The general atmospheric circulation model explains how air moves around the Earth, creating predictable pressure belts and wind systems that shape global climates and weather patterns.
The Earth’s heat imbalance
The driving force behind global atmospheric circulation is the uneven heating of the Earth's surface by the Sun. This happens because the Earth is a sphere and sunlight strikes different parts at different angles:
At the equator, sunlight hits the Earth directly, resulting in concentrated solar energy and high surface temperatures.
Towards the poles, the same amount of sunlight is spread over a larger area and travels through more atmosphere, making it less intense.
This imbalance causes differences in air temperature, which leads to differences in air pressure. Warmer air is less dense and rises, creating low pressure. Cooler air is denser and sinks, creating high pressure. The atmosphere responds to this imbalance by moving air from areas of high pressure to low pressure, attempting to reach equilibrium. This movement creates the global circulation system.
The three-cell model of atmospheric circulation
To understand global atmospheric circulation, scientists divide the atmosphere into three interconnected convection cells in each hemisphere. These cells transport heat energy from the equator to the poles and back again.
Hadley cell (0°–30° latitude)
The Hadley cell is the largest and most powerful of the three cells.
It extends from the equator to about 30° north and south.
At the equator, warm air is heated by intense solar radiation. This air rises, creating a low-pressure zone known as the Intertropical Convergence Zone (ITCZ).
As the air rises, it cools and condenses, forming large cumulonimbus clouds and producing heavy rainfall, especially in tropical rainforests.
The cooled air moves towards 30° N and S in the upper atmosphere.
When it reaches around 30°, the air sinks, causing high pressure and dry conditions. This creates desert climates like the Sahara and Australian Outback.
At the surface, air flows back toward the equator to replace the rising air, forming the trade winds.
Ferrel cell (30°–60° latitude)
The Ferrel cell is found between 30° and 60° in both hemispheres.
It is more complex than the Hadley cell and is indirectly powered by the other two cells.
In this cell, surface air flows poleward from the high-pressure areas at 30° and meets cold air moving from the poles at around 60°.
At 60°, the warmer, lighter air from the subtropics is forced to rise over the denser polar air, creating a low-pressure zone.
This rising air then moves back toward 30° at high altitudes, where it descends to complete the loop.
The Ferrel cell is responsible for much of the variable weather experienced in the temperate zones, including the UK and parts of North America.
Polar cell (60°–90° latitude)
The polar cell operates from 60° to the poles at 90°.
Cold air at the poles sinks, creating high pressure.
This air moves towards the equator at the surface and meets the warmer air from the Ferrel cell at around 60°.
The warmer air forces the cold air to rise, leading to the creation of a polar front, a major source of storm systems.
The rising air then moves back towards the poles at high altitude and sinks again, completing the polar circulation.
Each of these three cells forms a convection loop that redistributes heat and moisture, helping to regulate Earth’s climate system.
Major pressure belts
The movement of air within the three-cell system creates distinct bands of high and low pressure around the globe:
Equatorial low (0°) – Where warm air rises at the equator, creating a low-pressure zone.
Subtropical high (30° N/S) – Where air sinks, creating a high-pressure zone.
Subpolar low (60° N/S) – Where warm and cold air meet, causing rising air and low pressure.
Polar high (90° N/S) – Where cold air sinks, leading to a high-pressure zone.
These pressure belts are responsible for shaping global wind patterns and influencing the climate zones around the world.
Surface winds and the Coriolis effect
Air moves from areas of high pressure to low pressure, but the Earth’s rotation causes these winds to curve. This is known as the Coriolis effect:
In the Northern Hemisphere, winds are deflected to the right.
In the Southern Hemisphere, winds are deflected to the left.
This deflection causes the creation of three major global wind belts:
Trade winds (0°–30° latitude)
Air moves from the subtropical high at 30° towards the equatorial low.
In the Northern Hemisphere, these winds blow from the northeast, and in the Southern Hemisphere, from the southeast.
The winds converge at the ITCZ, bringing moisture and storms to tropical areas.
Westerlies (30°–60° latitude)
Air moves from the subtropical high at 30° toward the subpolar low at 60°.
These winds blow from the west to the east in both hemispheres.
They are responsible for much of the weather in temperate regions, such as the UK and northern USA.
Polar easterlies (60°–90° latitude)
Air flows from the polar high to the subpolar low.
These winds blow from the east to the west, and are generally cold and dry.
These global wind belts, influenced by pressure zones and the Coriolis effect, form the basis of global weather systems.
