Global Atmospheric Circulation (AQA GCSE Geography)

• The global atmospheric circulation can be described as a worldwide system of winds moving heat FROM the equator TO the poles to reach a balance in temperature

Wind formation

• Air always moves from high pressure to lower pressure, and this movement of air generates wind
• Winds are large scale movements of air due to differences in air pressure
• This pressure difference is because the Sun heats the Earth's surface unevenly
• Insolation that reaches the Earth's surface is greater at the equator than at the poles due to Earth's curvature and angle of the Earth's tilt

Angle of insolation

• Hot air rises and cooler air sinks through the process of convection
• The irregular heating of Earth’s surface creates various pressure cells, each generating different weather patterns

• The movement of air within each cell is roughly circular and moves surplus heat from equatorial regions to other parts the Earth
• The three-cell model shows global circulation: the Hadley, Ferrel and Polar cells

The 3-cell atmospheric wind model

• Each hemisphere has three cells (the Hadley cell, Ferrel cell and Polar cell) which circulates air from the surface through the atmosphere and back to the Earth's surface
• Hadley cell is the largest cell and extends from the equator to between 30° and 40° north and south
  • Trade winds that blow from the tropical regions to the equator and travel in an easterly direction
  • Near the equator, the trade winds meet, and the hot air rises and form thunderstorms (tropical rainstorms)
  • From the top of these storms, air flows towards higher latitudes, where it becomes cooler and sinks over subtropical regions
  • This brings dry, cloudless air, which is warmed by the Sun as it descends - the climate is warm and dry (hot deserts are usually found here)
• Ferrel cell is the middle cell, and generally occurs from the edge of the Hadley cell to between 60° and 70° north and south of the equator
  • This is the most complicated cell as it moves in the opposite direction from the Hadley and Polar cells; similar to a cog in a machine
  • Air in this cell joins the sinking air of the Hadley cell and travels at low heights to mid-latitudes where it rises along the border with the cold air of the Polar cell
  • This occurs around the mid-latitudes and accounts for frequent unsettled weather (particularly the UK)
• Polar cell is the smallest and weakest of the atmospheric cells. It extends from the edge of the Ferrel cell to the poles at 90° north and south
  • Air in these cells is cold and sink creating high pressure over the highest latitudes
  • The cold air flows out towards the lower latitudes at the surface, where it is slightly warmed and rises to return at altitude to the poles

Coriolis effect

• Each cell has prevailing winds associated with it 
• These winds are influenced by the Coriolis effect
• The Coriolis effect is the appearance that global winds, and ocean currents curve as they move
• The curve is due to the Earth's rotation on its axis, and this forces the winds to actually blow diagonally
• The Coriolis effect influences wind direction around the world in this way:
  • In the northern hemisphere it curves winds to the right
  • In the southern hemisphere it curves them left
• The exception is when there is a low-pressure system:
  • In these systems, the winds flow in reverse (anti clockwise in the northern hemisphere and clockwise in the southern hemisphere)

Global wind belts: Surface winds

• The combination of pressure cells, the Coriolis effect and the 3-cells produce wind belts in each hemisphere:
  • The trade winds: Blow from the subtropical high-pressure belts (30 degrees N and S) towards the Equator's low-pressure zones and are deflected by the Coriolis force
  • The westerlies: Blow from the sub-tropical high-pressure belts to the mid-latitude low areas, but again, are deflected by the Coriolis force
  • The easterlies: Polar easterlies meet the westerlies at 60 degrees S
• Global atmospheric circulation affects the Earth's climate
• It causes some areas to have certain types of weather more frequently than other areas:
  • The UK has a lot of low-pressure weather systems that are blown in from the Atlantic Ocean on south-westerly winds, bringing wet and windy weather