Carl-Gustaf Arvid Rossby (Swedish pronunciation: [kɑːɭ ˈɡɵ̂sːtav ˈǎrːvɪd ˈrɔ̌sːbʏ] 28 December 1898 – 19 August 1957) was a Swedish-born American meteorologist who first explained the large-scale motions of the atmosphere in terms of fluid mechanics. He identified and characterized both the jet stream and the long waves in the westerlies that were later named Rossby waves.