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Global Wind
Patterns

Understand how differential solar heating, the Coriolis effect, and pressure gradients drive Earth's atmospheric circulation and shape our climate.

AP Environmental Science

What Are Global Winds?

Global winds are large-scale, persistent air movements that flow across entire hemispheres. Unlike local breezes caused by small temperature differences, global winds arise from the unequal heating of Earth's surface by the Sun.

These winds redistribute heat energy from the tropics toward the poles, making most of Earth habitable. Without them, the equator would be far hotter and the poles far colder than they are today.

Differential Heating: The Engine

Equator (0°) 30°N 30°S North Pole South Pole Direct rays Oblique rays

Pressure Gradients Drive Wind

Warm air at the equator rises (low pressure at the surface), while cooler air at the subtropics sinks (high pressure at the surface). This pressure difference creates a pressure gradient force (PGF) that pushes air from high to low pressure.

Low Pressure (L)
Warm air rises, creating surface low pressure zones. Found near the equator and ~60° latitude. Associated with clouds and rain.
High Pressure (H)
Cool air sinks, creating surface high pressure zones. Found near ~30° latitude and the poles. Associated with clear, dry skies.

The Coriolis Effect

Earth rotates from west to east. Moving air does not rotate with the surface at the same speed, causing an apparent deflection: the Coriolis effect.

Northern Hemisphere
Moving air deflects to the right of its direction of travel.
Southern Hemisphere
Moving air deflects to the left of its direction of travel.

The Three-Cell Model

90°N 60°N 30°N 0° 30°S 90°S Polar Cell Ferrel Cell Hadley Cell (N) Hadley Cell (S) Ferrel Cell Polar Easterlies Westerlies NE Trade Winds SE Trade Winds Westerlies H (Polar High) L (Subpolar Low) H (Subtropical High) L (ITCZ) H (Subtropical High) H (Polar High)

The Hadley Cell: 0° to 30°

1
Intense solar heating at the equator warms air, causing it to rise (convection). This zone of rising air is the Intertropical Convergence Zone (ITCZ).
2
Rising air cools and spreads poleward at high altitude. Moisture condenses, producing heavy tropical rainfall.
3
Air sinks at ~30° latitude, creating subtropical high pressure zones. This descending air is warm and dry: the world's major deserts (Sahara, Arabian, Australian) lie here.
4
Surface air flows back toward the equator. The Coriolis effect deflects it, creating the trade winds: NE in the Northern Hemisphere, SE in the Southern Hemisphere.

Ferrel and Polar Cells

Ferrel Cell (30° to 60°)
An indirect circulation cell driven by the Hadley and Polar cells. Surface winds flow poleward and are deflected east by the Coriolis effect, forming the prevailing westerlies. Most mid-latitude weather systems occur here.
Polar Cell (60° to 90°)
Cold, dense air sinks at the poles (high pressure) and flows toward 60° latitude at the surface. Coriolis deflection creates the polar easterlies. Where polar and Ferrel air masses meet: the polar front.

Surface Wind Belts

Trade Winds
0° to 30°. Blow from east to west. NE trades (NH) and SE trades (SH). Very reliable: historically used for sailing.
Westerlies
30° to 60°. Blow from west to east. Carry weather systems across mid-latitudes. Strongest in the Southern Hemisphere ("Roaring Forties").
Polar Easterlies
60° to 90°. Blow from east to west. Cold and dry. Weakest of the three belts due to low temperature contrast near poles.

Trade Winds vs. Westerlies

Trade Winds Westerlies
Latitude 0° to 30° 30° to 60°
Direction East to West West to East
Cell Hadley Cell Ferrel Cell
Consistency Very steady Variable, wavy
Climate Role Tropical weather, ENSO Mid-latitude storms
Named for Historical trade routes Blow from the west

How to Predict Wind Direction

1
Identify the latitude. Determine which wind belt the location falls in: 0-30° (trade winds), 30-60° (westerlies), or 60-90° (polar easterlies).
2
Determine the hemisphere. The Coriolis effect deflects wind right in the NH and left in the SH.
3
Find the pressure gradient direction. Air flows from nearest high-pressure zone toward the nearest low-pressure zone.
4
Apply Coriolis deflection to the pressure gradient direction. The resulting deflected direction is the prevailing surface wind.
5
Name the wind by the direction it blows from (e.g., winds from the northeast = "NE trade winds").

Example: NE Trade Winds

HIGH PRESSURE (30°N Subtropical) LOW PRESSURE (0° ITCZ) PGF (southward) Coriolis deflects right NE Trade Wind (blows from the NE)

Example: Prevailing Westerlies

HIGH PRESSURE (30°N Subtropical) LOW PRESSURE (60°N Subpolar) PGF (poleward) Coriolis deflects right Westerly Wind (blows from the SW toward NE)

The ITCZ and Seasonal Shifts

Equator ITCZ: June-August (shifts north to ~15°N) ITCZ: December-February (shifts south to ~10°S) NE Trades SE Trades

Jet Streams: High-Altitude Winds

Jet streams are narrow bands of fast-moving air in the upper troposphere (~10 km altitude). They flow from west to east and strongly influence weather patterns and air travel.

Polar Jet Stream
Located at ~60° latitude, where polar and Ferrel cells meet. Speeds of 100 to 250 km/h. Guides mid-latitude storms. Meanders (Rossby waves) bring cold snaps and heat waves.
Subtropical Jet Stream
Located at ~30° latitude, near the boundary of Hadley and Ferrel cells. Generally weaker and steadier than the polar jet. Influences tropical cyclone paths.

Environmental Impacts

Desert Formation
Sinking air at ~30° creates arid conditions. The Sahara, Arabian, Sonoran, and Kalahari deserts all lie in subtropical high pressure zones.
Ocean Currents
Surface winds drive ocean circulation. Trade winds push equatorial currents westward. Westerlies push mid-latitude currents eastward. This creates the major ocean gyres.
ENSO Events
Weakening of SE trade winds causes El Nino: warm water shifts east across the Pacific, altering global weather. Strengthened trades cause La Nina: opposite pattern.

Common Mistakes

✕ Confusing Wind Names
Thinking "westerlies" blow toward the west. They blow from the west. Wind is named by its source direction.
✕ Coriolis Creates Wind
The Coriolis effect only deflects existing wind. It does not generate wind: the pressure gradient force does that.
✕ Mixing Up Deflection
In the NH, deflection is to the right. In the SH, it's to the left. Students often reverse these or apply NH rules globally.

Summary: Global Wind Belts

Feature Trade Winds Westerlies Polar Easterlies
Latitude 0° to 30° 30° to 60° 60° to 90°
Cell Hadley Ferrel Polar
Direction (NH) NE to SW SW to NE NE to SW
Strength Strong, steady Moderate, variable Weak, cold
Key Climate Effect Tropical rain, ENSO Mid-lat storms Polar front
Surface Pressure H(30°) to L(0°) H(30°) to L(60°) H(90°) to L(60°)

Question 1 of 5

In the Northern Hemisphere, surface winds in the Hadley cell are deflected to the right by the Coriolis effect. What are the resulting surface winds called?

Question 2 of 5

The world's major hot deserts (Sahara, Arabian, Kalahari) are concentrated near 30° latitude. Which atmospheric process best explains this pattern?

Question 3 of 5

The Coriolis effect is zero at the equator and strongest at the poles. Which formula describes the Coriolis acceleration?

Question 4 of 5

During an El Nino event, what happens to the SE trade winds in the Pacific?

Question 5 of 5

A city at 45°N latitude experiences frequent storms driven by the collision of warm and cold air masses. Which boundary is responsible?