( ( Although ( ( pressure ( and ( air density ) ) ) ( decrease ( as ( altitude increases ) ) ) ) ) ( , ( ( ( the composition ) ( of ( the atmosphere ) ) ) ( ( remains reasonably ) ( ( the same ) ( below ( ( ( ( ( about 50,000 ) ft. ) or ) ( 15 ( km . ) ) ) - ) ) ) ) ) ) )
(ROOT (S (SBAR (IN Although) (S (NP (NN pressure) (CC and) (NN air) (NNS density)) (VP (VB decrease) (PP (IN as) (NP (NN altitude) (NNS increases)))))) (, ,) (NP (NP (DT the) (NN composition)) (PP (IN of) (NP (DT the) (NN atmosphere)))) (VP (VBZ remains) (ADVP (RB reasonably)) (PP (NP (DT the) (JJ same)) (IN below) (NP (NP (QP (RB about) (CD 50,000)) (NN ft.)) (CC or) (NP (NP (CD 15)) (NP (NNP km) (. .))) (: -))))))
Although pressure and air density decrease as altitude increases, the composition of the atmosphere remains reasonably the same below about 50,000 ft. or 15 km. -
(ROOT (S (NP (NNP Air)) (VP (VBP pressure) (SBAR (S (NP (PRP it)) (VP (VBZ decreases) (PP (IN as) (NP (NN altitude) (NNS increases))))))) (. .)))