( Thus ( ( air ( mass types ) ) ( ( ( are ( ( polar and ) tropical ) ) ( with ( sub-classes ( of ( ( maritime or ) continental ) ) ) ) ) . ) ) )
(ROOT (S (ADVP (RB Thus)) (NP (NN air) (NN mass) (NNS types)) (VP (VBP are) (ADJP (JJ polar) (CC and) (JJ tropical)) (PP (IN with) (NP (NP (NNS sub-classes)) (PP (IN of) (ADJP (JJ maritime) (CC or) (JJ continental)))))) (. .)))
Thus air mass types are polar and tropical with sub-classes of maritime or continental.
(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)))) (. .)))