( Therefore ( , ( ( air pressure ) ( ( decreases ( as ( we ( increase altitude ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB Therefore)) (, ,) (NP (NN air) (NN pressure)) (VP (VBZ decreases) (SBAR (IN as) (S (NP (PRP we)) (VP (VB increase) (NP (NN altitude)))))) (. .)))
Therefore, air pressure decreases as we increase altitude.
(ROOT (S (NP (NNP Air)) (VP (VBP pressure) (SBAR (S (NP (PRP it)) (VP (VBZ decreases) (PP (IN as) (NP (NN altitude) (NNS increases))))))) (. .)))