( This ( ( is ( because ( ( the ( key ( energy source ) ) ) ( is ( ( the ( latent heat ) ) ( that ( 's ( released ( when ( ( water vapor ) ( condenses ( into ( ( ( cloud droplets ) and ) rain ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT This)) (VP (VBZ is) (SBAR (IN because) (S (NP (DT the) (JJ key) (NN energy) (NN source)) (VP (VBZ is) (NP (NP (DT the) (NN latent) (NN heat)) (SBAR (WHNP (WDT that)) (S (VP (VBZ 's) (VP (VBN released) (SBAR (WHADVP (WRB when)) (S (NP (NN water) (NN vapor)) (VP (VBZ condenses) (PP (IN into) (NP (NP (NN cloud) (NNS droplets)) (CC and) (NP (NN rain)))))))))))))))) (. .)))
This is because the key energy source is the latent heat that's released when water vapor condenses into cloud droplets and rain.
(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.