1.3.1 Global Atmospheric Circulation

• • 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 moves from high to low pressure, generating wind.
  • Wind is caused by differences in air pressure on a large scale.
  • Uneven heating of Earth's surface by the Sun creates these pressure differences.
  • Insolation is greater at the equator due to Earth's curvature and tilt angle.
  • Convection causes hot air to rise and cool air to sink.
  • Irregular heating of Earth’s surface creates various pressure cells, each generating different weather patterns.
  • Each pressure cell has circular air movement, transferring heat globally.
  • The three-cell model shows: 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.
• Wind Pressure Cell

  1. As the sun heats the surface, air expands and begins to rise.
  2. As the air rises, the surface pressure lowers.
  3. As the air rises into the atmosphere, it adds pressure.
  4. As the air cools in the atmosphere, it becomes denser and begins to sink, lowering the atmosphere pressure.
  5. As the air sinks, the surface pressure rises.
  6. There is a pressure difference now. To balance this, air rushes to the low pressure area. The greater the pressure difference, the stronger the winds.
• Explain the link between global air pressure and surface wind belts.

  Answer:

  • Sinking air causes high pressure (1) causing winds to move away/diverge [1] to meet in areas of low pressure [1] e.g. the Polar highs/easterlies meet the westerlies (low pressure) at 60 degrees N and S of the Equator [1] 
  • Winds blow from high pressure areas to low pressure areas [1]  such as the trade winds blowing from 30 degrees N and S towards the Equator [1]