( ( ( The boundary ) ( between ( two ( air masses ) ) ) ) ( ( is ( called ( a ( weather front ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN boundary)) (PP (IN between) (NP (CD two) (NN air) (NNS masses)))) (VP (VBZ is) (VP (VBN called) (NP (DT a) (NN weather) (NN front)))) (. .)))
The boundary between two air masses is called a weather front.
(ROOT (S (PP (IN In) (NP (NN weather) (NNS terms))) (, ,) (NP (NP (DT the) (NN boundary)) (PP (IN between) (NP (CD two) (NN air) (NNS masses)))) (VP (VBZ is) (VP (VBN called) (NP (NN front)))) (. .)))
In weather terms, the boundary between two air masses is called front.