( ( ( ( ( ( high values ) ( mean ( warm air ) ) ) , ) and ) ( ( low values ) ( mean ( cold air ) ) ) ) . )
(ROOT (S (S (NP (JJ high) (NNS values)) (VP (VBP mean) (NP (JJ warm) (NN air)))) (, ,) (CC and) (S (NP (JJ low) (NNS values)) (VP (VBP mean) (NP (JJ cold) (NN air)))) (. .)))
high values mean warm air, and low values mean cold air.
(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))))))) (. .)))