Analysis of Distribution of Temporal-Spatial Trend of Horizontal Visibility on the Southern Coasts of the Caspian Sea using Ridit Analysis

The present study aims to investigate the temporal-spatial trend of horizontal visibility in the southern coasts of the Caspian Sea. For this purpose, horizontal visibility and climatic parameters (relative humidity, temperature, precipitation, and wind speed) were obtained in 13 selected meteorological stations from 1951-2010. After the data screening, the temporal changes of the perspective were examined. Also, Ridit tests were applied to examine the trend of horizontal visibility, and a correlation coefficient was used to identify the relationship between the climatic parameters effective in reducing horizontal visibility. The results of the research showed that the average annual dispersion of horizontal visibility in the studied stations experienced relatively different periods; the first period from 1951 to 1976 period of horizontal increase, the second period from 1977 to 1985 period of noticeable decline, the third period from 1986 to 2002 fixed period (no fundamental change) and the fourth period from 2003 to 2020 relative increase. Among the stations, the highest negative correlation coefficient was experienced by Astara station, with a value of -0.47, and the lowest was experienced by Sari station, with a value of -0.07. In the correlation between horizontal visibility and precipitation in the studied stations, the maximum significant negative correlation was observed in the Manjil station (with a value of -0.15) and the least in the Maratape station (with a value of -0.012). The correlation between horizontal visibility and temperature is direct and positive in most stations, and the maximum positive correlation is assigned to the Astara station with a value of 0.38. It also showed a negative correlation between wind speed and horizontal visibility. The inverse correlation's maximum and minimum range fluctuates between -0.28 in Sari and -0.08 in Babolsar.