Jet streams are narrow bands of strong westerly winds (~100+ m/s) at upper levels, located where the thermal wind is strongest. The subtropical jet marks the poleward edge of the Hadley cell and subtropical anticyclones (~30° lat), while the polar jet forms at the baroclinic zone between mid and polar latitudes (~60° lat). Polar jet variability drives mid-latitude weather patterns and storm tracks.
From the thermal wind relationship, you know that when a horizontal temperature gradient exists in the atmosphere, the geostrophic wind must change with height — the wind shear is proportional to the temperature contrast. From planetary circulation cells, you know that Earth's atmosphere organizes into distinct latitudinal bands: the Hadley cell in the tropics, the Ferrel cell in the midlatitudes, and the Polar cell at high latitudes. Jet streams are the dramatic consequence of combining these two ideas — they form where the strongest temperature gradients meet the deepest atmosphere, concentrating kinetic energy into remarkably narrow ribbons of fast-moving air.
The subtropical jet stream sits near 30° latitude at roughly 10–12 km altitude, right at the poleward boundary of the Hadley cell. Here, air that rose at the equator and flowed poleward in the upper troposphere has been steadily deflected eastward by the Coriolis force. By the time it reaches the subtropics, it has accumulated so much eastward momentum that it forms a concentrated wind maximum — often reaching 50–70 m/s. The subtropical jet is relatively steady in position and strength because the Hadley cell itself is thermally direct and persistent. It is strongest in winter, when the equator-to-pole temperature gradient steepens.
The polar jet stream forms near 50–60° latitude where cold polar air masses collide with warmer midlatitude air — the polar front. This is where the temperature gradient is sharpest in the lower and middle troposphere, and the thermal wind equation dictates that the strongest wind shear and therefore the strongest upper-level winds will concentrate here. Unlike the relatively stable subtropical jet, the polar jet is wild and variable. It meanders in great sinuous waves called Rossby waves, dipping south to bring Arctic air into temperate regions (troughs) and bulging north to carry warm air poleward (ridges). These undulations are the steering mechanism for midlatitude weather — extratropical cyclones form and travel along the jet, and the position of the jet determines whether a given region experiences warmth or cold, drought or rain.
The polar jet's behavior has enormous practical consequences. When the jet is strong and relatively zonal (flowing mostly west to east), weather systems move briskly across the midlatitudes, and no single pattern persists for long. When the jet weakens and becomes highly meridional (large north-south undulations), weather patterns stall — a blocking ridge can park over a region for weeks, producing heat waves and drought, while an adjacent deep trough delivers persistent cold and flooding. The jet stream's position also matters for aviation: flying with a strong jet stream tail wind can cut hours off a transatlantic flight, while flying against it extends travel time significantly. Understanding where the jets are and how they are behaving is the starting point for nearly all midlatitude weather forecasting.
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