Visibility is an excellent indicator of air quality because its impairment results from light scattering and absorption by atmospheric particles and gases. In this study, the historical visibility database are explored during the last five decades to investigate annual, seasonal, and long term variations of the visibility in the city of Tehran. Correlation of the long-term visibility trend with any kind of precipitation, fog and high relative humidity are also examined.

Trends of visual range of best, median and worst visibilities at 10th, 50th and 90th percentiles of daily visibility data shows a downward trend for all the percentiles. There is a considerable decrease in visibility for the Tehran metropolitan area. This may be attributed to the lack of efficiencies in fuel consumption, vehicle adequacy, and transportation industry. Most decreasing trend of visibility is recorded during spring time followed by winter, fall and summer. Decreasing trends in visibility occur for all stations and aren't significantly dependent on the special weather events. The most sensitive season to meteorological filtering is fall and it is followed by spring and winter. Summertime visibility trend is not sensitive to meteorological screening.