The tropics gain more radiation than they lose, while high latitudes lose more than they gain. Atmospheric and ocean circulation keeps that regional imbalance from growing without limit by carrying energy poleward.

Pressure and density decrease with height

Air density is mass per unit volume,

$$\rho=\frac{m}{V}.$$

Air pressure is force per unit area. Under hydrostatic balance, it equals the weight of the atmospheric column above a point per unit horizontal area. Because there is less air overhead at greater altitude, pressure falls upward according to

$$\frac{dp}{dz}=-\rho g.$$

Pressure and density are related but not identical. Warm air can be less dense than cold air at the same pressure, and temperature changes how quickly pressure decreases with height. Most atmospheric mass lies near the surface because gravity compresses the lower atmosphere.

The pressure-gradient force starts the wind

Horizontal pressure differences create the pressure-gradient force (PGF):

$$\mathbf{a}_{\mathrm{PGF}}=-\frac{1}{\rho}\nabla p.$$

The minus sign means acceleration points from higher toward lower pressure. Isobars connect equal-pressure locations. Closely spaced isobars indicate a large pressure change over a short distance and therefore a strong PGF; widely spaced isobars indicate a weaker gradient.

Pressure units are related by

$$1\ \mathrm{mb}=1\ \mathrm{hPa}=100\ \mathrm{Pa}, \qquad 1\ \mathrm{kPa}=10\ \mathrm{mb}.$$

Winds are named for the direction they come from. A westerly blows from west to east.

Coriolis turns moving air

Earth’s rotation creates an apparent Coriolis deflection in an Earth-fixed frame:

• to the right of motion in the Northern Hemisphere;
• to the left in the Southern Hemisphere;
• zero at the equator and stronger toward the poles;
• stronger for faster motion.

Its latitude dependence is represented by

$$f=2\Omega\sin\phi,$$

where $\Omega$ is Earth’s rotation rate and $\phi$ is latitude. Coriolis does not start the wind; PGF does. In the idealized force, Coriolis acts perpendicular to motion, so it changes direction rather than doing work on the air.

The words “right” and “left” follow the moving object. A southbound object in the Northern Hemisphere is deflected to its right, which is westward.

Friction changes the balance near the surface

Above the friction layer, PGF and Coriolis can approach geostrophic balance. PGF points across isobars toward low pressure, Coriolis opposes it, and the wind flows roughly parallel to straight isobars.

Near the ground, surface roughness creates friction and turbulence. Friction slows the wind, which weakens Coriolis. PGF is no longer fully balanced, so the wind crosses isobars at an angle toward lower pressure.

This produces the characteristic surface patterns:

Rising air expands and cools, which can promote condensation, clouds, and precipitation. Sinking air compresses and warms, lowering relative humidity and often suppressing clouds. These are tendencies, not guarantees of a particular day’s weather.

Unequal heating creates global pressure belts

On a non-rotating idealized Earth, warm air would rise near the equator, move poleward aloft, sink over the cold poles, and return equatorward at the surface. Rotation and atmospheric eddies divide that simple loop into three circulation cells per hemisphere.

From equator to pole, the idealized pressure belts are:

The belts are discontinuous and migrate with the seasons. Continents, oceans, mountains, monsoons, and transient weather systems break them into regional pressure centres.

The ITCZ

The Intertropical Convergence Zone is where the northeast and southeast trade winds meet. Strong surface heating makes moist air buoyant, convergence adds lifting, and condensation releases latent heat that supports deep thunderstorms. The ITCZ shifts toward the summer hemisphere, generally farther over continents than oceans.

Subtropical highs

Air that rose in the tropics spreads poleward aloft and sinks near $20^\circ$–$35^\circ$ latitude. As it descends, compression warms it adiabatically and relative humidity falls. This suppresses clouds and helps produce subtropical desert belts.

Subpolar lows and polar highs

Near $60^\circ$, warmer westerly air meets cold polar air along the polar front. Convergence, fronts, and cyclones favour ascent and precipitation. Over the poles, cold dense air tends to sink, producing thermal highs and low absolute precipitation: polar deserts.

