Atmospheric visibility is a standard of human visual perception of the environment. It is also directly associated with air quality, polluted species and climate. The influence of urban atmospheric visibility affects not only human health but also traffic safety and human life quality. Visibility is traditionally defined as the maximum distance at which a selected target can be recognized. To replace the traditional measurement for atmospheric visibility, digital image processing schemes provide good visibility data, established by numerical index. The performance of these techniques is defined by the correlation between the observed visual range and the obtained index. Since performance is affected by non-uniform illumination, this paper proposes a new procedure to estimate the visibility index with a sharpening method. The experimental results show that the proposed procedure obtains a better correlation coefficient than previous schemes.