The Sun heats Earth unevenly: most strongly near the equator, where it is high in the sky, and least near the poles (as with the seasons). This uneven heating drives winds on a planet-wide scale, and Earth's spin bends them into steady belts.
- Near the equator, warm, moist air rises: a belt of low pressure, clouds and heavy rain.
- The air spreads toward the poles high up, cools, and sinks near 30° N and 30° S: high pressure and dry air, which is why many great deserts lie near 30°.
- At the surface, air flows back toward the equator as the trade winds. Between 30° and 60° the westerlies blow; near the poles, the polar easterlies.
Key idea
Uneven heating makes air rise at the equator and sink near 30°. Earth's spin bends moving air (the Coriolis effect): to the right in the Northern Hemisphere and to the left in the Southern, turning the flow into the trade winds, westerlies and polar easterlies.
Why it works
Earth spins toward the east, and the ground at the equator moves faster (about 1,670 km/h) than the ground nearer the poles. Air moving toward the equator from 30° N can't keep up with the faster ground beneath it, so it lags behind toward the west: seen from the ground, it curves to the right and arrives as a wind from the northeast. That is the northeast trade wind. In the Southern Hemisphere the curve goes the other way, making southeast trades.
Worked example: Trade winds
Which way do the trade winds blow just north of the equator?
- Surface air flows from about 30° N toward the equator, so it starts out moving south.
- In the Northern Hemisphere it is bent to the right, toward the west.
- So it comes from the northeast: the northeast trades.
Answer: From the northeast
Worked example: Deserts
Why are the Sahara and the Australian deserts near 30° latitude?
- Air that rose at the equator sinks near 30°.
- Sinking air warms and dries out, so clouds rarely form there.
Answer: Dry air sinks there
Common mistake
Thinking the Coriolis effect makes sinks drain in opposite directions in the two hemispheres. It is far too weak at the size of a sink, which drains whichever way the water was already swirling. It matters only for large, long-lasting motions such as winds, ocean currents and hurricanes.