AP® Environmental Science · Unit 4 · Topic 4.5
Global Wind Patterns
Global winds are driven by heat rising at the equator and sinking at the poles, organized into three circulation cells per hemisphere, with Earth's rotation bending straight north-south flow into the curving trade winds and westerlies we actually observe.
Sunlight hits the equator hardest, so air there heats up, rises, and spreads toward the poles at high altitude. It cools, sinks back down around 30 degrees north and south, and some of it flows back toward the equator at the surface, completing a loop called a Hadley cell. Two more loops, the Ferrel cell (30 to 60 degrees) and the polar cell (60 to 90 degrees), stack up toward each pole, each one powered by the temperature difference between the air masses on either side of it. Together these six cells (three per hemisphere) carry heat from the energy-rich equator toward the energy-poor poles.
None of that surface air travels in a straight line, because Earth is rotating underneath it. Air drifting from 30 degrees toward the equator is moving from a point on Earth with a slower eastward spin speed to one with a faster spin speed, so relative to the ground it falls behind and curves to the west, producing the northeast and southeast trade winds instead of straight north-south flow. This apparent bending, deflecting right in the northern hemisphere and left in the southern hemisphere, is the Coriolis effect, and it shapes every large-scale wind belt, including the westerlies in the Ferrel cells.
This cell structure also explains why deserts line up at 30 degrees latitude. That's where Hadley cell air, already wrung dry by rain near the equator, sinks back toward the surface, warming and holding almost no moisture, so very little rain falls. The same mechanics connect directly to ocean surface currents, which the trade winds help drive, and to seasonal shifts in rainfall belts as the sun's heating zone migrates north and south.
Key terms
- Hadley cell
- The circulation loop between the equator and 30 degrees latitude, where warm air rises at the equator and sinks at 30 degrees.
- Ferrel cell
- The mid-latitude circulation loop, roughly 30 to 60 degrees, sandwiched between the Hadley and polar cells.
- Coriolis effect
- The apparent deflection of moving air, right in the northern hemisphere and left in the southern, caused by Earth's rotation.
- Trade winds
- The steady northeast or southeast surface winds blowing from about 30 degrees toward the equator.
- Westerlies
- The prevailing winds blowing from the west in the mid-latitudes, roughly 30 to 60 degrees, driven by the Ferrel cell and Coriolis deflection.
Watch it explained
- How it worksWalk me through Hadley, Ferrel, and polar cells moving heat from the equator to the poles.
- How it worksHow does the Coriolis effect bend a straight north-south wind into a curving trade wind?
- Ties togetherHow do the descending arms of the Hadley cells put deserts at 30 degrees north and south?
- Common mistakeWhy isn't the Coriolis effect a real force pushing sideways on the air?
Check yourself
End of topic.