Climate and global circulation

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Unequal Heating and the Global Heat Engine

The Sun heats Earth's surface **unequally**: the equator receives more direct sunlight than the poles.
This temperature difference drives **atmospheric circulation**, which redistributes thermal energy from the tropics toward the poles.
The atmosphere acts as a **heat engine**: solar energy is the heat source, and space is the heat sink.
The work done by this engine moves air masses, balancing Earth's energy budget.

The Three-Cell Model of Atmospheric Circulation

In each hemisphere, the atmosphere is organized into three circulation cells: **Hadley cell**, **Ferrel cell**, and **Polar cell**.
These cells transport heat and moisture, creating global wind belts and pressure zones.
The cells shift slightly poleward in warmer periods but remain stable due to Earth's size and rotation.

The Hadley Cell

Warm, moist air rises at the **equator**, creating a low-pressure zone (Intertropical Convergence Zone, ITCZ).
Rising air moves poleward aloft, cools, and descends at about **30° latitude**, forming high-pressure zones (horse latitudes).
Surface air flows back toward the equator, deflected westward by the **Coriolis effect**, producing the **trade winds** (easterlies).
The Hadley cell is a **thermally direct circulation** and the most powerful atmospheric cell.

The Polar Cell

At about **60° latitude**, relatively warm air rises and moves poleward aloft.
At the poles, air cools, descends, and creates cold, high-pressure zones.
Surface air flows from the poles toward 60°, deflected westward, forming the **polar easterlies**.
The polar cell acts as a **heat sink**, moving cold air toward lower latitudes.

The Ferrel Cell

The Ferrel cell is a **thermally indirect** circulation between the Hadley and Polar cells (30°–60° latitude).
Surface air flows poleward and eastward, deflected by the Coriolis effect, producing the **prevailing westerlies**.
At about 60°, this air meets cold polar air, rises, and returns aloft toward 30°.
The Ferrel cell is driven by the other two cells and helps transfer heat poleward.

The Coriolis Effect

Earth's rotation causes moving air to be deflected: to the **right** in the Northern Hemisphere, to the **left** in the Southern Hemisphere.
This deflection creates the curved paths of global wind belts: trade winds, westerlies, and polar easterlies.
The Coriolis effect also influences ocean currents and the rotation of large storms.

Global Wind Belts and Pressure Zones

**Trade winds**: blow from east to west near the equator (0°–30°), caused by the Hadley cell.
**Prevailing westerlies**: blow from west to east in the mid-latitudes (30°–60°), caused by the Ferrel cell.
**Polar easterlies**: blow from east to west near the poles (60°–90°), caused by the Polar cell.
**Low pressure** zones: at the equator (ITCZ) and 60° latitude; **high pressure** zones: at 30° (horse latitudes) and the poles.

Ocean Circulation and Climate

Ocean currents are driven by **wind belts** and the Coriolis effect, redistributing heat globally.
Warm currents (e.g., Gulf Stream) transport heat from the tropics toward the poles; cold currents (e.g., California Current) bring cool water toward the equator.
Together, atmospheric and ocean circulation regulate regional climates and weather patterns.

Global atmospheric circulation cells (Northern Hemisphere). Arrows show rising air at equator and 60°, descending at 30° and poles, with surface wind belts.

Global wind belts and pressure zones. Low pressure at equator and 60°, high pressure at 30° and poles. Surface winds: trade winds, westerlies, polar easterlies.

