Investigation of urban atmospheric visibility by high-frequency extraction: Model development and field test

This work investigates atmospheric visibility by employing two statistical processes for high-frequency extraction: Sobel operator and fast Fourier transform (FFT). In replacing the traditional measurement methods for atmospheric visibility, the new approaches can provide accurate digital data urban visibility by establishing the numerical indices. The procedure is illustrated as follows. Digital images of urban areas in Kaohsiung, a city at the south of Taiwan, are analyzed according to brightness. High-frequency components of the image are extracted to calculate the index values by employing the Sobel operator and FFT. Finally, the correlation between each index and the visual range estimated by trained investigators are evaluated. A good correlation between two indices and the values obtained by visual investigation is affirmed by correlation coefficients (R2), 0.8139 and 0.7797, respectively. Furthermore, both indices are highly correlated with each other (R2=0.9173). Convenient transmission and the exchangeability of digital images of the real-time landscape facilitate the publication of results on the world wide web (WWW).