Long-term Fluctuations in Horizontal Visibilities over Northwest of Iran

Document Type : Research Paper

Authors

• Atefeh Hoseini Sadr ¹
• bromand salahi ²
• Gholam Hasan Mohammadi ³

¹ Geography Department, Payame-noor University, Tehran, Iran

² Professor of climatology, Faculty of Social Science, University of Mohaghegh Ardabili, Ardabil, Iran

³ Iranian Meteorological Organization (IRIMO), East Azerbaijan Province Central Bureau of Meteorology, Tabriz, Iran

Abstract

Highlights

The annual mean horizontal visibility in all stations has relatively similar fluctuations and its regional-mean variations can be classified into three different periods: The first period (1951-1985) was characterized by a significant decrease in the average horizontal visibility, with the regional mean decreasing from > 30 to 11 km. This period was therefore named the “severe decreasing period”. From 1986 to 2005, the average horizontal visibility remained at its lowest level without any significant trend. The regional mean of horizontal visibility variation was between 9.4 and 11.8 km during this period. Therefore, this period was named the “low and stable period”. From 2006, the regional mean of horizontal visibility has gradually increased with a gentle slope, reaching around 16 km. Therefore, this period was introduced as the “recent relative improvement”. Despite the relatively increasing trend during this period, the average horizontal visibility has still been significantly lower than in the 1950s and 1960s. Therefore, the trend of horizontal visibility throughout the study period has been decreasing, as confirmed by the Kendall test. The average Ridit value in northwest Iran was around ~0.85 in the early 1950s. However, it decreased significantly over the following 10 years and dropped to below 0.40 in 1976. The regional mean of Ridit values for all years before 1980 was higher than 0.5, indicating that the horizontal visibility during those years was higher than the reference distribution. Between 1985 and 2010, the regional mean of Ridit values (between 0.31 and 0.38) reached its lowest level. In the last 10 years of the study period (i.e., between 2011 and 2020), the regional mean of Ridit values gradually increased and reached higher than 0.5 since 2017, indicating a relative improvement in horizontal visibility in recent years. However, compared to the years between 1951 and 1980, it is still at a lower level. Therefore, the linear trend of the regional mean Ridit value throughout the study period is decreasing. The percentage of bad horizontal visibility in all stations from 1951 to 1975 was very low and mostly less than 10% of the total observations. From 1975 to 2010, the frequency of poor visibilities gradually increased and reach 10% - 20%. Since 2010, the percentage of bad horizontal visibility has relatively decreased and reached around 10%, indicating a relative improvement in horizontal visibility in recent years. The changes in horizontal visibility in northwest Iran are well reflected through the changes in the frequency of very good visibility. Until before 1980, except for the Tabriz station, very good visibility accounted for a high percentage (20% to 80%) of total horizontal visibility observations. However, the frequency of very good horizontal visibility decreased rapidly from the beginning of the study period. This study confirms the increasing trend of bad horizontal visibility and the decreasing trend of very good horizontal visibility, indicating a decreasing trend of horizontal visibility in northwest Iran. Among the weather phenomena affecting horizontal visibility degradation, smog (38/7%) had the most significant impact, followed by mist and dust storms with percentages of 30% and 17.5%, respectively.

Conclusion

In this study, three different fluctuation periods were identified in the changes in horizontal visibility. In the first period, horizontal visibility was at its highest level, and humidity, especially mist, was the main cause of the visibility reduction. However, in the second period, a considerable increase in smog led to a reduction in visibility. In the third period, due to the relative stability in the frequency of smog and an increase in the frequency of dust storms, there was a relative improvement in horizontal visibility. But, it is not completely improved. It can be concluded that the factors contributing to the decrease in horizontal visibility in northwest Iran have shifted from moisture phenomena to dust-related phenomena (aerosols and smoke) in the past 70 years. Variation of Ridit values indicates that it was higher than the reference distribution until 1986, and it has decreased to less than that since that year. Among the stations, those located in the vicinity of well-developed cities (Tabriz and Urmia) have declined below the reference distribution sooner than other stations. Despite the relative increase in Ridit values in recent years (since 2005), in most stations, the Ridit value has been below 0.5. Due to the location of most stations in the vicinity of well-developed cities and the impact of human activities on air pollution and subsequent reduction in visibility, the results of this study cannot be extended to all of northwestern Iran. However, to have a more accurate examination, data from stations located in natural environments also need to be investigated. As a result, this approach can provide a more realistic assessment of the trend of horizontal visibility in the study area. It has also been shown that multiple factors affect the fluctuations in horizontal visibility, including changes in meteorological factors and anthropogenic pollutants, especially in densely populated cities. Therefore, determining the extent of the impact of each of these factors and identifying the percentage of their impact in past decades can lead to a better understanding of what has happened in the past and help in using them for environmental management in the future

Keywords

• : Horizontal Visibility
• Fluctuation Periods
• Trend Analysis
• Ridit test
• Northwest of Iran

10.22034/gp.2023.57121.3156

Main Subjects

• Climatology