( ( Air pressure ) ( ( is ( ( the force ) ( ( exerted ( by ( ( the weight ) ( of air ) ) ) ) above ) ) ) . ) )
(ROOT (S (NP (NNP Air) (NN pressure)) (VP (VBZ is) (NP (NP (DT the) (NN force)) (VP (VBN exerted) (PP (IN by) (NP (NP (DT the) (NN weight)) (PP (IN of) (NP (NN air))))) (PP (IN above))))) (. .)))
Air pressure is the force exerted by the weight of air above.
(ROOT (S (PP (IN At) (NP (NP (DT the) (NN earth) (POS 's)) (NN surface))) (, ,) (NP (NP (DT the) (NN air) (NN pressure)) (VP (VBN exerted) (PP (IN on) (NP (PRP you))))) (VP (VBZ is) (NP (NP (DT a) (NN result)) (PP (IN of) (NP (NP (DT the) (NN weight)) (PP (IN of) (NP (NP (NN air)) (PP (IN above) (NP (PRP you))))))))) (. .)))