This study presents a comprehensive long-term evaluation of vertical radio refractivity and refractivity gradients over selected locations in North-Central Nigeria. Forty-one years (1980– 2020) of meteorological data obtained from the ERA5 reanalysis dataset of the European Centre for Medium-Range Weather Forecasts (ECMWF) were analyzed. Air temperature, atmospheric pressure, and relative humidity at heights of 12 m, 100 m, and 250 m above ground level (AGL) were used to compute radio refractivity using ITU-R formulations. Vertical refractivity gradients were subsequently derived and interpreted within established propagation regime classifications. Results reveal a consistent decrease in refractivity with increasing altitude across all locations. Wet-season refractivity values exceed dry-season values due to enhanced moisture contribution to the wet refractivity component. Mean vertical refractivity gradients were −125.65 N/km in Minna, −87.36 N/km in Lokoja, and −77.54 N/km in Jos. These values exceed the standard atmospheric gradient (≈−39 N/km) in magnitude, indicating persistent super-refraction conditions, particularly in lowland and humid environments. The findings demonstrate that regional topography and atmospheric moisture distribution significantly influence radio wave bending characteristics. Incorporating location-specific vertical refractivity gradients into terrestrial propagation models is essential for improving reliability and interference prediction in subtropical continental climates.
Lami et al. (Sun,) studied this question.