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The monthly and seasonal variation of surface refractivity and water vapour density were studied using thirty-nine years meteorological data for forty-eight stations in Nigeria. The factors which influence the transmission of radio signals operating within the troposphere are water vapour and refractivity. The results show that the surface refractivity and water vapour density generally have higher values during the rainy season than dry season at all station studied. Furthermore the results show that the value of surface refractivity and water vapour density varies from about 263 N-units and 3 g/m(3) in arid region of Nigeria (North East) to about 393 N-units and 23 g/m(3) in the coastal area of Nigeria (South West) respectively. For optimal performance of terrestrial radio link across Nigeria it is required to account for the variability of these parameters for optimal systems design.