عنوان مقاله [English]
The presence of fog reduces the horizontal visibility to less than 1000 meters and disrupts air transport services and can make it impossible for aircraft to land and take off. Climatology of fog can help better diagnosis and prediction of fog. In this study, METAR data from 2005 to 2020 were used to detect fog events at Rasht Airport and according to the classification algorithm of Tardif and Rasmussen (2007), the types of fog events were determined. Then the fog climate was studied during the period. The results showed that in terms of frequency, the most common type of fog at Rasht Airport was radiation fog with 58.84% and Cloud Base Lowering (CBL) fog with 26.74% of all fog occurrences. The rarest type of fog was advection fog with 3.49% of all occurrences at this Airport during 16 years of study. These results are consistent with Tajbakhsh (2015).
In terms of the duration of the fog occurrence, due to the long and heavy rains in this station, the radiation fog event was the longest type of fog events. Also, the duration of CBL fog event was shorter than other types of fog events. The second type of fog in terms of duration of occurrence was advection fog, which can be caused by the slow movement of the synoptic system to transfer moist air from the sea to the land (Tardif and Rasmussen, 2007). Advection and radiation fogs had minimum visibility of less than 100 meters for 50% of occurrences. Radiation fog had the lowest concentration so that only 50% of radiation fog events had visibility less than 400 meters.
Precipitation fog had the lowest concentration so that only 50% of precipitation fog events had visibility less than 400 meters. The highest incidence of fog during the study years was at 00 GMT. This stems from the fact, the radiative cooling is the strongest in the hours before sunrise. The strong reduction of the temperature increases the relative humidity near to 100%. This trend of changes in the number of hours with fog event during the day and night is consistent with the results of studies by Tajbakhsh (2015) at Rasht Airport, Cséplő, et al. (2019) in Hungary, and Tardif and Rasmussen (2007) in New York.
The monthly distribution of radiation fog showed that this type of fog event often occurs in autumn and winter to early spring (October to April). The minimum frequency of radiation fog was also observed in May to September. The maximum monthly frequency of advection fog was seen in early spring (March) and its minimum frequency was seen in April, June to September and November. Previous studies have shown that in early spring, with the increase of air temperature on land, the water temperature is lower than the air temperature on land and the conditions for the formation of advection fog are available (Roach, 1995; Cho, et al., 2000; Taylor, 1917; Klein and Hartmann, 1993). The monthly distribution of CBL fog showed that there was no occurrence of CBL fog in July to September. This type of fog starts in late autumn and continues until mid-spring. Boundary layer cooling is the most important process that causes fog in spring, while winter occurrences can be caused due to large-scale atmospheric systems (Tardif and Rasmussen, 2007). The monthly distribution of precipitation fog showed that the most occurrences are in winter season (January and February). Precipitation fog didn’t occurred in April to October. Since precipitation fog depends on large-scale factors (Tardif and Rasmussen, 2007), this type of fog is more common in autumn and winter.
In terms of annual changes in the occurrence of fog, there was no significant trend in the number of fog hours during the studied years. In terms of fog concentration, the number of semi-dense fog events (visibility equal or more than 100 meters and less than 500 meters) at Rasht Airport was higher than fog (visibility equal or more than 500 meters and less than 1000 meters) and dense fog (visibility less than 100 meters) in all months. Also, most fog events had a minimum visibility of 100 meters and then 200 meters.
Keywords: Fog climatology, Fog type, Radiation fog, Advection fog, CBL fog.