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Radio signal transmissions of frequencies above 30 MHz in the troposphere are prone to the fluctuations of weather and climate. This is because water vapor molecules with their permanent electric dipole moments account for the atmosphere having a complex dielectric constant and hence a complex refractive index (Hall, 1979).
Variation in the radio refractive index which in turn is caused by variation in pressure, temperature and water vapor pressure results in the refraction and scattering of electromagnetic waves propagating through the troposphere (Hall and Barclay, 1991). Therefore, in the propagation of radio waves through the atmosphere, the radio refractivity of air is an important parameter to be considered because its space-time distribution results in scattering, sub-refraction, super-refraction, ducting, and absorption phenomena (Batueva et al., 1998).
In the lowest layer of the atmosphere, the statistics of the vertical gradient of radio refractivity is an important parameter for the estimation of path clearance and propagation associated effects, such as ducting on transhorizon paths, surface reflection and multipath fading and distortion on terrestrial line-of-sight links (ITU-R, 2003). The radio engineer involved in the design of radio communication systems operating in the above frequency bands
(30 MHz and above), normally subjects long term data relating to atmospheric refractive index and its properties to statistical analysis in order to be able to predict these parameters (Aro and Willoughby, 1992).
Among the earliest radio scientists that worked in this field were Bean and Thayer (1959). They observed that a correlation existed between monthly means of surface refractivity, NS, and monthly means of refractivity decrease in the first kilometer, DN, above the ground. Also, Adebanjo (1977) determined an empirical relation of the form DN = –25 exp (0.0022 NS) to indicate the average change in refractivity between surface and the first kilometer over Nigeria. This relationship was based on the data for the same stations in Nigeria for which Owolabi and Williams (1970) computed NS values. The DN values were obtained from a world atlas of atmospheric radio refractivity published by Bean et al. (1966). Although the contributions of these scientists to radio science especially in the African region are invaluable, they could not explore the diurnal trend of refractivity because of a dearth of atmospheric data. Therefore, recent efforts (Adedeji and Ajewole, 2008; and Oyedum et al., 2009 and 2010) including this work explore the diurnal and seasonal trends of surface refractivity, temperature, pressure and humidity.
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