( ( air pressure ) ( ( is ( ( the force ) ( ( exerted ( on ( a surface ) ) ) ( by ( ( the weight ) ( of ( ( the air ) ( above it ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NN air) (NN pressure)) (VP (VBZ is) (NP (NP (DT the) (NN force)) (VP (VBN exerted) (PP (IN on) (NP (DT a) (NN surface))) (PP (IN by) (NP (NP (DT the) (NN weight)) (PP (IN of) (NP (NP (DT the) (NN air)) (PP (IN above) (NP (PRP it)))))))))) (. .)))
air pressure is the force exerted on a surface by the weight of the air above it.
(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))))))))) (. .)))