Calculation of relative frequency of freezing and thaw cycles to identity risky regions for important garden and crop plants in Khorasan Razavi province

Freezing and thaw cycles that appear due to consistent decrease and increase of temperature in the short-term period are the important factors of damage to crop and garden plants in susceptible area. In this research to calculate the relative frequency of freezing and thaw cycles in the Khorasan Razavi province, the daily minimum and maximum temperatures of 9 synoptic stations were used over 20 statistic years (1989-2008). Also 6 distinct range of temperatures including: the minimum temperatures lower and equal to -2 and the maximum temperatures greater and equal to 2 (A), the minimum temperatures lower and equal to -3 and the maximum temperatures greater and equal to 3 (B), the minimum temperatures lower and equal to -5 and the maximum temperatures greater and equal to 5 (C), the minimum temperatures equal to -2 and the maximum temperatures greater than 2 (D), the minimum temperatures equal to -3 and the maximum temperatures greater than 3 (E) and the minimum temperatures equal to -5 and the maximum temperatures greater than 5 were presented. Following processing by a computer program in the FORTRAN space, the averages of monthly, seasonal and annual number of days with this phenomenon were calculated for each station and then the relative frequencies of monthly and seasonal were obtained. Results showed that, relative frequency of freezing and thaw cycles in winter is highest, but cold-season cereals and fruit trees show lower damage than spring and autumn seasons, because of cold acclimation phenomenon in this season. Monthly investigation showed that in October, the probability of damage to crop plants such as cotton or sugar beet in regions such as Sabzevar, Gonabad and Kashmar is less than regions such as Mashhad, Sarakhs and Neyshabur. In March and April, fruit trees may have faced with damage in Neyshabur, Torbat heydariye and Golmakan. Also in these months, early implant of cotton is risky in regions such as Sabzevar, Kashmar and Neyshabur.