( ( Air pressure ) ( ( is ( ( the force ) ( ( exerted ( on you ) ) ( by ( ( the weight ) ( of ( ( tiny particles ) ( of air ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Air) (NN pressure)) (VP (VBZ is) (NP (NP (DT the) (NN force)) (VP (VBN exerted) (PP (IN on) (NP (PRP you))) (PP (IN by) (NP (NP (DT the) (NN weight)) (PP (IN of) (NP (NP (JJ tiny) (NNS particles)) (PP (IN of) (NP (NN air)))))))))) (. .)))
Air pressure is the force exerted on you by the weight of tiny particles of air.
(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))))))))) (. .)))