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

• 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

• 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 (the Hadley cell, Ferrel cell and Polar cell) that circulate air from the surface, through the atmosphere, and back to the Earth's surface again

• The Hadley cell is the largest cell and extends from the equator to between 30° and 40° north and south

  • Trade winds blow from the tropical regions to the equator and travel in an easterly direction

  • Near the equator, the trade winds meet, and the hot air rises and forms thunderstorms (tropical rainstorms)

  • From the top of these storms, air flows towards higher 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)

• The Ferrel cell is the middle cell and generally occurs from the edge of the Hadley cell to between 60° and 70° north and south of the equator

  • This is the most complex cell because it moves in the opposite direction from the Hadley and Polar cells, similar to a cog in a machine

  • Air in this cell joins the sinking air of the Hadley cell and moves at low altitudes to mid-latitudes, where it rises along the border with the cold air of the Polar cell

  • This occurs around the mid-latitudes, contributing to frequent unsettled weather

• The Polar cell is the smallest and weakest of the atmospheric cells, extending from the edge of the Ferrel cell to the poles at 90° north and south

  • Air in these cells is cold and sinks, creating high pressure over the highest latitudes

  • The cold air flows out towards the lower latitudes at the surface, where it is slightly warmed and rises to return at altitude to 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