Climatology study of fog at Mashhad airport

Climatology study of fog at Mashhad airportAbstract Low horizontal visibility caused by fog can affect air traffic and in some cases is the main cause of air accidents. Fog is a condition in which water droplets or ice crystals in the air layer near the Earth's surface reduce horizontal visibility to less than 1000 meters. Fog is one of the major causes of flight delays and accidents. In fact, fog is the second hazardous weather event affecting aviation activities (Gultepe et al., 2019). The effects of fog in the aviation industry can cost hundreds of millions of dollars due to flight delays and cancelations (Gultepe et al., 2017). Fog event at Mashhad airport has repeatedly delayed or canceled flights. Therefore, study the climatology of the fog event at this airport helps to better understand this phenomenon and improve fog forecasting. For this purpose, all types of fog events were detected and analyzed using the hourly observation data of METAR during the statistical period from 2001 to 2020 and based on Tardif and Rasmussen (2007) algorithm. Then the fog climate was studied during the period. The results showed that CBL fog is the most common type of fog in terms of frequency with 43.94% of all fog occurrences at Mashhad Airport. Precipitation fog is also the rarest type of fog among fog types with 22.72% of all fog occurrences. Advection fog, which is formed under the influence of the marine environment, was not observed at this airport. Given that radiation fog usually forms at night and usually dissipates after sunrise, this type of fog has been the longest-lasting type of fog at the airport during 16 years of study. Also, the duration of CBL fog was the shortest one compared to other types of fog events.The minimum visibility of radiation and CBL fog events at this airport was lower than other types of fog. Also, precipitation fog had the lowest concentration compared to other types of fog events, which is consistent with the results of Tardif and Rasmussen (2007). The most reports of fog occurrence at Mashhad Airport during the studied years was at midnight and before sunrise, which could be due to the radiation cooling at night and before sunrise, which reaches its maximum (Hoch et al., 2011; Cséplő et al., 2019; Zouzoua et al., 2021; Wærsted et al., 2017). This result is consistent with the study of Cséplő et al. (2019) and Tardif and Rasmussen (2007). Tajbakhsh (2015) has also reported the maximum occurrence of fog at 21, 00 and 03 UTC using 20-years synoptic data at Mashhad Airport. Also, the frequency of fog occurrence decreases rapidly after 03 UTC.The monthly distribution of fog types in Mashhad Airport showed that radiation fog events often occur in winter and CBL fog events often occur from late autumn to mid-spring. Boundary layer cooling is the most important process that causes fog event in spring, while winter fog occurrences can be related to large-scale atmospheric systems (Tardif and Rasmussen, 2007). The maximum monthly frequency of precipitation fog event at this airport is in the winter. The fog events in the autumn can also be related to these weather systems. Since precipitation fog events depend on large-scale factors (Tardif and Rasmussen, 2007), the maximum occurrence of this type of fog is observed in winter. In general, the frequency of fogs in December and January is higher than other months of the year. This result is consistent with Tajbakhsh (2015).In terms of annual changes in fog occurrences, there was no significant trend in the number of fog hours during the studied years (2001-2020). In terms of fog concentration, in all months of the year, the number of semi-dense fog events with minimum visibility of 100 to 500 meters was more than other types. Also, the number of fog events with minimum visibility of 100 meters and then 200 meters had the highest number. It shows the importance of climate investigation and prediction of the fog event at this airport from the point of view of the aviation industry, because dense fog events cause difficulties in the landing and takeoff of airplanes. Keywords: Fog climatology, Fog type, Radiation fog, Precipitation fog, CBL fog.