( Fronts ( ( are ( ( the boundaries ) ( between ( ( air masses ) ( of ( different temperature ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Fronts)) (VP (VBP are) (NP (NP (DT the) (NNS boundaries)) (PP (IN between) (NP (NP (NN air) (NNS masses)) (PP (IN of) (NP (JJ different) (NN temperature))))))) (. .)))
Fronts are the boundaries between air masses of different temperature.
(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.