Atmospheric Circulation (College Board AP® Environmental Science): Study Guide

Atmospheric circulation & global wind patterns

• Global atmospheric circulation can be described as the worldwide system of winds that move solar heat energy from the equator to the poles to reach a balance in temperature

Wind formation

• Air always moves from areas of higher 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

    • This is due to the Earth's curvature and the angle of the Earth's tilt

• For a full explanation of why insolation varies with latitude, see Solar Radiation & Earth's Seasons — Effect of latitude on insolation

• This irregular heating of the Earth’s surface creates pressure cells

  • In these pressure cells, hot air rises and cooler air sinks through the process of convection

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Image caption: Two Hadley cells — warm air rises at the equator (the ITCZ), flows poleward at altitude, and cool dry air descends around 30°N and 30°S (the horse latitudes), returning equatorward as the trade winds.

• The amount of water vapour that air can hold depends on its temperature — warm air can hold more water vapour than cool air

• As warm, moist air rises along the equatorial branch of a Hadley cell, it cools, and its capacity to hold water vapour drops

  • The vapour condenses out as clouds and heavy rainfall, which is why tropical rainforests are concentrated near the equator

• The opposite happens around 30°N and 30°S: descending air warms, regaining water-vapour capacity, so it dries out

  • This is why the world's major deserts (Sahara, Arabian, Atacama, Australian outback, Kalahari) cluster at these latitudes

• Air movement within the cell is roughly circular and moves surplus heat from equatorial regions to other parts of the Earth

• In both hemispheres (the Northern hemisphere and the Southern hemisphere), heat energy transfer occurs where different atmospheric circulation cells meet

  • There are three types of cell

  • Each cell generates different weather patterns

• These are the Hadley, Ferrel and Polar cells

  • Together, these three cells make up the tricellular model of atmospheric circulation:

The tricellular atmospheric wind model

• Each hemisphere has three cells (Hadley, Ferrel, Polar) that circulate air between the surface and the upper atmosphere

• The Hadley cell is the largest cell, extending from the equator to about 30°N and 30°S

  • Surface winds in this cell are the trade winds

  • In the Northern Hemisphere these blow as NE trade winds

  • In the Southern Hemisphere they blow as SE trade winds

  • The trade winds converge at the equator at the Intertropical Convergence Zone (ITCZ)

  • At the ITCZ, warm rising air produces heavy rainfall and thunderstorms

  • The descending branch around 30°N and 30°S forms the horse latitudes

  • The world's major hot deserts are concentrated at these latitudes

• The Ferrel cell is the middle cell, lying between roughly 30° and 60° north and south

• The Polar cell is the smallest and weakest, lying between roughly 60° and 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 the 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 (counterclockwise 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 produces 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