Seasonal changes in circulation patterns
The general circulation model is not fixed. It shifts with the seasons because of the Earth’s 23.5° axial tilt and its orbit around the Sun.
Seasonal migration of the ITCZ
In June–July, when the sun is overhead at the Tropic of Cancer (23.5°N), the ITCZ moves northward, bringing rainy conditions to regions like India and West Africa.
In December–January, the sun is overhead at the Tropic of Capricorn (23.5°S), and the ITCZ shifts southward, affecting parts of Brazil and Indonesia.
Example: South Asian Monsoon
In summer, the northward-shifted ITCZ draws moist ocean air inland. When this air hits land, it rises, cools, and causes torrential rainfall.
In winter, the ITCZ shifts south, and dry air from the land brings dry weather to much of the region.
This seasonal shift leads to the wet and dry seasons in many tropical regions and is crucial for agriculture and water supply.
Climatic zones influenced by atmospheric circulation
The interaction between the pressure belts, surface winds, and the Earth's surface results in distinct climate zones around the world:
Equatorial regions (0°–15° latitude)
Low pressure, high rainfall, and high humidity.
Dominated by the ITCZ and rising air of the Hadley cell.
Consistent temperatures year-round.
Examples: Amazon Rainforest (Brazil), Congo Basin (Africa), Borneo (Indonesia).
Subtropical regions (around 30° latitude)
High pressure, low humidity, minimal rainfall.
Caused by sinking air of the Hadley cell.
Hot, dry deserts with extreme temperatures.
Examples: Sahara Desert (Africa), Arabian Desert, Atacama Desert (South America).
Temperate regions (30°–60° latitude)
Variable weather due to the influence of westerlies and Ferrel cell.
Four distinct seasons.
Moderate rainfall.
Examples: United Kingdom, United States, parts of China and Australia.
Polar regions (60°–90° latitude)
Cold and dry year-round.
High pressure dominates due to sinking cold air.
Low precipitation, mostly in the form of snow.
Examples: Antarctica, Arctic regions like Greenland and Northern Canada.
Jet streams
Jet streams are fast-flowing currents of air found in the upper atmosphere at the boundaries of circulation cells, especially near the tropopause (around 10–15 km high).
Types of jet streams:
Polar jet stream – Found between the Ferrel and Polar cells, around 60° latitude.
Subtropical jet stream – Found between the Hadley and Ferrel cells, near 30° latitude.
These winds can reach speeds of over 200 mph (320 km/h) and play a key role in:
Steering weather systems.
Influencing high- and low-pressure systems.
Determining the development of storm systems.
Jet streams are stronger in winter due to larger temperature differences between air masses.
Intertropical Convergence Zone (ITCZ)
The ITCZ is a belt of low pressure found near the equator where the northeast and southeast trade winds converge.
Key characteristics:
Warm, moist air rises due to high surface temperatures.
This air cools and condenses, causing heavy rainfall and thunderstorms.
The ITCZ migrates with the seasonal position of the overhead sun, leading to seasonal rainfall patterns.
Regions close to the ITCZ experience frequent and intense rainfall, making it a crucial component of tropical weather systems and rainforest ecosystems.
Global examples illustrating atmospheric circulation
Amazon Basin (Equator)
Located within the ITCZ, the Amazon receives year-round rainfall.
Driven by constant uplift of warm air in the Hadley cell.
Sahara Desert (30°N)
Under the influence of descending dry air from the Hadley cell.
Experiences minimal rainfall and extreme daily temperatures.
United Kingdom (50°–60°N)
Affected by the westerlies and Ferrel cell.
Weather is variable, with regular changes in pressure systems.
Jet stream frequently brings storms and rain.
Antarctica (90°S)
Located beneath the polar high with descending cold air.
Conditions are extremely dry, making it the driest continent despite being covered in ice.
Understanding global atmospheric circulation helps explain the location of climatic zones, the movement of weather systems, and the seasonal changes in temperature and precipitation patterns around the world.
FAQ
Deserts form around 30° north and south due to the presence of subtropical high-pressure zones created by the descending limbs of the Hadley cells. At the equator, warm air rises, cools, and loses moisture through heavy rainfall. This now dry air spreads toward 30° latitude, where it descends. As air descends, it becomes warmer and drier, increasing its ability to absorb moisture from the land surface. This creates arid conditions and leads to minimal cloud cover and high daytime temperatures. Because the air is sinking, it also prevents the formation of precipitation. These areas experience very low annual rainfall, often less than 250 mm, and extremely high rates of evaporation, making them ideal for desert formation. Famous deserts like the Sahara, Kalahari, and Atacama lie in these zones. The consistency of this pattern globally highlights the role of large-scale atmospheric circulation in shaping regional climates.
