( ( A front ) ( ( is ( ( the boundary ) ( between ( ( two masses ) ( of ( ( moving air ) ( with ( different ( ( temperatures and ) humidity ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN front)) (VP (VBZ is) (NP (NP (DT the) (NN boundary)) (PP (IN between) (NP (NP (CD two) (NNS masses)) (PP (IN of) (S (VP (VBG moving) (NP (NN air)) (PP (IN with) (NP (JJ different) (NNS temperatures) (CC and) (NN humidity)))))))))) (. .)))
A front is the boundary between two masses of moving air with different temperatures and humidity.
(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.