( ( FRONT ( -LRB- ( ( weather -RRB- ) ( Boundary ( of ( two ( overlapping ( sir masses ) ) ) ) ) ) ) ) . )
(ROOT (S (VP (VB FRONT) (S (-LRB- -LRB-) (NP (NN weather)) (-RRB- -RRB-) (NP (NP (NNP Boundary)) (PP (IN of) (NP (CD two) (VBG overlapping) (NN sir) (NNS masses)))))) (. .)))
FRONT (weather) Boundary of two overlapping sir 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.