( ( A front ) ( ( is ( ( a boundary ) ( between ( ( air masses ) ( of different ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN front)) (VP (VBZ is) (NP (NP (DT a) (NN boundary)) (PP (IN between) (NP (NP (NN air) (NNS masses)) (PP (IN of) (ADJP (JJ different))))))) (. .)))
A front is a boundary between air masses of different .
(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.