( ( As ( ( a result ) ( of ( its ( low density ) ) ) ) ) ( , ( ( ( warm ( air low ) ) ( in ( the atmosphere ) ) ) ( ( tends ( to ( ( ( ( ( rise , ) expand ) , ) and ) cool ) ) ) . ) ) ) )
(ROOT (S (PP (IN As) (NP (NP (DT a) (NN result)) (PP (IN of) (NP (PRP$ its) (JJ low) (NN density))))) (, ,) (NP (NP (JJ warm) (NN air) (NN low)) (PP (IN in) (NP (DT the) (NN atmosphere)))) (VP (VBZ tends) (S (VP (TO to) (VP (VB rise) (, ,) (VB expand) (, ,) (CC and) (VB cool))))) (. .)))
As a result of its low density, warm air low in the atmosphere tends to rise, expand, and cool.
(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))))))) (. .)))