Determination of potential frost free season in some regions of North West

Dates of start and end of frost based on minimum shelter temperature equal or less than 0°C were determined for 20 synoptic stations at some areas in Tehran, Zanjan, Ghazvin, Ghom, Azarbaijan gharbi and Azarbaijan sharghi provinces, on the period of 1986-2000 years record. The cause of frosts was determined by study of weather parameter values at observation hours and in some cases by the aid of synoptic maps. Distribution percent of last spring and first fall frosts in both types were calculated. The "potential growing season" defined as the interval between last spring and first fall advection frosts, is found to be from 65 days in Sarab to 5 days in Tabriz longer than the "growing season" defined by the interval from last spring to first fall occurrences of minimum shelter temperature of 0°C or below. By regarding to the results of this study along with local situation and type of crops, it is possible to estimate the usefulness frost protection operations methods to meet the potential frost free season in the study regions.