Hadley, Ferrel, and Polar cells

Hadley cell: equator to about $30^\circ$

Air rises in the ITCZ, moves poleward aloft, sinks in the subtropical highs, and returns equatorward near the surface. Coriolis turns that return flow westward:

• northeast trades in the Northern Hemisphere;
• southeast trades in the Southern Hemisphere.

The Hadley cell is thermally direct: warm air rises and cooler air sinks.

Ferrel cell: about $30^\circ$–$60^\circ$

Surface air moves poleward from subtropical highs toward subpolar lows. Coriolis turns it eastward, producing the westerlies. The idealized return flow is equatorward aloft. Unlike a simple convection loop, the Ferrel cell is strongly maintained by travelling cyclones, anticyclones, and eddy heat transport, so it is thermally indirect.

Polar cell: about $60^\circ$–$90^\circ$

Cold air sinks over the poles, moves equatorward at the surface, and is turned westward into polar easterlies. It meets warmer air near the polar front, rises, and returns poleward aloft.

The full sequence from equator to pole is therefore:

$$\begin{aligned} \text{ITCZ} &\rightarrow \text{Hadley cell} \\ &\rightarrow \text{subtropical high} \rightarrow \text{Ferrel cell}\\ &\rightarrow \text{subpolar low} \rightarrow \text{Polar cell}\\ &\rightarrow \text{polar high}. \end{aligned}$$

Global pressure belts, cells, and surface winds

Swipe horizontally to see both hemispheres.

The figure combines three relationships that are easy to separate by accident: pressure-gradient flow begins at the highs, Coriolis turns the surface winds, and convergence or divergence at the pressure belts connects those horizontal winds to rising or sinking air. The Ferrel cell is the idealized middle branch; travelling weather systems supply much of its real transport.

Jet streams and storm steering

Jet streams are narrow bands of fast wind near the tropopause. They are mainly westerly and form where horizontal temperature and pressure gradients are strong. The subtropical jet lies near the poleward edge of the Hadley circulation; the polar-front jet lies near the boundary between warmer midlatitude and colder polar air.

Jets meander into poleward ridges and equatorward troughs. Divergence aloft removes mass from an atmospheric column, favouring falling surface pressure and ascent; convergence aloft adds mass, favouring rising surface pressure and descent. The exact effect depends on position within the wave and on changes in wind speed, so “ridge equals high” and “trough equals low” are only rough shortcuts.

The jets help steer midlatitude cyclones and weather systems. They are not rigid pipes: their position, strength, and waviness change continually.

Winds drive surface-ocean gyres

Persistent winds exert stress on the ocean surface. Trade winds drive tropical water mainly westward, while westerlies drive midlatitude water mainly eastward. Coriolis, Ekman transport, and continental boundaries organize these flows into basin-scale gyres.

• Northern Hemisphere subtropical gyres rotate clockwise.
• Southern Hemisphere subtropical gyres rotate counterclockwise.
• Subpolar gyres rotate in the opposite sense where basin geometry allows.

Warm western-boundary currents such as the Gulf Stream and Kuroshio carry tropical water poleward. They are narrow and fast. Cooler eastern-boundary currents such as the California, Canary, Peru/Humboldt, and Benguela currents generally carry water equatorward and are broader and slower. These currents redistribute heat and shape coastal temperature, humidity, fog, and precipitation.

Density drives overturning circulation

The density of seawater depends on temperature, salinity, and pressure:

$$\rho=\rho(T,S,p).$$

Cooling generally increases density, and increasing salinity also increases density. In parts of the high-latitude North Atlantic, surface water loses heat, becomes dense, and sinks. Cold deep water flows southward, joins circulation around Antarctica, spreads into other basins, and eventually returns toward the surface through distributed upwelling and mixing.

This thermohaline or overturning circulation is slow and three-dimensional. It transfers heat and exchanges surface water with the deep ocean. The common “global conveyor belt” is a helpful sketch, not one continuous pipe: winds, eddies, mixing, multiple water masses, and several sinking and upwelling regions all contribute.

Wind-driven gyres and density-driven overturning are linked but distinct. Together with the atmosphere, they move energy away from the tropical radiation surplus and toward high latitudes.