Atmospheric Circulation and Weather Systems include storms, rains, and clouds. Atmospheric Circulation is the movement of the air that distributes the thermal energy on the surface of the Earth on a large scale. It is a major factor in the global climate that also affects the moisture amount and the precipitation. On the other hand, the weather system is a movement of the warm and the cold air in the whole Earth. This movement is based on the pressure system.
In this article, we are going to discuss the Atmospheric Circulation and Weather Systems of the Earth in detail.
Chapter 9 Atmospheric Circulation and Weather Systems| Class 11 Geography Notes
Atmospheric Pressure
Atmospheric Pressure is the air or biometric pressure. It is a force that applies on the surface of the Earth by the present air above it. In simple words, it is also known as the air pressure. Atmospheric pressure and temperature have an inverse relation. When the temperature increases, the atmospheric pressure will decrease.
There are two distribution types of Atmospheric Pressure as mentioned below.
Vertical Variation of Pressure
Vertical Distribution of Atmospheric Pressure denotes the condition when the air pressure decreases with the increased altitude. This type of atmospheric pressure is always high at the sea level.
Horizontal Distribution of Pressure
Horizontal Distribution of Atmospheric Pressure is defined by the isobars. These isobars connect those places that have uniform atmospheric pressure on the Earth.
World Distribution of Sea Level Pressure
Near the equator, sea level pressure is typically low, forming what is known as the equatorial low-pressure zone. Moving away from the equator towards approximately 30° N and 30° S, high-pressure areas known as the subtropical highs are observed. Further towards the poles, around 60° N and 60° S, low-pressure belts called the subpolar lows are found. Near the poles themselves, pressure tends to be high, forming the polar high-pressure zones. It’s important to note that these pressure belts are not fixed and can vary over time.
To analyze the horizontal distribution of pressure, isobars are drawn on maps to connect places with equal pressure. Pressure readings are typically adjusted to sea level to eliminate the influence of altitude.
The pressure gradient force arises from differences in atmospheric pressure and results in the acceleration of air from regions of high pressure to regions of low pressure. The strength of the pressure gradient force depends on the spacing between isobars, with stronger gradients indicated by closely spaced isobars and weaker gradients by widely spaced isobars.
Frictional force impacts wind speed and is most significant near the Earth’s surface, extending up to elevations of 1-3 km. Over the surface of the sea, frictional effects are minimal.
The Coriolis force, named after the French physicist who described it in 1844, is a result of the Earth’s rotation on its axis. This force deflects the direction of moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Its effect on wind direction becomes more pronounced with increasing distance from the equator. The Coriolis force influences the movement of pressure systems, causing them to oscillate with the apparent movement of the sun. For instance, in the Northern Hemisphere, pressure systems tend to move southwards in winter and northwards in summer.
Forces Affecting the Velocity and Direction of Wind
Factors Influencing Wind Movement
The motion of air, termed wind, is primarily driven by differences in atmospheric pressure. Wind flows from areas of high pressure to areas of low pressure. Friction and the Coriolis force, influenced by the Earth’s rotation, also affect wind movement. Thus, near the Earth’s surface, horizontal winds respond to the combined effects of three forces: pressure gradient force, frictional force, and the Coriolis force. Additionally, gravitational force acts downward.
Pressure Gradient Force
Pressure gradient force arises from differences in atmospheric pressure and is responsible for accelerating air from regions of high pressure to regions of low pressure. The force is stronger where isobars (lines connecting places with equal pressure) are closely spaced and weaker where they are apart.
Frictional Force
Frictional force affects wind speed, with its greatest impact near the Earth’s surface and diminishing with altitude. Over the sea, frictional effects are minimal.
Coriolis Force
The Coriolis force, a consequence of the Earth’s rotation, deflects the direction of moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection increases with wind velocity and latitude. It is strongest at the poles and absent at the equator.
The Coriolis force acts perpendicular to the pressure gradient force, which is perpendicular to isobars. The interaction of these two forces causes wind to circulate around low-pressure areas. At the equator, where the Coriolis force is zero, winds blow perpendicular to isobars, preventing the intensification of low pressure and hindering the formation of tropical cyclones.
Wind Circulation Patterns
The velocity and direction of wind result from the balance between wind-generating forces. At upper atmospheric levels, winds are controlled mainly by the pressure gradient and the Coriolis force. When isobars are straight and friction is absent, winds blow parallel to isobars, forming geostrophic winds.
Around low-pressure areas, air converges and rises, leading to cyclonic circulation, while around high-pressure areas, air subsides and diverges at the surface, resulting in anticyclonic circulation. These wind circulations are influenced by larger atmospheric circulation patterns, such as the Hadley, Ferrel, and polar cells, which transport heat energy from lower to higher latitudes.
Effects of General Atmospheric Circulation on Oceans
The general atmospheric circulation influences ocean currents, initiating large and slow-moving oceanic currents. Oceans, in turn, contribute energy and water vapor to the atmosphere, affecting weather and climate. An important example is the El Niño-Southern Oscillation (ENSO) phenomenon, which disrupts weather patterns globally, leading to events like heavy rainfall in South America, droughts in Australia and India, and floods in China.
Seasonal and Local Wind Patterns
Wind patterns vary seasonally due to shifts in regions of maximum heating, pressure, and wind belts. Monsoons, particularly over Southeast Asia, exemplify significant seasonal wind modifications. Local winds, such as land and sea breezes, mountain and valley winds, and katabatic winds, are influenced by differential heating and cooling of land and water surfaces.
Air Masses and Fronts
Air masses, large bodies of air with relatively uniform temperature and moisture characteristics, form over homogeneous regions called source regions. These air masses, classified based on their source regions, interact at boundaries called fronts. Fronts, including cold, warm, stationary, and occluded fronts, are associated with abrupt changes in weather conditions and precipitation.
Extra Tropical Cyclones and Tropical Cyclones
Extra-tropical cyclones develop in middle and high latitudes and are characterized by frontal systems. They cover larger areas than tropical cyclones, affecting weather over land and sea. Tropical cyclones, originating and intensifying over warm tropical oceans, bring about large-scale destruction when they make landfall. Known by various names depending on their location, these storms are associated with warm sea surface temperatures, the Coriolis force, and favorable atmospheric conditions.
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FAQs on Atmospheric Circulation and Weather Systems
How does atmospheric circulation determine our weather?
Differential heating is the major reason why we have different weather patterns, jet streams, deserts and prevailing winds is all because of the global atmospheric circulation caused by the rotation of the Earth and the amount of heat different parts of the globe receive.
What is the weather circulation system?
Global Atmospheric Circulation is the movement of air around the planet. It explains how thermal energy and storm systems move over the Earth’s surface. Without the Earth’s rotation, tilt relative to the sun, and surface water, global circulation would be simple.
What are the two main causes of atmospheric circulation?
This pattern, called atmospheric circulation, is caused because the Sun heats the Earth more at the equator than at the poles. It’s also affected by the spin of the Earth.
Why is atmospheric circulation important?
Atmospheric circulation is an essential part of Earth’s climate system because it redistributes heat around the planet. These large scale wind circulations move in response to differences in temperature at the equator, the warmest region of the planet, and the poles, which are the coldest regions.
What powers our weather?
Energy from the Sun drives climate by heating Earth’s surface unevenly. Ice also reflects incoming sunlight, cooling the poles even more. The temperature difference sets the ocean and atmosphere in motion as they work together to distribute heat around the planet.
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