monitoring the occurrence of frost through an analysis of air masses in south west basins of iran

frost is one of the atmospheric phenomena which seriously threaten crop production. it also causes numerousaccidents in mountainous roads. in this research the spatial synoptic classification ssc method was employed toclassify the type of air masses. for the classification, such meteorological data as: temperature, dew point, mean sealevel pressure, cloudiness, direction and speed of wind were collected for a period of 45 years from 1961 to 2005. for aclassification of air masses a discernment method was applied and while typical characteristics of seed days being usedas input for this discernment function method. the results indicated that, spring season air masses, dry polar (dp) airmasses, moist polar (mp) and moist moderate (mm) air masses have contributed most to the occurrence of advectionfrosts respectively. dry polar air masses bore the lowest temperature and dew point, along with northerly and easterlywind components and a clear sky. these air masses caused the occurrence of the most severe and most extensiveadvection frosts in south west basins of iran.