عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Because of the passing of the air systems, temperature changes are often very sudden and plants cannot adapt themselves to these fluctuations and get damage.
Because low temperatures have harmful effects on the agricultural productions, so, the investigation of these temperatures are very useful for predicting and preventing probable damages. In this study, for determining the time of the freezes occurrence, daily minimum temperatures of Tabriz station were used for a 21-year period. The longest and extreme freezes, occurred during 7-12 Farvardin 1382 (March 27- April 1, 2003) and 17 Farvardin 1383(April 5, 2004). Synoptic maps in 1000, 850, 500 hpa were also used for detecting the synoptic patterns over these periods. The results showed that, the power and location of the pressure systems that prevail over the area during the freeze events were the main factors that control the freezes. Formation of low pressures and related troughs also have an important role in the spring freezes in area, and the main synoptic pattern that make the freezes is deepening the troughs of northern Caspian sea.