/ ˈtrɒpəsfɪə(r); NAmE ˈtroʊpəsfɪr; ˈtrɑːp-/ noun
[ sing. ] ( technical ) the lowest layer of the earth's atmosphere, between the surface of the earth and about 6-10 kilometres above the surface
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WORD ORIGIN
early 20th cent.: from Greek tropos turning + sphere .