( ( ( ( ( ( Warm air ) ( has ( ( a ( lower density ) ) ( than ( cool air ) ) ) ) ) , ) so ) ( ( warm air ) ( rises ( within ( ( ( cooler air ) , ) ( similar ( to ( hot ( air balloons ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (JJ Warm) (NN air)) (VP (VBZ has) (NP (NP (DT a) (JJR lower) (NN density)) (PP (IN than) (NP (JJ cool) (NN air)))))) (, ,) (IN so) (S (NP (JJ warm) (NN air)) (VP (VBZ rises) (PP (IN within) (NP (NP (JJR cooler) (NN air)) (, ,) (ADJP (JJ similar) (PP (TO to) (NP (JJ hot) (NN air) (NNS balloons)))))))) (. .)))
Warm air has a lower density than cool air, so warm air rises within cooler air, similar to hot air balloons.
(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))))))) (. .)))