Mechanism of Global Air Circulation

 

    Air circulation is one of the prominent features of the Earth. It is the type of circulation that happens when there is an uneven distribution of temperature. For example, when there is more heat in one place and more cold in another place, there will be movement of air between these two places to balance the heat. This is the same principle as the process of wind formation, but on a larger scale. Please visit my previous article to know more about the formation of the wind.

https://sciencetopic03.blogspot.com/2021/10/what-causes-winds-science-behind.html

 

Pressure difference

    Global air circulation is the movement of a large mass of air over the earth, to satisfy the Earth’s temperature. This major movement of air is mainly described by the concepts of high-pressure and low-pressure. A high-pressure zone is a type of zone, where a huge mass of air settles down on the earth’s surface, due to the presence of heavy moisture; whereas a low-pressure zone is a type of zone, where the gases in that place rise up, due to the loss of moisture by immense heat. Due to this concept of high and low pressure, and the rotation of the earth, the global air circulation takes place around the globe in three cells namely Hadley, Ferrel, and Polar cells. Among these, the Hadley cells are the bigger ones, in which the larger mass of air circulation takes place. Let’s see the operation of the air circulation in a detailed schematic representation.

Global Air Circulation

    By seeing the above figure we can understand, that there are three cells in both the northern and the southern hemispheres. The air circulation takes place in all over these cells. The above pictorial representation is the cross-section of the earth. Actually, the cells cover the whole part of the earth. Each hemisphere will have these three cells. Each cell has been identified in the above diagram in different colors. These cells are categorized based on the different zones on the earth, which are with respect to the different amounts of sunlight falling on the earth. The very center part of the earth, where the sunlight falls directly and produces more intense heat is called the tropical zone. Similarly while moving 30°N from the tropical zone we get the subtropical zone. There is a major heat difference between the Sub-tropical and the tropical zone which creates a huge amount of airflow between these two places. This airflow creates the Hadley cell in the atmosphere. Similarly, by moving 30°N from the subtropical zone we get the subpolar zone. The air movement between the subtropical and the subpolar zone creates the Ferrel cell. At last moving to the extreme pole, we will have the North Pole. The air circulation between the sub-polar zone and the extreme North Pole creates the Polar cell.

    The most crucial thing that needs to be considered in this global air circulation is the pressure difference. Each cell border creates a high pressure and a low-pressure zone depending upon the sunlight falling on it. Always the low-pressure zone is satisfied with the high-pressure zone, which means the air in the high-pressure zone moves to the low-pressure zone. Because there is more air in the high-pressure zone and there is no air in the low-pressure zone. Now as the earth faces the sun directly in the tropical zone, the atmosphere gets heated rapidly leading to the surface air to get warm and expand, therefore causing it to rise (as it is less dense). Now as the air goes up, it creates an empty space in the surface, causing low pressure; which means there is no air or anything to cause pressure on it. As the warm air goes upward, it meets the tropopause where the temperature gets slightly cooler. Due to this, the air molecules get condensed and become denser. Now due to Coriolis Effect, the condensed air moves to the sub-tropical zone from the tropical zone. Due to the heavy density of the condensed air, it falls over the subtropical zone where the temperature is slightly cooler. This circulation of air creates the Hadley cell.

Formation of Hadley Cell

    When the air sinks at the subtropical zone, it meets the earth’s surface and creates a divergence effect. Due to the divergence effect, it gets splits up into two directions as shown in the above figure. This diverged air again moves to the Hadley cell causing a chain reaction in the cell. Now as the cold air gets accumulated over the subtropical zone, it creates a high-pressure area over that zone. The important point to be noted is that, the air is driven by the heat of the sun, happens only at the Hadley cell. The remaining air movement over all the cells are caused by the convergence and the divergence effect fully. Now as one air movement is moving towards the north from the subtropical zone it meets the incoming polar current and converges at the bottom and rises up (Convergence effect) as there is no space for the air to go. Upon going, it again diverges by the existing polar air currents in the tropopause and the Coriolis Effect. So this rising cold air from the Sub-polar zone creates a low pressure, leading to precipitation. This air circulation creates the Ferrel cell. Finally, the air moving from the Ferrel cell to the pole sinks and diverted back to the Ferrel cell, creating the polar cell.

