( ( ( The amount ) ( of ( air ( at ( a ( given place ) ) ) ) ) ) ( ( is ( measured ( ( ( ( by ( its pressure ) ) , ) usually ) ( in ( ( millibars , ) ( abbreviated mb ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN amount)) (PP (IN of) (NP (NP (NN air)) (PP (IN at) (NP (DT a) (VBN given) (NN place)))))) (VP (VBZ is) (VP (VBN measured) (PP (PP (IN by) (NP (PRP$ its) (NN pressure))) (, ,) (RB usually) (PP (IN in) (NP (NP (NNS millibars)) (, ,) (NP (JJ abbreviated) (NN mb))))))) (. .)))
The amount of air at a given place is measured by its pressure, usually in millibars, abbreviated mb.
(ROOT (S (NP (NNP Millibar)) (VP (VBZ is) (NP (NP (DT the) (NN unit)) (VP (VBN used) (S (VP (TO to) (VP (VB measure) (NP (NN air) (NN pressure)))))))) (. .)))