( ( ( ( ( ( ( Continental tropical ) and ) ( maritime tropical ) ) , ) and ) ( continental ( ( ( polar and ) maritime ) polar ) ) ) ( ( are ( ( the ( four types ) ) ( of ( air masses ) ) ) ) . ) )
(ROOT (S (NP (NP (NP (NNP Continental) (NNP tropical)) (CC and) (NP (JJ maritime) (NNS tropical))) (, ,) (CC and) (NP (JJ continental) (ADJP (JJ polar) (CC and) (JJ maritime)) (NNS polar))) (VP (VBP are) (NP (NP (DT the) (CD four) (NNS types)) (PP (IN of) (NP (NN air) (NNS masses))))) (. .)))
Continental tropical and maritime tropical, and continental polar and maritime polar are the four types of air masses.
(ROOT (S (SBAR (IN If) (S (NP (DT an) (NN air) (NN mass)) (VP (VBZ forms) (PP (IN over) (NP (DT an) (NN ocean)))))) (NP (PRP it)) (VP (VBZ is) (VP (VBN called) (NP (DT a) (JJ maritime) (NN air) (NN mass)))) (. .)))