Formation of Ferrel and Polar cell

    These air movements around the cells create a continuous chain reaction and will cause a sustainable loop in distributing the temperature over the earth's surface. The same thing happens in the southern hemisphere but in the opposite direction which means the winds are directed in the opposite direction. There is also one of the important points to be noted in the moving of the air currents. Those are called jet streams. Jet streams are one of the prevailing winds in the world. These winds are created at the tropopause during the movement of the wind from one zone to another zone. These winds are one of the strongest winds in the world, which flows in the direction from west to east. By using this feature airplanes fly at the jet streams to travel fast and to save their fuel. There are also many more winds flowing on the earth which are categorized as primary (permanent winds), Secondary (seasonal winds), and Tertiary (local winds).

    The permanent winds are always permanent, and it flows all along the year over the earth's surface. The seasonal winds appear only for a specific period of the season. The local winds are also seasonal but only occur for a particular time of the day.

Permanent winds

Ø  Trade winds

Ø  Westerlies

Ø  Easterlies or the polar easterlies

     Periodic winds

Ø  Monsoon winds

Ø  Land breeze

Ø  Sea breeze

Ø Mountain and valley breeze

     Local winds

Permanent Winds or Planetary winds or prevailing winds

    Planetary winds are the type of winds that flow continuously over the surface of the earth. It flows all the days of a year. These winds are the surface air that is flowing inside the cells while the upper air is called jet streams.

Trade winds

    These winds flow from the subtropical region to the equatorial region. These winds are classified as Northeast trade winds and Southeast trade winds. This name was framed by the direction of origination of the winds. The northeast trade winds originate from the direction of the northeast direction and go to the southwest direction. This direction is due to the effect of the Coriolis Effect. Similarly in the southern hemisphere, the southeast trade winds originate from the southeast direction and proceed to the North West direction. 

Formation of Trade Winds

Westerlies

    The westerlies are classified as south-westerlies and north-westerlies. These winds flow from the subtropical region to the polar region. South westerlies are named such as; they originate from the southeast direction and proceed to the North West direction. Similarly in the southern hemisphere, the winds originate from the sub-tropics in the North West direction and proceed to the southeast direction.

Formation of Westerlies

Polar easterlies

    As the name suggests it is associated with the poles. And as the wind direction moves from the east direction it is determined as the easterlies. Thus the polar easterlies name comes. These winds originate from the North Pole and move to the sub-polar low.

Formation of Polar Easterlies

Global Air Circulation

Periodic winds

    Periodic winds are the type of winds that flows over the earth only during a specific duration. These winds occur due to the seasonal changes of the earth.

Monsoon winds

    Monsoon is a seasonal reversal of wind occurring due to different heating patterns of the land and sea. Monsoon occurs in various parts of the world, and the particularly noticeable is at the Indian subcontinent. It is similar to the land and sea breeze, but on a larger scale.

Land breeze

    Land breeze is the movement of air from the land to sea. During night time the land gets cooler than the sea. So there will be a high-pressure over the land and a low- pressure over the sea. This creates the airflow from the land to the sea; causing a land breeze.

Sea Breeze

    A sea breeze is just opposite to a land breeze, which flows from sea to land. During the daytime, the temperature will be higher. This creates the land to get hotter than the sea. Due to this, low-pressure will be created over the land and high-pressure will be created over the sea, which causes the airflow from land to sea.

Mountain and valley breeze

    These winds occur between the mountains and the valleys. During the daytime, the temperature of the air over the slope gets warmer than in the valleys. This creates a pressure difference and the airflow begins to move from the valley to the mountain slopes. Similarly, during night time vice versa happens, and airflow gets opposite; terming as a mountain breeze.

Local Winds

    Local winds occur in the local regions. These winds occur due to the smaller temperature difference between the two areas. There are several local winds around the world like Chinook in Northern America and Loo in the Northern plains of India.