( ( So ( for ( clouds ( to form ) ) ) ) ( , ( ( water vapor ) ( ( must ( be ( present ( in ( the atmosphere ) ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (IN So) (PP (IN for) (NP (NNS clouds) (S (VP (TO to) (VP (VB form))))))) (, ,) (NP (NN water) (NN vapor)) (VP (MD must) (VP (VB be) (ADJP (JJ present) (PP (IN in) (NP (DT the) (NN atmosphere)))))) (. .)))
So for clouds to form, water vapor must be present in the atmosphere.
(ROOT (S (NP (NNP Water) (NN vapor)) (VP (MD must) (VP (VB lose) (NP (NN heat) (NN energy)) (SBAR (IN before) (S (NP (NNS clouds)) (VP (MD can) (VP (VB form))))))) (. .)))
Water vapor must lose heat energy before clouds can form.