عنوان مقاله [English]
Atmospheric visibility is a key factor in everyday life mainly in aviation, industry and surface traffic. It has been defined as the greatest distance at which an observer can just see a black object viewed against the horizon sky which is known as the visual range. Furthermore, light extinction which is mostly due to absorption and scattering effects of aerosols in the atmosphere, can be calculated from visual range using Koschmieder formula. Visibility and extinction have similar units. In a non-polluted atmosphere, visibility ranges from 145 to 225 km, and in normal atmospheric condition it ranges from 10 to 100 km; yet in polluted areas it can be remarkably low down. Visibility is regularly measured at synoptic meteorological stations all over the world as a standard meteorological parameter. Skilled observers have been measuring the visual range using individual markers at known distance from the meteorological location against the horizon sky.
In this paper, airport visibility data for the period of 1970 to 2010 are examined in the four busiest airports in Iran including Tehran-Mehrabad, Mashhad, Shiraz and Isfahan. All data from the four airport stations have been used for analyzing the temporal variations. The analyses are based on daily average measurements, i.e. the average of 9, 12 and 15 UTC data. Midday values are usually used in studies of this kind as they are more representative of regional visibility levels, because early morning radiation fogs and high relative humidity which may reflect only the local environment would regularly have dispersed by midday. The historical trends of extinction coefficient based on visual range for the four aforementioned airports are computed. Trend is determined by a least square regression analysis of midday average extinction. In general, an upward atmospheric extinction trend is seen for all stations. Tehran-Mehrabad airport has the most increasing trend. The extinction was around 0.3 km-1 in early 70’s but it increases in the present and reaches up to around 0.4 km-1.
Airport visibility data that inherently undervalue the true visibility are most appropriately summarized by cumulative percentiles. The Nth cumulative percentile is the visibility that is equal or exceeds N percent of the time. Visibility data lends itself well to the treatment in this manner. Daily visibility observations are investigated during the last four decades at 10th and 90th cumulative percentiles to show the threshold visibility in each airport. The 10th and 90th cumulative percentiles of visibility are used to identify the frequency of ‘good’ and ‘poor’ visibilities, respectively. Results show that there is not a distinctive difference between the 40-year poor or good visibilities among all stations. However, Tehran airport has the least quantity in visual range than the other airports. The good visibility has the largest threshold value of around 20 km in Shiraz and Mashhad airports.
Monthly comparison of extinction for 40 years, shows that there is an about 2-km difference between winter months and the rest of the year. It can be due to the effect of weather or concentration of pollutants in different months. The results of correlation analysis indicate that the difference may be due to the variation in relative humidity value in different months. To minimize the effect of humidity, the days with relative humidity value of above 70 percent and cloudiness of above 4/8 of sky are removed from the visibility trend analysis. Detailed analyses show that the trends of the screened days are nearly parallel to the trends of raw data, but with a slight difference in each airport. Increase in extinction is also observed since 1970 when absolute values of extinction change. The extinction trend is not significantly changed in Tehran and Isfahan airport which may emphasis on the role of air pollution on atmospheric extinction.