( ( Air masses ) ( ( are ( ( considered maritime ) ( if ( they ( ( originate ( over ( ( ( the ocean ) and ) continental ) ) ) ( if ( they ( develop ( over land ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Air) (NNS masses)) (VP (VBP are) (VP (VBN considered) (ADJP (JJ maritime)) (SBAR (IN if) (S (NP (PRP they)) (VP (VB originate) (PP (IN over) (NP (NP (DT the) (NN ocean)) (CC and) (NP (JJ continental)))) (SBAR (IN if) (S (NP (PRP they)) (VP (VBP develop) (PP (IN over) (NP (NN land))))))))))) (. .)))
Air masses are considered maritime if they originate over the ocean and continental if they develop over land.
(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)))) (. .)))