( ( ( ( Cloud ( condensation nucleus ) ) : ) ( ( ( ( ( a particle ) , ) ( either ( ( liquid or ) solid ) ) ) , ) ( upon ( which ( ( water condenses ) ( to ( form ( cloud droplets ) ) ) ) ) ) ) ) . )
(ROOT (NP (NP (JJ Cloud) (NN condensation) (NN nucleus)) (: :) (NP (NP (NP (DT a) (NN particle)) (, ,) (ADJP (DT either) (JJ liquid) (CC or) (JJ solid)) (, ,)) (PP (IN upon) (SBAR (WHNP (WDT which)) (S (NP (NN water) (NNS condenses)) (VP (TO to) (VP (VB form) (NP (NN cloud) (NNS droplets)))))))) (. .)))
Cloud condensation nucleus : a particle, either liquid or solid, upon which water condenses to form cloud droplets.
(ROOT (S (NP (NNP Condensation)) (VP (MD must) (VP (VB occur) (PP (IN for) (NP (NNS clouds))) (S (VP (TO to) (VP (VB form)))))) (. .)))