Topic 4.5 Notes – Global Wind Patterns

What Global Wind Patterns Are

Global wind patterns are the large-scale, persistent movement of air in the atmosphere. They exist because the equator gets the most intense solar radiation, so Earth is heated unevenly.

Here’s the chain you need to know cold:

• Unequal heating causes temperature differences
• Temperature differences cause density differences in air
• Density differences create pressure differences
• Pressure-gradient force moves air from high pressure to low pressure
• Earth’s rotation causes Coriolis deflection, which bends that moving air

That last part gets tested a lot. Pressure differences directly cause wind. The Coriolis effect changes wind direction, but it does not start the wind.

This circulation matters because it moves heat from the equator toward the poles and helps redistribute water vapor, which affects rainfall patterns.

The Pressure Belts, Convection Cells, and Wind Belts

There are six major circulation cells total, with three in each hemisphere. The key latitude bands are approximate: 0°, 30°, 60°, and 90°.

This global circulation diagram pulls those pressure belts, convection cells, and wind belts together in one view.

Global pressure belts

• 0° equatorial low pressure
  Strong heating makes air rise. Rising air means low pressure, clouds, and lots of precipitation.
• About 30° subtropical high pressure
  Air sinks here. Sinking air means high pressure, divergence, and drier conditions.
• About 60° subpolar low pressure
  Air rises again, so this zone often has active weather.
• 90° polar high pressure
  Very cold, dense air sinks at the poles.

Two named regions show up a lot:

• ITCZ or Intertropical Convergence Zone sits near the equator, where trade winds meet and air rises.
• Doldrums are the weak, variable winds near the ITCZ.
• Horse latitudes are the subtropical high-pressure zones near 30°.

The three-cell model

• Hadley cells from 0° to 30°
  • Air rises at the equator
  • Moves poleward high in the atmosphere
  • Cools and sinks near 30°
  • Surface air returns toward the equator
• Ferrel cells from 30° to 60°
  • Surface air moves toward 60°
  • Air rises near 60°
  • Returns aloft toward 30°
  • This cell is less directly caused by heating than the other two
• Polar cells from 60° to 90°
  • Cold air sinks at the poles
  • Surface air moves toward 60°
  • Air rises near 60° and returns poleward aloft

Prevailing surface wind belts

• Trade winds from 30° to 0°
  • Northeast trade winds in the Northern Hemisphere
  • Southeast trade winds in the Southern Hemisphere
  • Overall east-to-west flow toward the ITCZ
• Prevailing westerlies from 30° to 60°
  • Blow from the west
  • Overall west-to-east movement
• Polar easterlies from 60° to 90°
  • Blow from the east
  • Overall east-to-west movement

How Atmospheric Circulation Happens

This whole system works in a clear sequence:

1. The equator receives the most intense solar radiation.
2. Earth’s surface heats the air above it.
3. Warm air expands, becomes less dense, and rises.
4. Rising and sinking air create bands of low and high pressure.
5. Surface air moves from high pressure toward low pressure.
6. Earth’s rotation deflects that motion through the Coriolis effect.
7. The result is organized wind belts and convection cells.

A common mistake is mixing up temperature and pressure. Temperature differences help create the pattern, but pressure differences are what directly move the air.

How the Coriolis Effect Changes Wind Direction

Earth rotates west to east, so moving air appears to curve on a rotating planet.

Coriolis effect on a rotating Earth

• In the Northern Hemisphere, air is deflected to the right
• In the Southern Hemisphere, air is deflected to the left
• The effect is weakest at the equator and stronger toward the poles

Be careful here. “Right” and “left” are relative to the air’s motion. It does not mean every wind in one hemisphere curves the same compass direction. The path depends on whether the air is moving toward the equator or toward the pole.

This is why winds don’t just blow straight north-south.

Why Global Wind Patterns Matter

Global wind patterns help keep Earth’s energy balanced by moving thermal energy away from low latitudes.

They also shape broad precipitation patterns:

• Equator tends to be warm and wet because rising air cools, condenses, and produces rain
• Around 30° tends to be dry because sinking air warms and lowers relative humidity
• Many deserts sit here, including the Sahara and Arabian deserts
• Around 60° has rising air and frequent weather activity

This model is idealized. It shows long-term average circulation, not daily local weather. Actual winds are modified by seasonal shifts in heating, continents and oceans, topography, land-water heating differences, short-term pressure systems, and El Niño/La Niña.