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Practice questions

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1.What is the primary energy source that drives atmospheric circulation?
Easy
- AThe Sun
- BEarth's rotation
- CThe Moon's gravity
- DOcean currents
2.In the Hadley cell, air rises at the equator and descends at approximately which latitude?
Medium
- A30°
- B60°
- C90°
- D0°
3.Which of the following correctly describes the direction of surface winds in the Hadley cell of the Northern Hemisphere?
Hard
- AFrom the northeast to the southwest
- BFrom the southeast to the northwest
- CFrom the southwest to the northeast
- DFrom the northwest to the southeast
4.What is the name of the high-pressure zone at about 30° latitude known for light winds and little precipitation?
Medium
- AHorse latitudes
- BDoldrums
- CPolar highs
- DSubtropical lows
5.The polar cell is driven by air rising at which latitude?
Easy
- A60°
- B30°
- C90°
- D0°
6.What causes the deflection of winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere?
Medium
- ACoriolis effect
- BGravity
- CPressure gradient force
- DFriction
7.Which of the following statements about the Ferrel cell is correct?
Hard
- AIt is a thermally indirect circulation cell.
- BIt is driven by strong convection at the equator.
- CIt extends from the equator to 30° latitude.
- DIt produces the trade winds.
8.What type of pressure system is found at the equator due to rising air?
Medium
- ALow pressure
- BHigh pressure
- CVariable pressure
- DNo pressure

Revision notes

## Climate and Global Circulation - **Unequal heating** by the Sun and Earth's rotation drive atmospheric and ocean circulation, redistributing heat from the tropics to the poles. - **Three circulation cells per hemisphere:** Hadley, Ferrel, and Polar cells. ### Hadley Cell - **Location:** 0°–30° latitude. - **Process:** Warm, moist air rises at the equator (low pressure), moves poleward aloft, cools, and descends at ~30° (high pressure). Surface air flows back toward the equator, deflected west by Coriolis → **trade winds** (easterlies). - Shifts north/south with the thermal equator. ### Polar Cell - **Location:** 60°–90° latitude. - **Process:** Air rises at ~60° (low pressure), moves poleward aloft, cools, and descends at the poles (high pressure). Surface air flows toward 60°, deflected west → **polar easterlies**. - Creates cold, dry conditions; record low −89.2°C in Antarctica. ### Ferrel Cell - **Location:** 30°–60° latitude. - **Process:** Surface air flows poleward from the horse latitudes (30°), deflected east → **prevailing westerlies**. Air rises at ~60° and returns aloft toward 30°. ### Key Concepts - **Coriolis effect:** Deflects moving air to the right in the Northern Hemisphere, left in the Southern. - **Horse latitudes:** High-pressure zones at ~30° with light winds and little rain. - **Jet streams:** Fast, narrow air currents at the boundary between cells (e.g., polar jet stream). - **Thermal equator:** Zone of maximum heating; shifts seasonally. - **Rossby waves:** Large-scale waves in the atmosphere that influence jet stream paths. ### Summary Table | Cell | Latitude | Surface Winds | Pressure | |---------|----------|---------------------|----------| | Hadley | 0°–30° | Trade winds (E) | Low at 0°, high at 30° | | Ferrel | 30°–60° | Westerlies (W) | High at 30°, low at 60° | | Polar | 60°–90° | Polar easterlies (E)| Low at 60°, high at 90° | *Note: Winds named from where they come (e.g., trade winds blow from east to west).*

Hadley cell: A large-scale atmospheric circulation cell that extends from the equator to about 30° latitude, driven by rising warm air at the equator and descending cool air at the subtropics.
Ferrel cell: A mid-latitude atmospheric circulation cell between about 30° and 60° latitude, characterized by surface westerlies and rising air at the polar front.
Polar cell: A high-latitude atmospheric circulation cell from about 60° to 90° latitude, with rising air at 60° and descending cold air at the poles, producing polar easterlies.
Coriolis effect: The deflection of moving air (or water) due to Earth's rotation; to the right in the Northern Hemisphere, to the left in the Southern Hemisphere.
Trade winds: Steady easterly winds that blow from the subtropical high-pressure belts toward the equator, part of the Hadley cell.
Polar easterlies: Cold, dry winds that blow from the polar high-pressure areas toward lower latitudes, deflected west by the Coriolis effect.
Prevailing westerlies: Winds that blow from the west in the mid-latitudes (30°–60°), part of the Ferrel cell.
Horse latitudes: Subtropical high-pressure belts at about 30°–35° latitude, characterized by light winds and little precipitation.
Jet stream: A narrow, fast-moving air current in the upper troposphere, often found at the boundary between circulation cells (e.g., polar jet stream).
Thermal equator: The zone of maximum solar heating on Earth, which shifts seasonally north and south of the geographic equator.
Rossby waves: Large-scale, meandering waves in the atmosphere that influence the path of jet streams and weather systems.

