Abstract: In relation to the irregular vertical distribution of atmospheric refractive index, the mean effective-earth-radius-factor, k, was determined for Akure (7.15 N, 5.12 E) from experimental data and also, some variation characteristics of air refractive index with space and time were discussed. The air temperature, the water-vapour pressure and the barometric pressure were measured at various heights above the earth`s surface from which the distribution of the refractive index were obtained. Also, the refractive index was statistically considered and it was found that the average refractivity gradient of the atmospheric layer of thickness H (m) approximately follows the normal distribution. Hence, by introducing H, the height range of the propagation path and defining the mean gradient of M-profile in this range, the conception of the effective earth radius factor, k, was brought out.