( Clouds ( ( form ( when ( ( ( ( warm ( air rises ) ) and ) ( the water ) ) ( vapor ( it ( ( contains condenses ) ( onto ( particles ( called ( cloud ( condensation nuclei ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Clouds)) (VP (VBP form) (SBAR (WHADVP (WRB when)) (S (NP (NP (JJ warm) (NN air) (NNS rises)) (CC and) (NP (DT the) (NN water))) (VP (VBP vapor) (SBAR (S (NP (PRP it)) (VP (VBZ contains) (NP (NNS condenses)) (PP (IN onto) (NP (NP (NNS particles)) (VP (VBN called) (NP (NN cloud) (NN condensation) (NNS nuclei)))))))))))) (. .)))
Clouds form when warm air rises and the water vapor it contains condenses onto particles called cloud condensation nuclei.
(ROOT (S (NP (NNP Condensation)) (VP (MD must) (VP (VB occur) (PP (IN for) (NP (NNS clouds))) (S (VP (TO to) (VP (VB form)))))) (. .)))