolar low-pressure areas. And the cell located between the polar high-pressure zone and subpolar low-pressure zones is called the polar cell.

These three cells play an important role in atmospheric circulation. The details of these three cells are described below.

George Hadley was the first to mention the presence of this type of cell on both sides of the equator due to heat, so the tropical cell is known as Hadley cell. It is located between 0° to 30° latitudes in both hemispheres.

### Characteristics

• It is called a direct thermal cell.
• The heat is transferred from the tropics and subtropics towards the Polar Regions through these cells.
• Trade winds can be observed in these cells.
• Here the wind blowing against the trade winds in the upper sky is known as anti-trade wind.

## 2. Ferrel cell

• The second cell in the tricellular model is the Ferrel cell.
• It is located between 30° to 60° in both hemispheres.
• The Ferrel cell is called the indirect thermal cell because of the importance of the earth’s rotational force in its formation.
• The predominance of westerly winds can be observed in Ferrel cells.

Airflow in these cells is often interrupted by temperate cyclones and anticyclones.

• It plays an important role in transporting heat from low latitudes to high latitudes.
• The air front covers a large part of this cell. It is known as the polar front. This air front is formed at the confluence of subtropical winds and cold polar winds.

## 3. Polar cell

These cells are located between the polar high-pressure zone and the subtropical low-pressure zone in both hemispheres. It extends from 60° to the poles in both hemispheres.

### Origin

The polar wind flows parallel to the earth’s surface from the polar high-pressure zone to the subtropical low-pressure zone. This polar wind comes at a latitude of 60-65° and collides with the westerly wind and creates the front. Here the eastern polar wind is influenced by the rotation speed of the earth and goes upwards. When it reaches the middle part of the troposphere, it begins to flow north and south. As it travels towards the poles, this wind blows in the northern hemisphere from the south-west to the west. The same thing occurred in the southern hemisphere. Eventually, this wind lands in the Arctic and Antarctic regions and completes the cycle.

### Characteristics

• It is located in the middle of the pole from 60°.
• The polar cell is affected by the polar air.
• Cold temperatures play a major role in the formation of these cells, so they are called direct thermal cells.
• In the upper part of this region, the wind blows in the form of westerly wind (2) & (1).

## Importance and role

1. Transport heat over the earth’s surface affects the water cycle.
2. Helps the formation of clouds and precipitation.
3. Brings our daily weather.
4. Its long-term pattern helps to determine regional climate and ecosystem (3).

## Q&A

### 1. What role do midlatitude cyclones serve in the general circulation of the atmosphere?

The role of mid-latitude of cyclones are

• Transportation of moisture and heat toward the pole.
• Transportation of warm air towards tropics.
• Mid-latitude cyclones help maintain the balance of the atmosphere by decreasing the temperature gradients of the hemisphere.

### 2. Which below is an assumption of the single-cell model of the general circulation of the atmosphere?

Earth’s surface is uniformly covered with water. This is to remove any land-sea interactions.

### 3. What is the general circulation of the atmosphere?

General circulation of the atmosphere is an atmospheric phenomenon where heat is distributed all over the earth’s surface. This process is caused by the movement of planetary winds. Atmospheric circulation is regulated by various forces. These are

• Gravitational forces of the earth
• Coriolis force &
• Friction forces

### 4. What are the general patterns of circulation in the ocean and atmosphere and what drives them?

Ocean circulation means the movement of water in the ocean. It occurs due to various patterns mentioned below.

• Gyers means circulation moves cold surface water from the poles to the equator and the warm water is sent back toward the poles.
• Upwelling is the movement of surface currents along a coastline that leads to the circulation process. It occurs due to the Coriolis force and commonly occurs on the West coast of the continents where the surface water moves towards the equator and is instead cold water that moves up to the surface.
• Thermohaline circulation is the largest circulation of water on the planet resulting from changes in temperature and salinity.

The sun eventually carries out atmospheric circulation through the uneven heat of the earth’s surface. This is because the earth is a sphere. So solar radiation becomes weaker at higher latitudes because it has to travel more in the atmosphere to reach the surface.

### 5. What is the general circulation of the atmosphere and how does it work?

The movement of planetary air by which heat is distributed all over the earth’s surface is known as the general circulation of the atmosphere.

General circulation of the atmosphere creates wind which moves from a high-pressure zone to a low-pressure area. It also leads to high rainfall areas, like a tropical rainforest, and dry air areas, like deserts.

Reference

1. Savindra Singh. Climatology. Pravalika Publications, Allahabad. Chapter 6: General atmospheric circulation. Page No: 95- 117.