Between the cells of the global circulation, where cold polar air meets warmer air from the south, the westerlies aloft become a narrow, fast river of wind: the jet stream. It is the geostrophic wind at its strongest.
Key idea
The polar jet stream blows from west to east about 9 to 12 km up, above the boundary between polar and warmer air, often faster than 160 km/h (100 mph), and fastest in winter. It does not run straight: it meanders in great waves. Under a trough (a southward dip) the weather is cool and often stormy; under a ridge (a northward bulge) it is warm and settled. Storms tend to form and travel along the jet.
Why it works
Cold air is denser and more compressed than warm air, so pressure drops faster with height over cold air than over warm air. Climb to airliner height and the pressure over the warm side is much higher than over the cold side at the same altitude. That big pressure difference over a short distance drives a strong geostrophic wind, with low pressure (the cold side) on its left: a west wind. The sharper the temperature contrast, the faster the jet, which is why it is strongest in winter, when the difference between the tropics and the poles is greatest.
Worked example: Riding the jet
A jet flies 4200 km from Los Angeles to New York at an airspeed of 900 km/h, with a 150 km/h tailwind from the jet stream. How long does it take? How long would the return take against the same wind?
- Eastbound ground speed: $900 + 150 = 1050$ km/h; time $4200 \div 1050 = 4$ h.
- Westbound: $900 - 150 = 750$ km/h; time $4200 \div 750 = 5.6$ h.
- About an hour and a half longer flying west, which is why airlines plan routes around the jet.
Answer: 4 hours east, 5.6 hours west
Worked example: Trough or ridge?
In January the jet stream dips far south over the eastern United States and bulges north over the West. Where is it likely to be unusually cold?
- A southward dip is a trough, which lets polar air flow south under it.
- A northward bulge is a ridge, with warm air underneath.
- Cold in the East (under the trough), mild in the West (under the ridge).
Answer: In the East
Common mistake
Picturing the jet stream as a steady, straight band. It meanders in waves that slowly drift and change shape, and the waves steer weather systems: cold and stormy under troughs, warm and settled under ridges. Check: a forecast map of the jet almost always shows wiggles, and the storms are near the troughs.
A second, subtropical jet blows higher (about 12 to 14 km) near 30° latitude, at the poleward edge of the Hadley cell.