Grade 10-11 (age 15-17)

high school Atmospheric Circulation

Near the equator the Earth takes in more solar energy than it gives off, and near the poles it is the other way around, so air starts moving to carry the extra heat poleward. Because the Earth rotates, that flow cannot make one loop from equator to pole; in the Northern Hemisphere it breaks into three loops, the Hadley, Ferrel, and polar cells. Low-pressure belts form where air rises, at the equator and near 60°, and high-pressure belts form where it sinks, near 30° and at the pole; between them, the Coriolis effect turns surface winds into the trade winds, the westerlies, and the polar easterlies. In the diagram, changing the observer latitude shows which cell that latitude belongs to and which wind blows at the surface there.

Air that carries surplus heat to the poles

🌬️ Why does the atmosphere circulate?

①Near the equator the Sun is high, so each square meter receives a lot of energy; near the poles sunlight arrives at a low angle and delivers much less

②Averaged over a year, latitudes below about 38° have an energy surplus and latitudes above it have a deficit

③The equator does not keep heating up and the poles do not keep cooling because the atmosphere and the oceans carry the surplus heat to high latitudes

④The global pattern of air movement that does this heat transport is the general circulation of the atmosphere

Three circulation cells

Observer latitude35°N

💡 Which way each cell turns

①Hadley cell (0° to 30°): air warmed at the equator rises, flows poleward aloft, sinks near 30°, and returns toward the equator along the surface

②Polar cell (60° to 90°): cold air sinks at the pole, flows along the surface toward 60°, and rises there

③These two are direct cells, rising where it is warm and sinking where it is cold

④Ferrel cell (30° to 60°): an indirect cell turning the opposite way between the two direct cells, with surface air flowing poleward

⑤Sinking air near 30° rarely forms clouds, so it is dry there, which is why great deserts such as the Sahara lie near this latitude

The Coriolis effect and surface winds

Coriolis effect

Earth's rotation → moving objects veer to the right in the Northern Hemisphere

They veer to the left in the Southern Hemisphere. The effect is zero at the equator, grows with latitude, and is strongest at the poles.

Surface winds (Northern Hemisphere)

0° to 30°: trade winds (NE) / 30° to 60°: westerlies (SW) / 60° to 90°: polar easterlies (NE)

Surface air heading toward the equator veers right and becomes an easterly, and air heading poleward becomes a westerly.

🔄 A way to picture the Coriolis effect

①Roll a ball straight across a spinning turntable, and to someone standing on the turntable the ball seems to curve

②The Earth spins too, so seen from the ground, air moving north or south veers to the right in the Northern Hemisphere

③Air heading toward the equator veers right, which is west, and becomes a wind blowing from the east

④Air heading toward the pole veers right, which is east, and becomes a wind blowing from the west

⑤The Coriolis effect changes only the direction of the wind, not its speed

Pressure belts and Korea's weather

Pressure belts by latitude

equatorial low (0°) → subtropical high (30°) → subpolar low, polar front (60°) → polar high (90°)

Belts where air rises are low pressure, belts where it sinks are high pressure, and surface winds blow from the highs toward the lows.

📍 What it means that Korea lies in the westerlies

①Korea lies at about 33° to 43° N, inside the belt of westerlies

②So lows and highs usually pass from west to east, and the weather changes from the west first

③That is why Korean forecasters watch the clouds and pressure troughs over China first

④The same wind carries yellow dust from inland China to the Korean Peninsula in spring

Worked Examples and Exam Practice

Example 1

Name the surface wind at about 15° N and give its direction.

Latitudes 0° to 30° belong to the Hadley cell, whose surface flow runs from near 30° toward the equator.

Air heading toward the equator veers right in the Northern Hemisphere, so it becomes the trade wind blowing from the northeast.

▸ trade winds (from the NE)

With the observer latitude at 15° in the diagram, the Hadley cell loop brightens and the line below reads trade winds (from NE).

Example 2

Korea lies at about 37° N. Which cell and wind belt is it in, and in which direction does its weather mostly move?

37° is between 30° and 60°, so Korea is in the Ferrel cell, where the westerlies blow at the surface.

The westerlies blow from west to east, so lows and highs travel from west to east and the weather changes in that order.

▸ the westerly belt of the Ferrel cell; weather moves from west to east

That is why Korean forecasts so often say a pressure trough is approaching from the west.

CSAT-style problem

Which statement about the general circulation of the Northern Hemisphere is correct?

①The Coriolis effect is strongest at the equator

②The Hadley cell spans latitudes 0° to 30°

③The trade winds blow from the southwest

④The Ferrel cell is a direct cell

⑤A high-pressure belt forms at the equator

▸ ② The Hadley cell spans latitudes 0° to 30°

The Hadley cell is a direct cell in which air rises at the equator and sinks near 30°, so it spans latitudes 0° to 30°.

The Coriolis effect is strongest at the poles (①), the Northern Hemisphere trade winds blow from the northeast (③), the Ferrel cell is indirect (④), and rising air at the equator makes a low-pressure belt there (⑤).

Summary

The core of the general circulation

uneven energy by latitude + Earth's rotation (Coriolis) → three cells + pressure belts + surface winds

It is the planet-wide movement of air that carries surplus heat from low latitudes to high latitudes.

🎯 Exam Points

①The three cells are the Hadley cell (0° to 30°, direct), the Ferrel cell (30° to 60°, indirect), and the polar cell (60° to 90°, direct)

②In the Northern Hemisphere, the trade winds blow from the NE, the westerlies from the SW, and the polar easterlies from the NE

③The Coriolis effect acts to the right in the Northern Hemisphere, is zero at the equator, and is strongest at the poles

④From the equator to the pole, the pressure belts alternate low, high, low, high

⑤Korea lies in the westerlies, so its weather generally moves from west to east

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Last updated 2026-10-17

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