The jet stream is a narrow band of fast flowing air at high altitudes generally flowing from the west to east over the mid-latitudes. Jet streams are found at or just below the top of the troposphere, but the term is often wrongly used for all upper-level winds. Usually the jet stream marks the boundary between cold polar air to its north and warmer air to its south (northern hemisphere).
Atmospheric scientists had theorized the existence of jet streams at least as early as 1937, but it was for US bomber pilots to confirm it.
In November, 1944, the Japanese began launching unmanned bomb-carrying balloons, which should travel on the theorized jet stream winds across the Pacific Ocean to North America. Some ballons reached the US and Canada. They would have proved the existence of the jet stream - if the Japanese ever had learned that they've actually made it.
World War II was almost over when the United States introduced the first high-altitude bomber, an airplane called the Boeing B-29. It could fly at altitudes well above 7 kilometres (22,000 ft). When the B-29s were being put into service from a Pacific island base, two air force meteorologists were assigned to prepare wind forecasts for aircraft operations at such altitudes.
To make their prediction, the meteorologists primarily used surface observations and what is known in meteorology as the thermal wind. The thermal wind relationship states that if you stand with your back to the wind, and the air is colder to your left and warmer to your right, the wind speed on your back will get stronger as you ascend in the atmosphere. Using this relationship, the meteorologists predicted a 168-knot wind blowing from the west.
Their commanding officer could not believe the forecast, believing the fo recast speed much too high. However, on the next day, the B-29 pilots reported wind speeds of 170 knots from the west as predicted! The jet stream, as it would come to be known, was discovered.