Flashcards

What drives atmospheric circulation?

Unequal heating of Earth's surface by the Sun and the rotation of Earth drive atmospheric circulation.

What are the three major atmospheric circulation cells in each hemisphere?

The Hadley cell, Ferrel cell, and polar cell.

Where is the Hadley cell located?

The Hadley cell is located near the equator, extending from about 0° to 30° latitude in each hemisphere.

Describe the movement of air in the Hadley cell.

Warm, moist air rises at the equator, moves poleward in the upper troposphere, cools and descends at about 30° latitude, then flows back toward the equator along the surface.

What causes the trade winds?

The trade winds are the surface winds in the Hadley cell that blow from the east toward the equator due to the Coriolis effect.

What are the horse latitudes?

The horse latitudes are high-pressure zones at about 30° to 35° latitude where winds diverge, typically with light winds and little precipitation.

Describe the polar cell.

At about 60° latitude, air rises, moves poleward in the upper atmosphere, cools and descends at the poles, then flows back toward 60° latitude along the surface.

What are polar easterlies?

Polar easterlies are surface winds in the polar cell that blow from the east (from the poles toward 60° latitude) due to the Coriolis effect.

What is the Coriolis effect?

The Coriolis effect is the deflection of moving air (or water) due to Earth's rotation, causing winds to curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

How does the Coriolis effect influence global wind patterns?

It causes surface winds in the Hadley cell to blow from the east (trade winds), in the Ferrel cell from the west (westerlies), and in the polar cell from the east (polar easterlies).

What is the role of atmospheric circulation in redistributing heat?

Atmospheric circulation moves thermal energy from the equator toward the poles, balancing Earth's temperature distribution.

What is the thermal equator?

The thermal equator is the zone of greatest heating on Earth, which shifts north and south with the seasons.

How does the Hadley cell shift seasonally?

The Hadley cell shifts northward in June/July and southward in December/January, following the Sun's heating.

What are Rossby waves?

Rossby waves are large-scale waves in the atmosphere that influence the path of the polar jet stream.

What is the polar jet stream?

The polar jet stream is a fast-moving air current in the upper atmosphere, located between the Ferrel and polar cells, that influences weather patterns.

How does ocean circulation relate to atmospheric circulation?

Ocean circulation works together with atmospheric circulation to redistribute thermal energy around Earth.

What causes the rising of air at the equator?

Intense solar heating at the equator warms the air, causing it to expand, decrease in density, and rise.

Why does descending air create high pressure?

Descending air becomes denser and compresses, increasing surface pressure, which typically leads to clear skies and dry conditions.

What is a heat engine in the context of atmospheric circulation?

Earth's atmosphere acts as a heat engine driven by solar energy, with the Sun as the heat source and space as the heat sink, producing motion that redistributes heat.

How do tectonic uplift and plate tectonics affect atmospheric circulation over long timescales?

Tectonic uplift can alter jet streams, and plate tectonics can shift ocean currents, potentially changing major circulation patterns over hundreds of millions of years.

What is the approximate power of the Hadley cell as a heat engine?

The Hadley system is estimated to have a power of about 200 terawatts.

Why can't long-range weather predictions be made beyond ten days?

Small-scale weather systems are chaotic, and due to chaos theory (butterfly effect), predictions become unreliable beyond about ten days.

What is the lowest temperature recorded on Earth, and where was it measured?

−89.2 °C at Vostok Station in Antarctica, measured in 1983.

How does the polar cell contribute to cold surface temperatures in Antarctica?

The polar cell, combined with terrain and katabatic winds, creates very cold conditions at the surface.

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