( Fronts ( ( ( A weather ) front ) ( ( is ( ( a boundary ) ( between ( two ( distinct ( air masses ) ) ) ) ) ) . ) ) )
(ROOT (S (NP (NNS Fronts)) (ADVP (NP (DT A) (NN weather)) (RB front)) (VP (VBZ is) (NP (NP (DT a) (NN boundary)) (PP (IN between) (NP (CD two) (JJ distinct) (NN air) (NNS masses))))) (. .)))
Fronts A weather front is a boundary between two distinct 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.