What Is the Jet Stream? The River of Wind That Steers the Weather

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.

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, while a deep trough can pour Arctic air south, as into 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

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.

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.

Most turbulence on a flight comes from rising air in and around clouds, which pilots can see and weather radar can detect. The jet stream produces a different kind, called clear-air turbulence, that gives no warning. It happens where wind speed changes sharply over a short distance, typically at the edges of the jet above and below its fast core. The layers of air slide past each other at different speeds, and the boundary between them can roll up into invisible waves and eddies, much like the curling crests that wind raises on the surface of the sea.

Because there is often no cloud to mark these eddies, ordinary radar cannot see them, and aircraft occasionally drop suddenly in a clear blue sky. Forecasters therefore map zones where strong wind shear is likely and send warnings to airlines, and pilots report bumpy patches so that aircraft behind them can change altitude. This is one practical reason passengers are told to keep seat belts fastened while seated even when the sign is off. Most injuries in such events happen to people who were not strapped in, and the aircraft itself is built to withstand far rougher air.

The first person to measure these winds systematically was a Japanese meteorologist, Wasaburo Ooishi, who in the 1920s released large numbers of small balloons from an observatory at Tateno, north-east of Tokyo, and tracked them with a surveying telescope as they rose. He found strong westerly winds high above Japan, especially in winter, and published the results. But he wrote his reports in Esperanto, hoping to reach an international readership, and they were largely ignored outside Japan. His findings were known at home, however, and in the last year of the Second World War the Japanese military put them to use.

Between November 1944 and April 1945 Japan launched thousands of hydrogen balloons made of tough paper, each carrying incendiary or high-explosive bombs, to ride the winter jet across the Pacific in about three days. An altimeter-controlled device dropped sandbags when a balloon sank too low and released gas when it rose too high. Several hundred are known to have reached North America, landing as far apart as Alaska and Mexico, but they did little damage. One that came down in Oregon in May 1945 killed a woman and five children on a picnic, the only deaths from enemy action on the American mainland during the war.

Airlines plan routes around the jet stream every day. Over the North Atlantic, the strong westerly winds at cruising height mean that flights from North America to Europe are usually noticeably shorter than the same journey in the opposite direction, often by an hour or so. Planners shift the exact route daily to ride the strongest winds eastbound and avoid them westbound.

When the jet stream is unusually strong, the effect can be dramatic. In February 2020 a British Airways jumbo jet flew from New York to London in under five hours, helped by exceptionally powerful winds associated with a storm, reaching ground speeds far above its normal cruising speed. Westbound passengers on the same days faced slower journeys and sometimes refuelling stops.

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.