( ( ( ( Two ( factors double ) ) : ) ( ( The mass ) ( of ( ( air ( deflected downwards ) ) ( per ( unit ( of time ) ) ) ) ) ) ) . )
(ROOT (NP (NP (CD Two) (NNS factors) (JJ double)) (: :) (NP (NP (DT The) (NN mass)) (PP (IN of) (NP (NP (NN air) (NN deflected) (NNS downwards)) (PP (IN per) (NP (NP (NN unit)) (PP (IN of) (NP (NN time)))))))) (. .)))
Two factors double: The mass of air deflected downwards per unit of time.
(ROOT (S (NP (NP (DT The) (JJS best) (NN explanation)) (PP (IN for) (SBAR (WHADVP (WRB how)) (S (NP (NN air) (NNS masses)) (VP (VBP move) (PP (IN across) (NP (DT the) (VBN united) (NNS states)))))))) (VP (VBZ is) (SBAR (IN that) (S (NP (DT the) (VBG prevailing) (NNS westerlies)) (VP (VP (VBP move) (NP (NN air) (NNS masses)) (PP (IN from) (NP (NN west))) (PP (TO to) (ADJP (JJ east))) (PP (IN across) (NP (DT the) (VBN united) (NNS states)))) (CC but) (VP (MD may) (VP (VB be) (VP (VBN deflected) (PP (IN by) (NP (DT the) (NN jet) (NN stream)))))))))) (. .)))