Global Circulation
Air patterns
- Air moves (circulates) around the world as a system of winds
- These winds carry heat from warmer low latitudes to cooler high latitudes and back again, through a system of looped cells, either side of the equator
- This figure of 8 pattern, distributes heat around the globe
Wind formation
- Air moves due to pressure differences, known as a 'pressure gradient'
- The bigger the difference, the stronger the winds
- At the equator, the Sun heats the Earth's surface and air becomes warmer
- The warm air begins rising due to expanding air molecules, which are less dense (heavy) than the air around it
- As the air rises, it creates a low-pressure zone below the rising air
- The area above the rising air, becomes an area of high pressure
- As more air rises, it pushes the air apart which begins to cool
- Cool air is denser, and begins to sink
- This sinking air leaves behind an area of low pressure
- As it descends back to the Earth's surface, it starts to form an area of high pressure at the surface
- There is now a pressure difference at the surface and this draws the wind back to the area of low pressure
- Therefore, air always moves from areas of high pressure to areas of low pressure
Diagram of a typical wind pressure cell
Hot air rises and cooler air sinks through the process of convection
Insolation
- Insolation that reaches the Earth's surface is greater at the equator and at the poles
- This is due to the Earth's natural curvature and its angle of tilt
Diagram of global distribution of insolation
The irregular heating of Earth's surface, generates several pressure cells. Each cells produces a different weather pattern
The 3-cell atmospheric wind model
- Air movement 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
Global atmospheric circulation diagram
The three-cell wind model
- Each hemisphere contains three atmospheric cells, known as the Hadley, Ferrel and Polar cell
- These cells circulate air from the surface up to the high atmosphere and back down to the Earth's surface
- Hadley cell is the largest cell that starts at the equator and reaches as far as 40° north and south (depending on time of year)
- Warm trade winds travel in an easterly direction from tropical regions to the equator
- As these trade winds meet near the equator, warm air is forced upwards, which quickly cools and condenses forming tropical rainstorms
- From the top of these storms, air flows towards the high 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)
- Polar cell is the smallest and weakest that reaches from the edge of the Ferrel cell to the poles at 90° north and south
- Cold air sinks forming high pressure over high latitudes
- This cold air flows at the surface, towards the low latitudes
- The air is warmed slightly which encourages it to rise and return, at altitude, to the poles
- Ferrel cell sits in the middle at the edge of the Hadley cell between 60° and 70° north and south of the equator
- Unlike the Polar and Hadley cells, the Ferrel cell flows in the opposite direction (creating a figure of 8 type movement)
- Air joins the sinking air of the Hadley cell and flows at low atmospheric height to the mid-latitudes where it then rises along the border with the cold air of the Polar cell
- This is the reason why the UK frequently has unsettled weather
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° north and south) 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° south
Worked example
Explain the link between global air pressure and surface wind belts.
[4 marks]
- The command word here is 'explain', so you will need to include what/where and why
Answer:
- Sinking air causes high pressure [1] causing winds to move away/diverge [1] to meet in areas of low pressure. [1] For example, the Polar highs/easterlies meet the westerlies (low pressure) at 60° north and south of the equator. [1] Winds blow from high pressure areas to low pressure areas [1] such as the trade winds blowing from 30° north and south towards the equator. [1]
