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