Jet stream

A jet stream forms where large horizontal temperature contrasts create a strong pressure gradient near the top of the troposphere. Earth’s rotation turns this flow into predominantly west-to-east winds through the Coriolis effect, while the contrast between warm and cold air determines the stream’s strength.¹

The principal streams are the polar-front jet and the subtropical jet in each hemisphere. They are not continuous tubes: their cores meander, split, merge, and sometimes weaken into disconnected segments. The strongest winds are concentrated in a narrow region called the jet core, but the surrounding high-wind zone can extend across hundreds of kilometres.

Jet streams steer and reshape large weather systems by marking boundaries between contrasting air masses. A trough, or southward dip, often supports rising air and unsettled weather, while a ridge, or northward bulge, is more commonly associated with sinking air and persistent warmth or dryness.

Airlines exploit the wind for faster eastbound flights and avoid strong headwinds when practical. The stream itself does not produce all surface weather, but its position influences storm tracks, temperature changes, and the development of some cyclones. Clear-air turbulence can occur near sharp wind-speed or wind-direction changes around the jet, even when no clouds are visible.²

Jet streams generally strengthen in winter because the temperature contrast between low and high latitudes is greater. Their latitude and shape also shift with seasonal heating, land–sea contrasts, mountain ranges, and tropical convection.¹

On other planets, jet-like atmospheric bands can be far larger and faster than those on Earth. Jupiter has alternating eastward and westward jets extending around the planet, while Saturn’s atmosphere includes a particularly powerful equatorial jet. On Earth, planetary waves generated by mountains and uneven heating can make the polar jet unusually wavy, allowing cold or warm air to reach places far from its usual latitude.³

Jet streams are three-dimensional and occur at several levels, rather than as one universal wind belt. A weaker, higher-altitude phenomenon called the polar-night jet develops in the winter stratosphere, while the subtropical jet can help transport dust, volcanic material, and moisture over long distances.⁴

Scientists study whether Arctic amplification is altering the waviness or persistence of the mid-latitude jet, but the size and consistency of these effects remain subjects of active research. The quasi-biennial oscillation in the tropical stratosphere can also influence the circulation far beyond the equator. These links show why a surface forecast cannot be interpreted from jet position alone.⁵

Jet core

The zone within a jet stream where wind speed is greatest.

Polar-front jet

A mid-latitude jet associated with the boundary between polar and warmer air masses.

Subtropical jet

A generally higher-latitude jet produced largely by the poleward transport of angular momentum in the Hadley circulation.

Trough

A relatively elongated region of lower pressure or geopotential height that dips toward lower latitudes.

Clear-air turbulence

Aircraft turbulence occurring without the visible clouds that commonly signal turbulent air.