( Fronts ( ( Boundaries ( separating ( air masses ) ) ) . ) )
(ROOT (S (NP (NNS Fronts)) (VP (VBZ Boundaries) (S (VP (VBG separating) (NP (NN air) (NNS masses))))) (. .)))
Fronts Boundaries separating air masses.
(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.