( It ( too ( ( ( decreases slowly ) ( as ( you ( ( increase ( in altitude ) ) ( because ( ( relative humidity ) ( ( is ( ( the ( relative amount ) ) ( of ( water vapor ) ) ) ) ( that ( air ( can ( contain ( for ( ( ( a ( given temperature ) ) and ) pressure ) ) ) ) ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (NP (PRP It)) (ADVP (RB too)) (VP (VBZ decreases) (ADVP (RB slowly)) (SBAR (IN as) (S (NP (PRP you)) (VP (VB increase) (PP (IN in) (NP (NN altitude))) (SBAR (IN because) (S (NP (JJ relative) (NN humidity)) (VP (VBZ is) (NP (NP (DT the) (JJ relative) (NN amount)) (PP (IN of) (NP (NN water) (NN vapor)))) (SBAR (IN that) (S (NP (NN air)) (VP (MD can) (VP (VB contain) (PP (IN for) (NP (NP (DT a) (VBN given) (NN temperature)) (CC and) (NP (NN pressure))))))))))))))) (. .)))
It too decreases slowly as you increase in altitude because relative humidity is the relative amount of water vapor that air can contain for a given temperature and 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))))))) (. .)))