The tropics take in more sunlight than they radiate away, and the poles radiate more than they take in. The atmosphere carries the difference poleward. On a spinning planet it can't do that in one straight loop, so each hemisphere splits into three cells: Hadley, Ferrel and polar. Their edges set where the deserts, the rain belts and the jet streams sit.
Warm air rises at the equator, flows poleward high up, sinks near 30°, and returns along the surface. That loop is the Hadley cell. Near the poles a smaller polar cell turns the same way. Between them the Ferrel cell turns the other way, driven by the storms of the middle latitudes. Coriolis turns every surface wind sideways. Slow the planet down and watch the Hadley cell widen.
How to read this: on the left (top on phones), Earth seen from space with its surface winds as amber arrows. On the right, a side-on slice from the south pole to the north pole. Each loop of moving dots is one "cell" of air turning over.
Try this: click Not spinning, then Earth, then Fast, and count the loops.
Moving air is turned to the right in the Northern Hemisphere and to the left in the Southern. Surface air flowing toward the equator ends up blowing from the east: the trade winds and the polar easterlies. Air flowing poleward ends up blowing from the west: the westerlies.
The cell edges here use a toy rule, φedge ≈ 30° × √(ΩEarth/Ω), chosen to match Earth and to widen as spin slows. It shows the trend only. Simple theory for a smooth, storm-free atmosphere predicts that the cell's width scales roughly as 1/Ω, and full simulations give a weaker dependence.
Each layer of air carries the weight of everything above it, so pressure falls with height. How fast it falls depends on temperature. Warm air is less dense, so a warm column has to rise further to shed the same weight. Start two columns at the same surface pressure, one tropical and one polar. At any height above the ground the warm column has more air overhead, and that difference pushes air toward the pole aloft.
How to read this: two tall columns of air, one warm and one cold. Each green line joins points with the same air pressure, and its number is that pressure in hPa (the ground is 1000). The lines sit higher over the warm column, so at the same height the warm side has more air above it and pushes toward the pole.
Try this: click No contrast: the lines go flat and the push vanishes.
With Rd = 287 J/(kg·K) and g = 9.81 m/s², a column at 288 K () has H = 27,600 ft. Pressure falls by a factor of e (about 2.7) every scale height. Both columns here start at and hold one temperature all the way up, which is an idealization: the real atmosphere cools with height.
The top chart is the reason the circulation exists: a sunlight surplus near the equator and an infrared deficit toward the poles. The bottom chart is the reason it can't do the whole job in one loop. Air that leaves the equator high up keeps its angular momentum, so it spins up eastward as it moves poleward. Slide the latitude and follow it.
How to read this: top, sunlight in versus heat out at each latitude, in W/m² (watts per square meter, a rate of energy flow). Bottom, how fast air leaving the equator would be blowing by the time it reached that latitude. Equal widths are equal areas of Earth, so the latitudes bunch up toward the pole.
Try this: press Release a parcel and watch the bottom dot climb far past the green band.
The curves are an idealized annual mean: sunlight from a standard fit to annual insolation, an albedo that rises toward the poles, and outgoing infrared tuned so the globe balances. The horizontal axis is spaced by sin φ, so equal widths are equal areas of Earth. In this model surplus turns to deficit near 33°, and the poleward heat flow peaks there at 6.3 PW. Satellite-based estimates of the real peak are near 6 PW.
Real jets are far slower. Friction at the ground and the large storms of the middle latitudes trade angular momentum with the rest of the atmosphere, and a flow sheared this strongly would be unstable anyway. Past about 30° the smooth loop breaks down into the swirling weather of the W02 · Highs & lows. Those storms carry the rest of the heat poleward and drive the Ferrel cell.