Global Atmospheric Circulation Model

The global atmospheric circulation model explains how air moves around the Earth and distributes heat from the equator to the poles.

Wind Formation:

Air always moves from areas of high pressure to lower pressure, creating wind. Winds are large-scale movements of air caused by differences in air pressure. These pressure differences occur because the Sun heats the Earth’s surface unevenly.

Angle of Insolation:

Insolation is the amount of solar energy that reaches the Earth’s surface. Due to the Earth’s curvature and tilt, the amount of insolation is greater at the equator and decreases towards the poles. As a result, hot air rises at the equator, while cooler air sinks at the poles. This process is called convection.

The Three-Cell Model

The global atmospheric circulation is often represented by the three-cell model:

  • Hadley cell
  • Ferrel cell
  • Polar cell

Each hemisphere has these three cells. They circulate air from the surface through the atmosphere and back to the Earth’s surface.

The Hadley Cell

The Hadley cell is a large atmospheric circulation system. It spans from the equator to between 30° and 40° from the equator, in both directions – up north and down south. The winds in this system usually move from the tropics towards the equator.

In the northern half of our planet, these winds move in a way that makes them come from the northeast, so we call them northeast trade winds. On the other half of our planet, down south, these winds seem to come from the southeast, so we call them southeast trade winds. This is because of certain forces and friction that affect the direction of the wind.

When the trade winds meet around the equator, they create thunderstorms by causing hot air to rise. The air from the top of these storms then flows towards higher latitudes, cools down, and sinks over subtropical regions. 

As a result, dry and cloudless conditions prevail, leading to warm and dry climates that are often found in hot deserts.

The Ferrel Cell

The Ferrel cell is positioned between the Hadley and Polar cells and covers from the edge of the Hadley cell to between 60° to 70° north and south. It operates in the opposite direction to the Hadley and Polar cells, serving as a cog in a machine.

The air within this cell merges with the descending air of the Hadley cell and flows at low altitudes towards mid-latitudes. As it meets the cold air of the Polar cell, it ascends. This leads to frequent unstable weather, especially in areas such as the UK.

The Polar Cell

The Polar cell is the smallest and weakest atmospheric cell that stretches from the Ferrel cell’s boundary to the poles at 90° north and south.

This cell contains cold air that sinks, leading to high pressure over the northernmost and southernmost latitudes. As the frigid air moves towards lower latitudes at the surface, it slightly warms up and rises to return to the poles at higher altitudes.

Coriolis Effect

The Coriolis effect is an important factor that affects global winds and ocean currents. As the Earth rotates on its axis, it causes moving air (wind) to appear as if it’s curving instead of moving in a straight line.

It causes winds to blow diagonally instead of straight along their pressure gradient.

  • In the northern hemisphere, winds are deflected to the right
  • In the southern hemisphere, winds are deflected to the left.

However, when there are low-pressure systems, winds flow in reverse. This results in counterclockwise flow in the northern hemisphere and clockwise flow in the southern hemisphere.

Global Wind Belts

The combination of pressure cells, the Coriolis effect and the three cells give rise to distinct wind belts in each hemisphere.

The trade winds blow from the subtropical high-pressure belts (around 30° latitude) towards the low-pressure zones near the equator and are deflected.