The radiowaves propagation is critically affected by the behavior of the refractive index of air and for reason to install telecommunication systems the knowledge of this phenomenon is necessary. This work is the study of the refractive index from daily data of temperature, pressure and relative humidity from the surface until 5800m. Of altitude which were obtained by radionsonde in 8 cities of Brazil. After having taken the average of these data every month the refractivity of air variation with altitude was obtained. The gradient was caculated from the daily data and the probability distributions of effective earth radius and refractivity gradient throughout the year were taken. Then, a comparison of the annual behavior of refractivity near the ground , refractivity gradient and the gradient probability distribution was made among stations of each climatic regions. Finally, a correlations between the annual variation of surface refractivity and its gradient was analysed.