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Sailors knew for centuries that steady winds blow from the north-east in the northern tropics and from the south-east in the southern tropics. Explaining why took a combination of heat and rotation, and one of the clearest early explanations fits on four printed pages. George Hadley's 1735 paper is short enough for a student to follow, and its key step is arithmetic you can check. This post walks through it, reworks his numbers with modern values and points out the common mistakes.
Who was George Hadley?
Very little of Hadley's life is needed to understand his idea, and we will stick to what the paper itself shows. It was printed in the Philosophical Transactions of the Royal Society, volume 39 (1735), pages 58 to 62, under the title "Concerning the Cause of the General Trade-Winds", by "Geo. Hadley, Esq; F.R.S.", so he was a Fellow of the Royal Society. The full text is freely readable in a scan on the Internet Archive.
What was missing from earlier explanations?
Hadley opens by saying the causes of the trade winds "have not been fully explained by any of those who have wrote on that Subject", because they did not consider carefully enough "the Share the diurnal Motion of the Earth has in the Production of them".
Everyone agreed the Sun was the original cause. Hot air near the equator becomes lighter, and cooler air from north and south flows in to replace it. But Hadley points out that heating alone would make air rush in from the north and south, not from the east. Something else must turn the wind.
How did Earth's rotation explain the direction?
Hadley's key idea is that the ground at the equator moves east faster than the ground at the tropics, because the equator is a bigger circle and both make one turn per day. He gives the ratio of the equator to the tropics as "nearly in the Proportion of 1000 to 917", so their difference in circuit is about 2,083 miles.
Air moving from the tropics towards the equator starts with the slower eastward speed of the tropics. As it arrives over faster-moving ground, it falls behind. Relative to the ground it moves westward, and combined with its motion towards the equator that makes a north-east wind north of the equator and a south-east wind south of it.
He also completes the loop. Air heated at the equator rises, spreads north and south in the upper air, cools and sinks again beyond the tropics, where it arrives moving east faster than the ground beneath it, giving westerly winds. The NCERT Class 11 chapter on atmospheric circulation describes the same pattern, rising air at the Inter Tropical Convergence Zone, sinking at the subtropical highs and returning as the easterlies, and calls it the Hadley cell.
Worked example: Hadley's numbers, checked today
Use the NCERT value for Earth's radius, 6,378 km, from the Interior of the Earth chapter, and take the tropics at 23.5°.
Equator circumference: 2π × 6,378 ≈ 40,074 km.
Circle at 23.5°: multiply by cos 23.5° = 0.917. That is Hadley's 1000 to 917.
Difference per day: 40,074 × (1 − 0.917) ≈ 3,324 km.
As a speed: 3,324 km ÷ 86,400 s ≈ 0.0385 km/s, or about 38 m/s, which is roughly 138 km/h.
Hadley's 2,083 miles is about 2,083 × 1.609 ≈ 3,352 km, close to our 3,324 km. His speed, 2,083 miles in a day of 1,440 minutes, is 1.45 miles a minute, which matches his "above 1 Mile and ⅓ in a Minute".
The real trade winds are much gentler. Hadley's own explanation was that the air "must have gained some Motion Eastward from the Surface of the Earth or Sea" on the way, reducing its relative speed.
What mistakes do students make?
Saying "the Earth spins under the air" without direction. Be specific: the ground at the equator moves east faster, so air from higher latitudes lags and appears to come from the east.
Using the full radius at all latitudes. The radius of the circle at latitude φ is R cos φ.
Forgetting the return flow. Air that converges at the surface must rise and leave aloft, or pressure would build up.
Hadley's argument is a first step. The deflection of moving air by rotation is treated more completely through the Coriolis effect, which our post on Gustave Coriolis explains. If you want to see rotating-frame motion rather than read about it, Fermi's Simulation Lab has interactive simulations for ideas that are easier to see than read.
What can students learn from Hadley's way of working?
Hadley took a known cause, solar heating, and asked what it could not explain. He then added one physical idea, rotation, and checked it with a number. When the number came out too large, he did not hide it; he named the missing effect.
That is a useful habit for any derivation: after you get an answer, compare it with what is observed and explain the difference.
Try it: repeat the calculation for air moving from 30° latitude to the equator, and compare the speed difference with the one at 23.5°.
