Atmospheric Circulation: Distribution of Pressure, Winds, and ITCZ

The movement of air around the earth’s surface is known as Atmospheric Circulation. It explains how storm systems and thermal energy move over the planet. 

What is the Atmospheric System?

• The atmospheric pressure is the weight of a column of air per unit area extending from the mean sea level to the top of the atmosphere. It is quantified in millibar units.
• The uneven distribution of temperature on Earth’s surface leads to variations in atmospheric pressure. When air is heated, it expands, and when it cools, it gets compressed. 
• This causes air to move from areas of high pressure to low pressure, setting it in motion. The movement of wind helps distribute heat and moisture across the planet, thereby maintaining a consistent temperature overall. 
• Moist air rises vertically, cools, and forms clouds, which eventually result in precipitation. Gravity causes the air near the surface to be denser, resulting in higher pressure. The measurement of air pressure is done using a mercury barometer.

Vertical Variation of Pressure

• In the lower atmosphere, the pressure diminishes quickly as the altitude increases. Approximately 1 mb of pressure is lost for every 10 m increase in elevation. However, the rate of decrease may vary. 
• While the vertical pressure gradient force is significantly greater than the horizontal pressure gradient force, it is typically counterbalanced by an almost equal and opposite gravitational force. As a result, we do not encounter strong upward winds.

Horizontal Variation of Pressure

• The wind direction and velocity are greatly influenced by even small differences in pressure. To analyse the horizontal distribution of pressure, isobars are employed, which are lines connecting locations with equal pressure. 
• To account for the impact of altitude on pressure, measurements are taken at weather stations and then adjusted to sea level for comparison purposes. Weather maps display the distribution of pressure at sea level. 
• Pressure systems can be identified by studying the patterns of isobars. A low-pressure system is characterized by one or more isobars surrounding the center with the lowest pressure. 
• Similarly, a high-pressure system is enclosed by one or more isobars with the highest pressure at its center.

The Forces which affect the atmospheric Circulation by the Velocity and Direction of Wind:

• Pressure Gradient Force
  • The force generated by variations in atmospheric pressure can be attributed to differences in pressure. The pressure gradient refers to the rate at which pressure changes in relation to distance. A stronger pressure gradient occurs when isobars (lines connecting points of equal pressure) are closely spaced, while a weaker pressure gradient is observed when isobars are farther apart.
• Frictional Force
  • The speed of the wind is influenced by it. The greatest impact is observed at the surface, and its effects typically reach up to an altitude of 1 – 3 km. In contrast, there is minimal friction over the surface of the sea.
• Coriolis Force
  • The movement of the Earth around its axis influences the wind’s direction. This phenomenon is known as the Coriolis force, named after the French physicist who first explained it in 1844. 
  • In the northern hemisphere, the wind is deflected towards the right, while in the southern hemisphere, it is deflected towards the left. The degree of deflection increases with higher wind speeds. The Coriolis force is directly linked to the latitude angle, being strongest at the poles and non-existent at the equator.

Pressure and Wind 

• The speed and path of the wind are determined by a combination of factors. In the upper atmosphere, approximately 2 – 3 km above the Earth’s surface, the wind is not influenced by surface friction. 
• Instead, it is primarily influenced by the pressure gradient and the Coriolis force. When the lines connecting areas of equal pressure (isobars) are straight and there is no friction, the pressure gradient force is counterbalanced by the Coriolis force. 
• As a result, the wind blows parallel to the isobars in this scenario, and it is referred to as the geostrophic wind.

• In atmospheric Circulation, the circulation of wind around a low pressure system is known as cyclonic circulation, while around a high pressure system it is referred to as anticyclone circulation.
• The direction of winds around these systems varies depending on their location in different hemispheres. The wind circulation near the Earth’s surface around lows and highs is often closely linked to the wind circulation at higher altitudes. 
• Typically, air converges and rises over areas of low pressure, while over areas of high pressure, air subsides from above and diverges at the surface. 
• In addition to convergence, several factors such as eddies, convection currents, orographic uplift, and uplift along fronts contribute to the upward movement of air, which is crucial for cloud formation and precipitation.

General Atmospheric Circulation

• The pattern of planetary winds mainly depends on-
  • Latitudinal variation of atmospheric heating
  • Emergence of pressure belts
  • The movement of belts followss the apparent path of the sun
  • The distribution of continents and oceans
  • The rotation of the earth. 
• The movement of the planetary winds is known as the general atmospheric circulation. This atmospheric circulation also affects the movement of ocean water, which in turn has an impact on the Earth’s climate. The given below figure provides a description of the general atmospheric circulation.

Inter Tropical Convergence Zone (ITCZ)

• The air in the atmospheric Circulation at the Inter Tropical Convergence Zone (ITCZ) rises due to convection caused by high solar radiation, leading to the formation of a low pressure area. The winds from the tropics converge at this zone of low pressure. As the converged air rises, it forms a convective cell and reaches the top of the troposphere, reaching an altitude of approximately 14 km. 
• From there, it moves towards the poles, resulting in the accumulation of air around 30-degree N and S latitudes. Some of the accumulated air descends towards the Earth’s surface, creating a subtropical high. 
• Another reason for the air to sink is the cooling that occurs when it reaches latitudes of 30-degree N and S. Near the surface, the air flows towards the equator as easterlies. The easterlies from both sides of the equator converge at the Inter Tropical Convergence Zone (ITCZ). 
• These atmospheric circulations, from the surface upwards and vice versa, are known as cells. In the tropics, this type of cell is called the Hadley Cell. 
• In the middle latitudes, the atmospheric circulation consists of sinking cold air from the poles and rising warm air from the subtropical high. These winds at the surface are referred to as westerlies, and the cell is known as the Ferrel cell. 
• At polar latitudes, the cold and dense air descends near the poles and moves towards the middle latitudes as polar easterlies. This cell is called the polar cell. These three cells establish the general pattern of atmospheric circulation. 
• The transfer of heat energy from lower latitudes to higher latitudes plays a crucial role in maintaining this overall atmospheric circulation.
• The atmosphere’s general circulation also has an impact on the oceans. The large-scale winds in the atmosphere create slow-moving currents in the ocean. 
• Conversely, the oceans contribute energy and water vapour to the air. These interactions occur gradually across a significant portion of the ocean.

Atmospheric Circulation and its Effects on Oceans

• The warming and cooling of the Pacific Ocean play a crucial role in the overall atmospheric circulation. The central Pacific Ocean gradually carries warm water towards the coast of South America, replacing the cool Peruvian current. This occurrence of warm water near Peru’s coast is commonly referred to as El Nino. 
• The El Nino event is closely connected to pressure changes in the Central Pacific and Australia. This change in pressure conditions across the Pacific is called the southern oscillation. The combined effect of the southern oscillation and El Nino is known as ENSO. 
• During years when ENSO is particularly strong, significant weather variations occur worldwide. The dry west coast of South America experiences heavy rainfall, while Australia, India, and at times China can face droughts and floods. This phenomenon is closely monitored and utilized for long-range forecasting in many parts of the world.

Source: Distribution of Pressure

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