Global circulation - cells

• Different parts of the earth heat up differently with the equator receiving more rates of heat from the Sun than the poles
• The purpose of global circulation is to redistribute this heat
• If the earth did not rotate and was a simple landmass with no oceans we would have a single circulatory cell in each hemisphere where hotter air would rise at the equator and flow toward the poles, the air would sink as it cools and then returned towards the equator
• The unequal distribution of land and ocean and the speed of the Earth's rotation make this circulation system more complicated giving us a 3-cell pattern which exists in both the northern and southern hemispheres
• Hadley cells


  • Largest cells at the equator
  • Warmer less dense air rises to about 18 kilometers and spreads out underneath the tropopause
  • The tropopause acts as a lid to the lowest part of our atmosphere which contains all of our weather
  • The warm air spreads out towards the poles gradually cooling and sinking as it moves before descending to the surface and flowing back to the equator
• Polar cells


  • Smallest cells
  • Cold dense air descending in the polar regions flows at low levels to about 60 to 70 degrees north or south
  • As the air leaves the polar regions it starts to warm and rise returning to the poles at high levels
• Ferrel cells


  • Between the Hadley and polar cells
  • Not driven by temperature
  • Flow in the opposite direction to the Hadley and polar cells acting like a gear
• The circulating cells not only transport heat from the equator to the poles but also results in semi-permanent areas of high and low pressure due to the rising and descending parts of the circulation cells
• Where air is rising an area of low pressure is created so these areas seem much more rainfall, this is why the largest areas of rainforests are found near the equator and why the United Kingdom has a relatively wet climate
• Where air is descending an area of high pressure forms giving largely clear skies and little rainfall which leads to the desert regions
• Not all deserts are hot, Antarctica sits under the descending branch of the Polar cell and is also classed as a desert with more precipitation falling in the Sahara
• Antarctica is the largest and driest deserts overall
• The rotation of the earth gives us jet streams and prevailing winds