| − | The lower layer of earth's atmosphere, which exists from ground level to roughly 80 kilometers (50 miles) above sea level.<ref>Wile, Dr. Jay L. ''Exploring Creation With Biology''. Apologia Educational Ministries, Inc. 1998</ref> | + | The lower layer of earth's atmosphere, which exists from ground level to roughly 80 kilometers (50 miles) above sea level.<ref>Wile, Dr. Jay L. ''Exploring Creation With Physical Science''. Apologia Educational Ministries, Inc. 1999, 2000</ref> |