( ( Weather fronts ) ( ( ( ( mark ( ( the ( boundary ( or ( transition zone ) ) ) ) ( between ( two ( air masses ) ) ) ) ) and ) ( have ( ( an ( important impact ) ) ( upon ( the weather ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Weather) (NNS fronts)) (VP (VP (VBP mark) (NP (NP (DT the) (NN boundary) (CC or) (NN transition) (NN zone)) (PP (IN between) (NP (CD two) (NN air) (NNS masses))))) (CC and) (VP (VBP have) (NP (NP (DT an) (JJ important) (NN impact)) (PP (IN upon) (NP (DT the) (NN weather)))))) (. .)))
Weather fronts mark the boundary or transition zone between two air masses and have an important impact upon the weather.
(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.