This paper investigate the percentage of anomalous occurrence in tropic region of West Africa using meteorological data obtained from Era interim archive. The data covered the period of 36 years span from 1979 to 2014. Refractivity gradient, G are calculated from meteorological parameters (temperature, relative humidity and pressure) via surface refractivity and refractivity at 100m above the sea level. Relative incidence, β_o, which percentage of time refractivity gradient at 100 m is less than -100 N-units/km were estimated from cumulative distribution of refractivity gradient. The result shows the occurrence of seasonal variation of refractivity gradient across the observed stations with high variability in Jos. Super refraction and ducting condition are prevalent in Akure and Lagos stations, whereas sub refraction and ducting conditions are predominant in Jos during the rainy and dry season respectively. Minimum values of relative incidence of about 10% and 78% at Akure and Lagos respectively were obtained around 12 noon. Highest relative incidence of 80%, 98% and 97% were obtained at Jos, Lagos and Akure respectively in the mid night. Positive correlation of 0.29 and 0.66 exist between surface refractivity and β_o in Lagos and Akure respectively, moreover negative correlation of -0.84 is observed in Jos.
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