The Jet Stream
Narrow bands of strong upper-atmosphere winds that form at temperature boundaries and steer mid-latitude weather.
Jet streams are narrow bands of strong westerly winds in the upper troposphere, typically at altitudes of 9 to 16 kilometers. Two are prominent: the polar jet, forming at the boundary between cold polar air and warmer mid-latitude air, and the subtropical jet, forming at the poleward edge of the Hadley circulation. Speeds commonly reach 150 to 300 km/h and can exceed 400 km/h in the strongest cores.
The physical cause is the temperature difference between the equator and the poles. Because pressure gradients steepen with height where horizontal temperature contrasts are strong — the thermal wind relationship — the geostrophic wind increases with altitude, and the Coriolis effect turns the flow. The result is a fast, meandering westerly current rather than a straight wind: the jet traces troughs and ridges that shift position from day to day.
The jet stream steers weather in the mid-latitudes. Storm tracks follow its path, and its position determines where low-pressure systems develop and move. When the jet's waves — Rossby waves — become slow or blocked, weather patterns persist, producing prolonged heat waves, cold snaps, and flooding rains. In aviation, aircraft flying eastward ride the jet stream to save time and fuel, while westward flights route around it.
Jet streams are part of the general circulation of the atmosphere, transporting energy poleward and mixing air between latitudes. Their behavior on longer timescales is linked to climate variability and is an active research area, particularly how a warming Arctic may alter the jet's speed and waviness. Forecasters track jet position continuously because it is the single best predictor of where storms will go.
Tags
atmosphere meteorology weather wind
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