عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Since the occurrence of frost has been caused much damage to agriculture and horticulture production in the city of Karaj. Therefore the city authorities' attention to recognition of frost circumstances accurance, can prevent conditions in which severe damages would force to farmers. So recognizing conditions in frosty must be considered by authorities of this city to prevent intense damages.it has been used 25 year meteorology synoptic least temperature of Karaj to determine the date of the first frosty in autumn and the last frosty in spring In this research, the zero threshold has generally been used at first date of the first autumn frosty with two or three days duration and the last frosty of the spring in each farming year were determined, Then were studied these values aspect of occurrence probabilities and different statistical distribution for beginning frosty in autumn and the last frosts in spring. According to performed studies were known over 6 probability function, log Pearson III distribution on data of autumn and two parametric Gama distribution on data of spring with the least deviation square average the best estimation. According to results of this research the probability of the first frost of autumn before 7 Dec and before 20 Nov is 20 percentage also the probability of the last frosty of spring after 11 April and before 17 March is 20 percentage. According to obtained results in this research the best period for wheat cultivation based on thermal needs of autumn wheat to start germination(370 degree – days) in Karaj city is 20 to 27 Oct. Based on the obtained information in this research the spring frosty (at time of germination and flowering ) on performance of wheat has important influence but in autumn frosty despite delay in growing plant it has not important influence on plant performance and the damage of plant relates to time of cultivation and frost intensity and the coincidence of germination and cold.