The Sun heats the tropics far more than the poles (sun angle), and the atmosphere carries the extra heat poleward. Put that together with rising air in lows and sinking air in highs and the Coriolis turn, and you get the planet-wide pattern of rain belts, deserts and prevailing winds.
🔑 Key idea
In each hemisphere, three loops of circulation carry heat poleward. In the Hadley cell, air rises near the equator, flows poleward high up, and sinks near 30° latitude. In the Ferrel cell (30° to 60°), surface air flows poleward; in the polar cell, cold air sinks at the pole and flows toward 60°. Rising air makes the rainy belts near the equator and near 60°; sinking air makes the dry belts near 30° and at the poles. Coriolis turns the surface flows into the trade winds (0–30°), the westerlies (30–60°) and the polar easterlies.
💡 Why it works
Near the equator, strongly heated air rises in a band of towering clouds and heavy rain, the Intertropical Convergence Zone (ITCZ). High up, that air spreads poleward and cools by giving off infrared, and it is turned eastward by the Coriolis effect, so it piles up and sinks around 30°. Sinking air warms by compression and its relative humidity drops, so clouds can't form: the subtropical highs and the great deserts (the Sahara, the Arabian, the Kalahari, the Australian and the Sonoran) lie under them. Surface air returning toward the equator is turned right in the north and left in the south: the northeast and southeast trade winds.
The three cells and the surface winds. Change the month: the rainy belt (ITCZ) follows the Sun, north in July and south in January, bringing wet and dry seasons to the tropics.
✏️ Worked example: The winds at 15° S
Which way does the prevailing surface wind blow at 15° S?
15° S is in the Hadley cell, where surface air flows back toward the equator: northward.
In the Southern Hemisphere, moving air turns left. Air moving north turned left heads northwest.
It blows from the southeast: the southeast trade winds.
Answer: From the southeast
✏️ Worked example: Why is the Sahara dry?
The Sahara lies near 20–30° N. Use the cells to explain its dryness.
Air that rose over the equator flows north high up and sinks around 30° N.
Sinking air warms about 10 °C per km and its relative humidity drops far below 100%.
Clouds can't form in sinking, warming air, so it seldom rains.
Answer: It sits under sinking air of the Hadley cell
⚠️ Common mistake
Thinking the great deserts are hot and dry because they are close to the Sun. Every place on Earth is essentially the same distance from the Sun; the deserts near 30° are dry because air sinks there, warming and drying as it comes down. Check: the equator gets even more sunlight but is one of the rainiest places on Earth, because air rises there.
The pattern shifts with the seasons. The ITCZ follows the high Sun, moving north in the northern summer and south in the southern summer. Over South Asia that shift, strengthened by the hot land, reverses the winds: moist ocean air flows in for the summer monsoon rains, and dry winds blow out from the land in winter.
Practice
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