( Fronts ( are ( boundaries ( between ( contrasting ( air masses ) ) ) ) ) )
(ROOT (S (NP (NNS Fronts)) (VP (VBP are) (NP (NP (NNS boundaries)) (PP (IN between) (S (VP (VBG contrasting) (NP (NN air) (NNS masses)))))))))
Fronts are boundaries between contrasting 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.