( ( As ( you ( increase altitude ) ) ) ( , ( ( the ( outside ( air pressure ) ) ) ( ( decreases ( ( as well ) ( , ( but ( at ( a ( known rate ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN As) (S (NP (PRP you)) (VP (VB increase) (NP (NN altitude))))) (, ,) (NP (DT the) (JJ outside) (NN air) (NN pressure)) (VP (VBZ decreases) (UCP (ADVP (RB as) (RB well)) (, ,) (CC but) (PP (IN at) (NP (DT a) (JJ known) (NN rate))))) (. .)))
As you increase altitude, the outside air pressure decreases as well, but at a known rate.
(ROOT (S (NP (NNP Air)) (VP (VBP pressure) (SBAR (S (NP (PRP it)) (VP (VBZ decreases) (PP (IN as) (NP (NN altitude) (NNS increases))))))) (. .)))