Visibility trends in Tehran during 1958–2008

Visibility, in the absence of certain weather conditions (e.g., fog and rain), 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 airport visibility database is explored during the last five decades for the city of Tehran, Iran. Seasonal and long-term variations of the visibility are investigated using cumulative percentiles. The correlation of the long-term visibility trend with precipitation, fog and high relative humidity are also examined.The trends of visual range of the best, median, and worst visibilities at 10th, 50th and 90th percentiles of daily visibility data indicate an overall downward trend for all the percentiles. Results show a larger decrease in visibility for the Tehran metropolitan area than the sparsely populated and less polluted parts of the city. The largest trend in decreasing visibility is observed during winter time followed by spring, fall and summer. Decreasing trends in visibility occur for all stations and do not significantly depend on the special weather events.

► Historical visibility database are explored to investigate visibility trend.
► We examined correlation of visibility trend with precipitation, fog and humidity.
► More decrease for Tehran metropolitan area than sparsely populated parts.
► The most decreasing trend of visibility is observed during winter time.