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Abstract: Visibility, Air Pollution Index (API) and meteorological parameters over Nanjing during 2004 are analyzed using multiple statistic methods to study the characterization of visibility and relevant affecting factors. The mean value of visibility during the study year is 8.8km, with 57.9% of daily average less than 10km. Annual average visibility exhibits a significant diurnal variation with minimum of 6.6km at 07:00 Local Time (LT) and maximum of 10.7km at 16:00 LT. Seasonal variations is weak: 9.2km in spring (March–May), 9.8km in summer (June–August), 8.2km in autumn (September–November), and 8.1km in winter (December–February), respectively. No significant weekend effect in visibility is found. Moreover, wavelet analysis reveals the significant periodic variations in visibility, with major periods of synoptic scale (2–8days), and quasi-two-week scale (8–17days). Using a high pass filtering technique to eliminate the seasonal trend, correlation analysis and principal component analysis (PCA) are carried out to investigate the relations between visibility and relevant factors and to understand the dominated processes/causes to the synoptic changes in visibility. Our results highlight the significant impact of synoptic weather on air pollution and visibility in Nanjing. [Copyright &y& Elsevier]