( ( ( An ( air mass ) ) ( by definition ) ) ( ( is ( ( ( a ( large dome ) ) ( of air ) ) ( which ( has ( similar ( horizontal ( temperature ( and ( moisture characteristics ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT An) (NN air) (NN mass)) (PP (IN by) (NP (NN definition)))) (VP (VBZ is) (NP (NP (DT a) (JJ large) (NN dome)) (PP (IN of) (NP (NN air))) (SBAR (WHNP (WDT which)) (S (VP (VBZ has) (NP (JJ similar) (JJ horizontal) (NN temperature) (CC and) (NN moisture) (NNS characteristics))))))) (. .)))
An air mass by definition is a large dome of air which has similar horizontal temperature and moisture characteristics.
(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.