Weather, Climate and Ecosystems » Global atmospheric circulation and pressure systems

What you'll learn this session

Study time: 30 minutes

The global atmospheric circulation system and how it works
Major pressure belts around the Earth
The three main circulation cells: Hadley, Ferrel and Polar
How the Coriolis effect influences wind patterns
The relationship between pressure systems and weather
How global circulation affects climate zones and ecosystems

Introduction to Global Atmospheric Circulation

Have you ever wondered why deserts form where they do? Or why the UK gets so much rain from the west? The answers lie in how air moves around our planet. Global atmospheric circulation is like Earth's weather engine - it's the system that moves heat and moisture around the globe, creating the different climate zones we live in.

Key Definitions:

Atmospheric circulation: The large-scale movement of air around the Earth that distributes heat and moisture.
Pressure systems: Areas of high or low atmospheric pressure that influence weather patterns.
Convection: The process where warm air rises, cools and then sinks, creating circulation.

Why Does This Matter?

Global atmospheric circulation affects everything from where deserts form to which areas get the most rainfall. It influences the ecosystems that can develop in different regions and impacts human activities like agriculture, settlement patterns and even energy production. Understanding these patterns helps us predict weather and understand climate change impacts.

What Drives Global Circulation?

The sun doesn't heat the Earth evenly. Areas near the equator receive more direct sunlight than the poles, creating temperature differences. These differences drive the movement of air around the planet.

☀ The Sun's Uneven Heating

The equator receives more direct sunlight than the poles. This creates a simple pattern: warm air rises at the equator, moves toward the poles, cools, sinks and then returns to the equator. But Earth's rotation complicates this pattern!

↻ The Coriolis Effect

Because Earth rotates, moving air gets deflected - to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is called the Coriolis effect. It's why winds don't just flow north and south but create circular patterns.

The Three Circulation Cells

Instead of one big circulation loop from equator to pole, Earth actually has three pairs of circulation cells in each hemisphere:

↑ Hadley Cells

Location: Equator to about 30° N/S

Warm air rises at the equator, moves poleward, cools and sinks at around 30° latitude, then returns to the equator along the surface.

Creates: Tropical rainforests near the equator and hot deserts at 30° N/S

↓ Ferrel Cells

Location: 30° to 60° N/S

These mid-latitude cells circulate in the opposite direction to Hadley cells. Air rises at 60° and sinks at 30°.

Creates: Temperate climates with variable weather

↑ Polar Cells

Location: 60° to 90° N/S

Cold air sinks at the poles, flows toward 60° latitude, rises and then returns to the poles at high altitude.

Creates: Cold polar climates

Global Pressure Belts

The rising and sinking of air in these circulation cells creates alternating bands of high and low pressure around the Earth:

Equatorial Low Pressure (ITCZ): At the equator, intense heating causes air to rise, creating a band of low pressure called the Inter-Tropical Convergence Zone (ITCZ).
Subtropical High Pressure: At around 30° N/S, sinking air creates high-pressure zones. These are associated with the world's major deserts.
Sub-polar Low Pressure: At about 60° N/S, the meeting of warm and cold air masses creates areas of low pressure.
Polar High Pressure: Cold, dense air sinking at the poles creates high-pressure zones.

🌪 Low Pressure Systems

When air rises, it creates low pressure at the surface. Low pressure systems are associated with:

Clouds and precipitation
Unstable, often stormy weather
Converging surface winds
Rising air motion

☀ High Pressure Systems

When air sinks, it creates high pressure at the surface. High pressure systems are associated with:

Clear skies and dry conditions
Stable, calm weather
Diverging surface winds
Sinking air motion

Major Wind Belts

The combination of circulation cells and the Coriolis effect creates several major wind patterns around the globe:

Trade Winds: Blow from the subtropical highs (30° N/S) toward the equatorial low (ITCZ). They blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.
Westerlies: Blow from the subtropical highs (30° N/S) toward the subpolar lows (60° N/S). They blow from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.
Polar Easterlies: Blow from the polar highs toward the subpolar lows (60° N/S). They blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.

Case Study Focus: The UK's Weather

The UK sits in the path of the mid-latitude westerlies, which bring weather systems from the Atlantic Ocean. This explains why the UK often experiences:

Prevailing winds from the southwest
Frequent rainfall, especially in western regions
Mild winters and cool summers compared to other places at similar latitudes
Changeable weather as different pressure systems move across the country

The western side of the UK (Wales, Scotland, Northern Ireland) receives more rainfall than the east because the westerly winds drop most of their moisture as they rise over the western hills and mountains.

Seasonal Shifts in Circulation

The global circulation system isn't fixed - it shifts seasonally as the sun's position changes:

🌞 Summer

During the Northern Hemisphere summer:

The ITCZ shifts northward
Monsoon rains reach South and Southeast Asia
Mediterranean regions experience hot, dry conditions

❄ Winter

During the Northern Hemisphere winter:

The ITCZ shifts southward
Monsoon rains reach northern Australia
Mediterranean regions become wetter

Impact on Ecosystems

Global atmospheric circulation directly influences the distribution of the world's major ecosystems:

Tropical Rainforests: Found near the equator where the ITCZ brings heavy rainfall year-round.
Hot Deserts: Found at around 30° N/S where sinking air in the subtropical high pressure belts creates dry conditions.
Mediterranean Ecosystems: Found on the western sides of continents at around 30-40° latitude, with dry summers and wet winters.
Temperate Forests and Grasslands: Found in mid-latitudes where the westerlies bring moderate rainfall.
Tundra and Ice Caps: Found in polar regions where cold air sinks and creates stable, dry conditions.

Climate Change and Global Circulation

Climate change is affecting global atmospheric circulation in several ways:

The Hadley cells are expanding poleward, potentially shifting desert regions
Jet streams are becoming more erratic, causing more extreme weather events
The ITCZ is shifting, altering rainfall patterns in tropical regions
Ocean circulation patterns are changing, which affects atmospheric circulation

Key Takeaways

Global atmospheric circulation is a complex system that:

Redistributes heat from the equator to the poles
Creates distinct pressure belts and wind patterns
Determines the location of the world's major climate zones and ecosystems
Shifts seasonally, creating monsoons and other seasonal weather patterns
Is being affected by climate change, with potential impacts on weather patterns worldwide