The Jet Stream is formed by powerful high-altitude rivers of air which circle the globe and help determine our climate. The existence of these winds was first documented in Japan in the 1920s, but only became more widely known during World War Two, when American airmen encounter high-speed winds on bombing missions over Japan. At the same time, the Japanese military also began to use these powerful transcontinental winds to carry innovative balloon bombs all the way to the West Coast of America. Using archive recordings we tell the story of the discovery and speak to Professor Tim Woollings from Oxford University, the author of Jet Stream: A Journey Through Our Changing Climate. Photo: B-29 bombers passing Mount Fuji on their way to Tokyo, April 1945 (Getty Images)