Variations of Surface Radio Refractivity and Radio Refractive Index Gradients in the sub-Sahel

Abstract

Monthly variations of radio refractive index near the ground surface have been computed for a period of five years from 2000 to 2004 over Ilorin (8o 32”N, 4o 34”E). The data used was obtained from the University of Ilorin atmospheric observatory operating under the radiometric network of the Baseline Surface Radiation Network (BSRN). Also, seasonal variations of radio refractivity with height of up to 10 km atmospheric layer above ground over Minna (9o 37’’N, 6o 30’’E) have been evaluated. The data used here are those obtained from daily radiosonde ascents made at 1200hrs local time for a period of five years from 1979 to 1983. The refractivity-altitude variation over Minna have been plotted with those of four other low latitude stations and an equation of the form N = NS exp (-0.11h) has also been deduced to predict values of refractivity, N for these stations up to a height, h of about 2 km above ground, for a given surface refractivity, NS. The values of refractive index gradients computed showed that the atmosphere over Minna was subrefractive during the dry season and superrefractive during the wet season periods of the years.