Ocean currents work closely with global atmospheric circulation to redistribute heat around the Earth’s surface. When surface winds like the trade winds and westerlies blow across the oceans, they push water, creating surface currents. These currents move warm water from the equator toward the poles and cold water from the poles toward the equator. For example, the Gulf Stream carries warm water from the Gulf of Mexico across the Atlantic, warming western Europe, while the California Current brings cooler waters to the western coast of North America. These ocean currents affect coastal climates, with warm currents leading to milder, wetter conditions, and cold currents resulting in cooler, drier environments. Additionally, the interaction between atmospheric circulation and ocean currents contributes to phenomena like El Niño and La Niña, which can cause extreme weather patterns globally. Thus, ocean currents amplify and modify the effects of atmospheric circulation on climate.
The Coriolis effect is strongest at the poles and weakest at the equator, due to the rotation of the Earth. It causes moving air (and water) to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. At the equator, there is minimal Coriolis force, which is why air rises vertically in the Intertropical Convergence Zone without much horizontal deflection. As latitude increases, the Coriolis effect becomes more pronounced. This deflection creates the global wind belts: trade winds, westerlies, and polar easterlies. For example, air moving toward the equator from the subtropical highs is deflected westward, creating the northeast trade winds in the Northern Hemisphere and southeast trade winds in the Southern Hemisphere. Without the Coriolis effect, winds would flow straight from high to low pressure, but with it, we see curved and spiral patterns that define cyclones, anticyclones, and global wind direction.
The Polar Front is a boundary zone between cold polar air and warmer air from the mid-latitudes, typically found at around 60° latitude in both hemispheres. This front is the meeting point of the cold polar easterlies and the warmer westerlies from the Ferrel cell. At this boundary, the warmer, less dense air is forced to rise above the denser, colder air, creating a zone of low pressure. This uplift leads to the development of mid-latitude depressions or cyclones, which are responsible for much of the changeable and stormy weather seen in temperate regions. The Polar Front plays a crucial role in driving the jet streams, particularly the polar jet stream, which sits just above this boundary and helps steer weather systems across continents. Changes in the position or strength of the Polar Front can result in major shifts in regional weather, such as colder winters or stormier seasons in Europe or North America.
Jet streams are narrow bands of fast-moving air located at high altitudes (around 10–15 km above the Earth's surface) near the boundaries between atmospheric cells, especially between the Ferrel and Polar cells. These winds can reach speeds over 200 mph (320 km/h) and play a critical role in steering surface weather systems, such as cyclones and anticyclones, across the globe. Meteorologists closely monitor the position and strength of jet streams because they affect the development, movement, and intensity of weather systems. For example, if the polar jet stream dips south, it can bring cold Arctic air into lower latitudes, leading to cold snaps or snowstorms. Conversely, when it moves north, it can result in unusually warm conditions. Jet streams also influence air travel, reducing flight times when aircraft travel with the wind, and increasing turbulence when winds are unstable. Their dynamic behavior is a key focus in weather forecasting and climate modeling.
Practice Questions
Explain how the global atmospheric circulation model influences the climate of tropical and desert regions.
The global atmospheric circulation model influences climate by creating pressure belts that determine weather patterns. At the equator, intense solar heating causes air to rise, forming a low-pressure zone called the Intertropical Convergence Zone (ITCZ). This rising air cools, condenses, and produces heavy rainfall, resulting in a hot, wet tropical climate. Around 30° north and south, the air that rose at the equator sinks, forming high-pressure zones. This descending air is dry, leading to arid conditions and desert climates. These zones include the Sahara Desert and Australian Outback, both examples of hot, dry environments shaped by global circulation.
Describe the three-cell model of atmospheric circulation and explain how it helps to transfer heat around the Earth.
The three-cell model consists of the Hadley, Ferrel, and Polar cells, which move air and heat around the planet. In the Hadley cell, warm air rises at the equator, moves poleward at high altitude, then sinks at 30° latitude, transferring heat away from the equator. The Ferrel cell moves air from 30° to 60°, while the Polar cell circulates cold air from the poles to 60°. These cells work together to redistribute thermal energy from warmer to cooler regions. This system helps balance global temperatures and maintains the Earth’s overall climate stability through consistent circulation patterns.
