Climate as a Global System (Edexcel GCSE Geography: B (1GB0))

• The global atmospheric circulation can be described as a worldwide system of winds moving solar heat energy 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 the Earth's curvature and angle of the Earth's tilt

Diagram showing how angle of insolation spreads solar radiation over a wider area at the poles than the equator

• Hot air rises and cooler air sinks through the process of convection
• This irregular heating of the Earth’s surface creates pressure cells
• Each cell generates different weather patterns

A typical wind pressure cell system showing distribution of pressure at Earth's surface and upper atmosphere

• Air movement within the cell is roughly circular and moves surplus heat from equatorial regions to other parts of the Earth
• In both hemispheres, heat energy transfer occurs where 3 atmospheric circulation cells meet
• These are the Hadley, Ferrel and Polar cells and are shown via the tri-cellular model:

Heat energy flow and surface winds in the tri-cellular model

The tri-cellular atmospheric wind model

• Each hemisphere has three cells (the Hadley cell, Ferrel cell and Polar cell) which circulate 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 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 forms 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 in 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 sinks 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

Ocean conveyor belt

• Ocean currents redistribute heat energy around the globe
• Currents (warm or cold) act like 'rivers' of water in the sea 
• Cold currents move towards the equator and warm currents towards the poles
• Each ocean has its own pattern of current:
  • E.g. the warm Atlantic Ocean waters of the low latitudes are moved to high latitudes via the North Atlantic Drift
• All ocean currents are triggered by the prevailing surface winds of the global atmospheric circulation, with the Coriolis effect pushing the water direction
• Circulation is through convection currents driven by cold water freezing into ice at the poles
• The polar cold waters are denser, saltier sea water which sinks to the ocean floor
• Water then flows in behind it at the surface, which forms a current
• The deep ocean currents then flow towards Antarctica along the western Atlantic basin, then splits off into the Indian and Pacific Oceans where the water begins to warm up
• The warming makes the water less dense so it loops back up to the surface in the South and North Atlantic Ocean
• The warmed surface waters continue to flow around the globe and eventually return to the North Atlantic and the cycle begins again
• This movement of water is known as the thermohaline circulation and drives the ocean conveyor belt

Image of the thermohaline circulation

Pressure differences

• Air moves in the atmosphere either towards the ground (subsidence) or up into the atmosphere (convection)
• These movements influence air pressure and rainfall
• The sea and land heat up differently
  • Sea:
    • Forms high pressure in summer and low pressure in winter
    • Takes longer to heat and cool
    • Air is denser and cooler in summer but warmer in winter
  • Land:
    • Generally forms areas of lower pressure in summer and higher in winter
    • Heats quickly in summer and the air is lighter and rises
    • Cools quickly in winter

Table Showing Influence of Air Movement on Weather Conditions

Air Movement	Cause	Weather Conditions
Subsidence (sinking air)	Occurs in areas with low-intensity solar radiation, such as the poles or at high altitudes where the air is very cold. Air becomes denser and sinks towards the ground. As air sinks, it begins to warm and can therefore, hold more moisture preventing clouds from forming.	Forms high pressure areas where the air is descending. Brings clear skies or very thin clouds. Creates arid or semi-arid conditions due to very little precipitation.
Convection (rising air)	Occurs in areas with high levels of solar radiation. Ground heats the air above and rises. As air rises, it cools, and condenses into water droplets which form clouds.	Low pressure areas are created as air is moving upwards. Thick heavy cloud cover with heavy rainfall creates wet tropical regions.

• A broad pattern of latitudinal high and low pressure belts are created via the horizontal bands of the Hadley, Ferrel and Polar cells
• However, the distribution of land and sea, affects the location of these pressure zones and so the pattern is not symmetrical in each hemisphere, despite the mirroring of the cells

Pattern of latitudinal high and low pressure belts created by the Hadley, Ferrel and Polar cells