Statistical and Synoptically Study of Dense Fog Event in Rasht Airport

To better understand of dense fog events, to improve the forecast accuracy, some statistical and synoptically analyses studied by 800m and less horizontal visibility in the Rasht Airport of Iran during years 2000-2009. For this reason, the fog climatology was developed and in this framework some parameters such as horizontal visibility, temperature, dew point, saturation deficit, relative moisture and winds speed on onset hour and in the duration of the event have been analyzed. Frequency percent of fog events was calculated for all month during 2000-2009. Frequency of occurrence increases in the colder months (highest in February).The occurrence frequency of long duration fog events between 1800 UTC to 2100 UTC was more than 2100 UTC to 0300 UTC; and the peak end of fog events was nearby sunrise. The horizontal visibility at the onset time was lower (100m) for long duration fog events. In the most events, saturation deficit and relative humidity at the fog onset time were calculated zero and 80% respectively. Low wind speeds (0-0.2 ms-1) have long been known to favor fog development. Synoptically study on fog events shows that the most important patterns in long duration fog events development are blocking pattern, zero zone and inversion in cold season.