jet stream

Japanese meteorologist Wasaburo Oishi discovered the jet stream in the 1920s by tracking high-altitude weather balloons over Mount Fuji. He published his findings in Esperanto, a language almost no one read, and the discovery went largely unnoticed in the West. The Japanese military, however, paid attention—and later exploited the jet stream to launch balloon bombs against the American mainland during World War II.

German meteorologist Heinrich Seilkopf coined the term Strahlströmung (jet current) in 1939. The English translation jet stream entered wide use during World War II, when American B-29 bomber crews flying over Japan encountered headwinds exceeding 200 miles per hour at 30,000 feet. Missions that should have taken hours took far longer. The invisible river in the sky was disrupting military operations.

The jet stream is a narrow band of fast-moving air at the boundary between the troposphere and stratosphere, typically between 30,000 and 40,000 feet. It flows west to east at speeds of 60 to 250 miles per hour. The polar jet stream, driven by the temperature differential between arctic and temperate air masses, determines where storms track across the mid-latitudes. When the jet stream dips south, cold air follows. When it retreats north, warmth floods in.

Commercial aviation routes are planned around the jet stream. Eastbound flights from North America to Europe ride the jet stream and arrive hours faster than westbound flights fighting against it. A flight from New York to London might take six hours; the return takes seven or eight. The jet stream saved the airline industry billions in fuel costs once pilots learned to ride it.