( ( When ( ( these winds ) ( are strong ) ) ) ( , ( they ( ( deflect ( ( moister air ) ( ( ( moving up ) ( from ( ( ( the ( Gulf coast ) ) and ) ( cold ( air masses ) ) ) ) ) ( ( moving down ) ( from ( the Arctic ) ) ) ) ) ) ; ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (DT these) (NNS winds)) (VP (VBP are) (ADJP (JJ strong))))) (, ,) (NP (PRP they)) (VP (VBZ deflect) (NP (NP (NN moister) (NN air)) (VP (VBG moving) (PRT (RP up)) (PP (IN from) (NP (NP (DT the) (NNP Gulf) (NN coast)) (CC and) (NP (JJ cold) (NN air) (NNS masses)))) (S (VP (VBG moving) (PRT (RP down)) (PP (IN from) (NP (DT the) (NNP Arctic)))))))) (: ;)))
When these winds are strong, they deflect moister air moving up from the Gulf coast and cold air masses moving down from the Arctic;
(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)))))))))) (. .)))