Global Circulation (OCR GCSE Geography B)

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

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 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

• 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

Global Circulation & Weather Extremes

• Global atmospheric circulation affects the Earth's climate

• It creates differences in air pressure and 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

  • The UK is classified as a temperate weather system as there are no extremes of weather

• Pressure systems or atmospheric air pressure is measured in millibars

• Normal range of air pressure is between:

  • 1050mb high pressure

  • 980mb low pressure

• The lower the pressure system, the higher/stronger the winds become

Atmospheric Pressure Zones

• Extreme weather conditions occur where atmospheric cells meet

• Winds

  • Winds are relatively weak within pressure belts/zones

  • Between pressure zones/belts, winds are strong

  • The larger the pressure difference, the stronger the winds

• Precipitation 

  • Precipitation occurs when warm, moist air rises, cools and condenses

  • Air rising in low pressure areas causes frequent precipitation

  • In high pressure zones, little precipitation occurs - desert regions

• Temperature

  • The Equator receives the highest levels of sunlight

  • These areas are the warmest, but also the wettest

  • In high pressure zones, temperatures are high during the day, as there is a lack of cloud cover

  • However, at night temperatures can drop below freezing, due to the lack of cloud cover

  • The polar regions are very low as little sunlight reaches these areas

Extreme Weather Conditions in Contrasting Countries

• Extreme weather depends on location; what is extreme weather for the UK may be normal elsewhere

Comparison of Weather Between Contrasting Countries