A river of fast wind high in the sky, and why flying east is quicker.
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A plane from New York to London usually gets there quicker than the same trip back. It’s the same distance, so what’s different? The answer is invisible, high up in the sky.
About 10 km up, where big planes fly, there’s a narrow river of very fast wind: the jet stream. It blows from west to east, all the way round the world, sometimes faster than 400 km/h! (The map draws it smooth. The real one wiggles and changes every day.)
The Sun heats the equator much more than the poles. The jet stream forms along the border where cold polar air (blue) meets warmer air (red). The Earth’s spin swings it round from west to east.
Flying east, the jet stream blows the plane along from behind. That’s called a tailwind. Tap the button to fly home the other way.
The jet stream can bend into big waves, like a winding river. Where it dips south, cold Arctic air comes too. Where it bumps north, warm air goes north. (Real waves are messier than these.)
The waves usually move along from west to east, so the weather keeps changing. But sometimes they get stuck. In winter 2013–14, a dip sat over the middle of the USA for months, full of icy Arctic air, while the west coast stayed warm and dry.
Here the jet stream has dipped a long way south over Europe.
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The globe pictures are simplified to show the idea. Earth images: NASA Blue Marble (public domain).