عنوان مقاله [English]
Information on atmospheric visibility can be critical for fields including aviation industry. In order to predict visibility, forecasting fog is essential. However, literature review suggests that there is no silver bullet solution but rather several algorithms exists to forecast fog. Fog is an important hazardous weather event that affects aviation, transportation and marine traffic. Visibility forecast is very important for transportation, especially for air traffic where its accuracy is prominent. Fog formation is complex; its occurrence is widely variable in space and time, forming under a wide range of meteorological circumstances. In all cases it forms as a result of air near the surface becoming saturated and being cooled below its dew point. A weather observer is obliged to report fog (FG) when the obstruction to vision consists of water droplets or ice crystals and the visibility has been reduced to less than 1000 m. Mist (BR) is reported when the obstruction, due to ice crystals or water droplets, reduces the visibility to at least 1000 m but not more than 5000 m. More than one process may be at work during the formation of fog: advection of temperature or moisture can play a role in the formation of radiation fog, while radiative cooling can be a factor in the formation of other fog types. Table 2 classifies fog types into two main categories (I and II) depending on whether the wind speed at the time of fog onset was below 3 m/s or more than 3 m/s . This essentially distinguishes radiation fog from advection fog. If the wind speed at t was equal to or more than 3 m/s, cases were classified as either advection, cloud-base-lowering, or unknown events. To establish whether there was a sudden onset of fog, as is the case with advection events, clear skies or traces of low cloud with bases below 183 m (600 ft) had to be present in the hour before onset. CBL fog 1 was associated with a drop in cloud base over a period of 5 h prior to fog onset with an initial cloud base equal to or less than 914 m (3000 ft). If the wind speed at t was less than 3 m/s, CBL fog has the same definition as CBL fog 1, but also occurs under circumstances where the wind speed is less than 3 m/s. In the absence of cloud cover or with traces of very low cloud bases, 300 ft., three scenarios were investigated to quantify the occurrence of radiation (RAD) fog: RAD fog 1 accounts for radiation events that occur under cloudless conditions at CTIA with dropping air temperatures prior to onset. RAD fog 2 accounts for radiation events that are associated with cooling in the 4-h period prior to onset, and slight warming in the hour leading to onset. RAD fog 3 events represent circumstances where cloud cover was present below 914 m (3000 ft) at the time of fog onset , but cloud bases were increasing while the surface temperature was dropping (vii in Fig. 2). Although radiation fog events by definition only take place at night, the decision tree does not test whether the fog onset time was within the hour prior to sunset and before sunrise. Results, however, show that all events classified as radiation fog occurred at night. Once all the case was classified as a cloud-base-lowering, radiation. Considering the importance of poor visibility due to fog for aircraft landing operations at an airport, it has attempted to investigate the fog types in some important Iranian airports during 1995-2014 in this study. Mehrabad, Ahwaz, Mashad, Rasht, Isfahan, Kermanshah, Tabriz and Bandarabbas airports are considered for selected airports regarding number of foggy conditions of flight traffic. On the other hand, statistic of more relevant meteorological parameters in foggy condition are presented for selected airports in this study. Also, mean weather patterns including mean sea level pressure, wind speed and direction cloud amount are studied in selected duration over Iran. Using these patterns helps recognize the fog type according fog algorithm. Totally 2787 foggy reports are studied. Some of the most important results show that more than 80% of foggy conditions occurred in winter times (December, January and February). The most fog observations are reported in in 00, 03 and 21 UTC (before sunset and after sunrise). Mean wind (direction and speed) patters show that calm conditions in western parts of Iran. Westerly winds are dominant over central and eastern parts of Iran in 5 to 7 knots. A cyclonic circulation can be seen in north east section where Mashad airport is located. Advection fog can be described with the cyclonic circulation in Mashad airport but it needs more local investigations. Radiation fog (type 2) which is associate with cloud base less than 300 ft and are reported in 40% of Kermanshah, 50% of Rash and 35% Isfahan airport observations. Radiation fog (type 1) is the most popular fog type in Ahwaz, Bandar Abbas and Mashad airports respectively. The more important fog type is cloud base lowering fog (CBL) in Mehrabad and Tabriz airport than the other types but their numbers of reports are few. Advection fog isn’t a considerable fog type in theses airports except in Mashhad.