The jet stream is a narrow band of very strong winds in the upper troposphere, typically at altitudes of 9–12 km (near the 300 hPa level). Wind speeds within the core routinely reach 150–300 km/h, with extremes exceeding 400 km/h. The jet stream flows roughly from west to east in mid-latitudes and profoundly shapes the weather patterns below it.
Two main jet streams exist in each hemisphere. The polar jet (around 50–60°N latitude) marks the boundary between cold polar air and warmer subtropical air, and is the one most relevant to European weather. The subtropical jet (around 30°N) is weaker and sits at a higher altitude. Both jet streams meander in large wave patterns called Rossby waves: troughs (southward dips) bring cold air intrusions, while ridges (northward bulges) bring warm, settled weather.
When Rossby waves amplify and stall, blocking patterns develop that lock weather in place for days or weeks — causing heatwaves, cold spells, or prolonged drought. The jet stream also steers depressions and their fronts across the Atlantic towards Europe, so its position determines whether a region experiences stormy or quiet weather. Aviation exploits the jet stream: eastbound flights ride it for fuel savings and shorter times, while westbound flights avoid it. Climate change is suspected of weakening the jet stream and increasing blocking events, potentially making extreme weather more persistent.