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The seasonal variations of radio refractivity gradients within approximately 1 km atmospheric layer above ground are presented for three existing radiosonde stations in Nigeria, viz: Oshodi, a coastal area in the south; Minna, a middlebelt savannah region and Kano, a semi-arid, sub-sahelian area to the north. The data utilized are those obtained from daily radiosonde ascents made at 1200 h local time for a period of 9 years from 1975 to 1983. In addition to the conventional approach of computing the monthly means and standard deviations, harmonic analysis has also been applied to study the seasonal and spatial distribution of the gradients. Results show that the variations in each zone are influenced by the characteristic topographical features, and the prevailing atmospheric conditions which are dependent on the seasonal north-south movement of the Inter Tropical Discontinuity (ITD). The dry Harmattan season has been found to favour super-refraction at the coast with mean gradients reaching -55 N units/km during this period, but dropping to slightly lower value of -51 N units/km in the rainy months. Mean gradients at Minna and Kano during the dry season are -37 N and -32 N units,,km while the wet months exhibit values of -52 N and -44 N units, km, respectively. At all three stations, the first three harmonic amplitudes are sufficient to account for over 90% of the variation. Seasonal averages of refractivity gradients and K factors have been established for each of the three tropical climatic zones, which should be useful for calculating more accurately than before, the propagation parameters for any of the three zones. (C) 2002 Published by Elsevier Science Ltd .