( ( As ( altitude increases ) ) ( , ( ( ( ( the ( atmospheric ( pressure decreases ) ) ) and ) ( fewer ( oxygen molecules ) ) ) ( ( are ( available ( in ( the ( thinner air ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN As) (NP (NN altitude) (NNS increases))) (, ,) (NP (NP (DT the) (JJ atmospheric) (NN pressure) (NNS decreases)) (CC and) (NP (JJR fewer) (NN oxygen) (NNS molecules))) (VP (VBP are) (ADJP (JJ available) (PP (IN in) (NP (DT the) (JJR thinner) (NN air))))) (. .)))
As altitude increases, the atmospheric pressure decreases and fewer oxygen molecules are available in the thinner air.
(ROOT (S (NP (NNP Air)) (VP (VBP pressure) (SBAR (S (NP (PRP it)) (VP (VBZ decreases) (PP (IN as) (NP (NN altitude) (NNS increases))))))) (. .)))