What Is the Jet Stream? The River of Wind That Steers the Weather
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A flight from New York to London takes about an hour less than the return, and the reason is a river of air, roughly ten kilometres up, flowing east at speeds that can pass 400 kilometres an hour, which the eastbound plane rides and the westbound one fights. The jet stream was discovered by the pilots of the Second World War, who found their bombers over Japan making no headway into it, and it has turned out to be the main engine of the weather in the middle latitudes, the reason a British summer is decided by a bend in the wind over Canada.
Why it exists
The tropics are hot and the poles are cold, and the atmosphere moves heat from one to the other. The sharpest temperature contrast in the northern hemisphere is at the boundary between the warm air of the subtropics and the cold air of the polar regions, and where that boundary is steepest the pressure difference aloft is greatest, so the air is driven from the warm side towards the cold. The Earth's rotation then turns the moving air to the right, in the northern hemisphere, until it is flowing not towards the pole but along the boundary, from west to east. The faster the temperature changes across the boundary, the faster the wind, and it is fastest just below the top of the troposphere, where the contrast is sharpest.
There are two main jets in each hemisphere: the polar jet, at around 50 to 60 degrees latitude and the stronger of the two, and the subtropical jet at around 30, higher and weaker. Both are strongest in winter, when the pole is coldest and the contrast greatest, and both wander north in summer with the sun.
How it bends
The jet does not run straight round the globe. It meanders in great waves, called Rossby waves after the meteorologist who explained them in 1939, with typically four to six ridges and troughs around the hemisphere, and the waves are what make the weather. Where the jet bulges north, in a ridge, warm air is carried poleward and the surface beneath sits under high pressure, clear and settled; where it dips south, in a trough, cold air spills toward the equator and low pressure, cloud and rain follow. The whole pattern slowly drifts east, so that a place under a ridge this week may be under a trough the next. The main things the jet stream governs:
- •The track of storms: mid-latitude depressions form along the jet and are carried by it, which is why Atlantic storms cross Ireland and Britain rather than Portugal
- •Heatwaves and cold snaps: a ridge parked over a region gives it days or weeks of the same weather, as over Europe in the summer of 2003 and Texas in the winter of 2021
- •Droughts and floods: a jet that stalls delivers the same rain to the same place, or none at all, for a season
- •Flight times: airlines route eastbound flights into the core of the jet and westbound ones around it
When it stalls
The wave pattern usually moves along at a walking pace, and the trouble comes when it stops. A blocking pattern, in which a ridge locks in place for a week or more, produces the persistent extremes that do the most damage: the Russian heatwave of 2010 and the floods in Pakistan the same summer were two ends of the same stalled wave. Why the jet blocks is only partly understood. There is evidence, argued over, that the Arctic warming faster than the rest of the planet is weakening the temperature contrast that drives the jet, making it slower and wavier and more prone to stalling, which would mean a warming climate delivers not only hotter summers but stuck ones; other researchers find the signal too weak to separate from natural variation.
Finding it
Meteorologists locate the jet with weather balloons launched twice a day from hundreds of stations, which radio back wind speed and direction as they rise, and with satellites that track the movement of high cloud, and the position of its waves is the first thing a forecaster looks at. The reason a forecast for next week is possible at all is that the jet's waves evolve slowly enough to be predicted several days out; the reason a forecast for next month is not is that small errors in the waves grow until the pattern is unrecognisable. The pilot who found it over Japan in 1944 had no such maps, only a ground speed that did not make sense, and the name jet stream was given by a German meteorologist who had seen it in balloon data in the 1930s, when almost nobody had flown high enough to feel it.
The takeaway
The jet stream is a band of fast westerly wind near the top of the troposphere, created where warm subtropical air meets cold polar air and turned along the boundary by the Earth's rotation. It meanders in slow-moving Rossby waves whose ridges bring settled weather and whose troughs bring storms, steers the depressions of the mid-latitudes, sets flight times across oceans, and when it stalls produces the heatwaves, cold spells and floods that last for weeks.