( ( Maritime ( tropical ( air masses ) ) ) ( ( are ( much ( more common ) ) ) . ) )
(ROOT (S (NP (JJ Maritime) (JJ tropical) (NN air) (NNS masses)) (VP (VBP are) (ADJP (RB much) (RBR more) (JJ common))) (. .)))
Maritime tropical air masses are much more common.
(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)))) (. .)))