( ( In weather ) ( , ( ( all fronts ) ( ( are ( ( ( ( boundaries ( between ( masses ( of air ) ) ) ) ( with ( different densities ) ) ) , ) ( usually ( caused ( by ( temperature differences ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN In) (NP (NN weather))) (, ,) (NP (DT all) (NNS fronts)) (VP (VBP are) (NP (NP (NNS boundaries)) (PP (IN between) (NP (NP (NNS masses)) (PP (IN of) (NP (NN air))))) (PP (IN with) (NP (JJ different) (NNS densities))) (, ,) (VP (ADVP (RB usually)) (VBN caused) (PP (IN by) (NP (NN temperature) (NNS differences)))))) (. .)))
In weather, all fronts are boundaries    between masses of air with different densities, usually caused by temperature    differences.
(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.