Earth Science

Atmospheric Processes

Circulation in the Atmosphere

Study Tip
Think of the atmosphere as a fluid. Within this fluid would be the convective currents caused by temperature and density differences.

Convection Cells

Within the troposphere are convection cells.

  • Air heated in a relatively warm ground region rises, creating a low pressure zone.
  • Air from the surrounding area is sucked into the space left by the rising air.
Study Tip
Think of the atmosphere as a fluid. Within this fluid would be the convective currents caused by temperature and density differences.
  • Air flows horizontally at top of the troposphere through advection.
  • Air cools until it descends.
  • Air reaches the ground and creates a region where relatively cool, dense air is sinking (high pressure zone).
  • Air flowing from areas of high pressure to low pressure creates winds.
  • The greater the pressure difference is between pressure zones, the faster the wind speed is.

Air Pressure Zones

  • Warm air can hold more moisture than cool air.
  • As the warm air rises, it cools and it cannot hold all the water it contains. Water vapor may condense into clouds or precipitation.
  • When cool air descends, it warms up and evaporates water on the ground.
  • Air moving between high and low pressure zones creates global wind belts, and smaller pressure systems create localized winds that affect the weather and climate of a local area.

Atmospheric Circulation and Convection

  • As more solar energy hits the equator, warmer air starts to form a low pressure zone.
  • As less solar energy hits the poles, colder air starts for form a high pressure zone.
  • This would a convection cell in each hemisphere, due to the differences in pressure. However, the Earth rotates, and the Coriolis Effect creates three convection cells in each hemisphere, instead of one.
Air Circulation

Earth Science

  • Northern Convection Cells (Polar, Ferrell, Hadley)
  • The Polar Cell (between 60ᵒN and the North Pole) is where cold air descends.
  • The Ferrell Cell (between 30ᵒN and 60ᵒN) shares its southern, descending side with the Hadley Cell, and its northern, rising side with the Polar Cell.
  • The Hadley Cell (between 0ᵒ and 30ᵒN) is the lowest northern convection cell.
  • In the southern hemisphere, there are three mirrored convection cells.
Concept Check
  • How are convection cells created, and how do they affect the weather?
  • What are the main types of convection cells?
  • How does the Coriolis Effect complicate things?