( ( Most measurably ) ( , ( ( as ( altitude increases ) ) ( , ( air ( ( ( ( becomes ( less dense ) ) , ) ( decreasing ( atmospheric pressure ) ) ) . ) ) ) ) ) )
(ROOT (S (NP (JJS Most) (NN measurably)) (, ,) (PP (IN as) (NP (NN altitude) (NNS increases))) (, ,) (NP (NN air)) (VP (VBZ becomes) (ADJP (RBR less) (JJ dense)) (, ,) (S (VP (VBG decreasing) (NP (JJ atmospheric) (NN pressure))))) (. .)))
Most measurably, as altitude increases, air becomes less dense, decreasing atmospheric pressure.
(ROOT (S (NP (NNP Air)) (VP (VBP pressure) (SBAR (S (NP (PRP it)) (VP (VBZ decreases) (PP (IN as) (NP (NN altitude) (NNS increases))))))) (. .)))