( ( In ( areas ( of ( low ( air pressure ) ) ) ) ) ( , ( ( the air ) ( ( ( ( is ( ( ( less dense ) and ) ( relatively warm ) ) ) , ) ( which ( causes ( it ( to rise ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN In) (NP (NP (NNS areas)) (PP (IN of) (NP (JJ low) (NN air) (NN pressure))))) (, ,) (NP (DT the) (NN air)) (VP (VBZ is) (ADJP (ADJP (RBR less) (JJ dense)) (CC and) (ADJP (RB relatively) (JJ warm))) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBZ causes) (S (NP (PRP it)) (VP (TO to) (VP (VB rise)))))))) (. .)))
In areas of low air pressure, the air is less dense and relatively warm, which causes it to rise.
(ROOT (S (NP (NP (DT The) (JJR lower) (NN density)) (PP (IN of) (NP (JJ warm) (NN air)))) (VP (VBZ causes) (S (NP (PRP it)) (VP (TO to) (VP (VB rise) (PP (IN above) (NP (JJ cold) (NN air))))))) (. .)))