( ( ( The decrease ) ( of ( ( air density ) ( with altitude ) ) ) ) ( ( is ( accompanied ( by ( ( a decline ) ( in ( air pressure ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN decrease)) (PP (IN of) (NP (NP (NN air) (NN density)) (PP (IN with) (NP (NN altitude)))))) (VP (VBZ is) (VP (VBN accompanied) (PP (IN by) (NP (NP (DT a) (NN decline)) (PP (IN in) (NP (NN air) (NN pressure))))))) (. .)))
The decrease of air density with altitude is accompanied by a decline in air 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))))))) (. .)))