In principle, the EM wave propagation characteristics during quiet or disturbed air conditions are straightforwardly determined by the state of atmospheric refractivity. Nevertheless, atmospheric refractivity varies in time and space more or less randomly. Therefore the statistics of atmospheric refractivity and related propagation effects are of main interest. This work clearly investigates the major difference between radio refractivity changes for quiet and disturbed days during dry and rainy season for Minna. Radio refractivity values were calculated from measured meteorological parameters (relative humidity, temperature and pressure) at Minna, in 2008. The result showed that, there is a sharp decrease in radio refractivity at the early hour of the day (6:00hr) during dry season, while during rainy season, the sharp decrease occurred at the noon time (13:00hr). The result equally showed that radio refractivity has higher values ~388N-units during rainy season than during dry season with the highest value of ~318N-units. This is due to decrease in relative humidity during dry season than during rainy season.