( ( On occasions ) ( , ( ( arctic ( air-mass properties ) ) ( ( are important ) ; ) ) ) )
(ROOT (S (PP (IN On) (NP (NNS occasions))) (, ,) (NP (JJ arctic) (JJ air-mass) (NNS properties)) (VP (VBP are) (ADJP (JJ important))) (: ;)))
On occasions, arctic air-mass properties are important;
(ROOT (S (NP (NP (CD One)) (PP (IN of) (NP (NP (DT the) (NNS properties)) (PP (IN of) (NP (DT an) (NN air) (NN mass)))))) (VP (VBZ is) (SBAR (IN that) (S (NP (DT an) (NN air) (NN mass)) (VP (VBZ has) (NP (NP (JJ similar) (NN temperature) (CC and) (NN moisture) (NNS properties)) (PP (IN throughout) (NP (PRP it)))))))) (. .)))
One of the properties of an air mass is that an air mass has similar temperature and moisture properties throughout it.