Risk analysis of first and last frost occurrences in central Alborz region, Iran

Central Alborz is one of the important agricultural regions of Iran. Occurrence of the first frost in fall and the last frost in spring causes damage to the crops in this region every year. Information about the probable dates of frost occurrence helps farmers in preventing or reducing the damages caused by frost. Six stations, with 34 years of daily minimum temperature data, were selected at various elevations. Dates of frost occurrences in three degrees of severity (mild, moderate, and severe) and frost-free periods were obtained for each year. Appropriate distributions were determined for each frost series (dates of frosts and frost-free periods) on the basis of relevant statistical methods. It was found that the Pearson type III distribution was most appropriate. Probability distribution was constructed for each frost series. Frost occurrences up to a given date and maximum lengths of frost-free periods, with their probabilities, were determined. Correlations between the dates of frost occurrence and elevation, and between the maximum lengths of frost-free periods and elevation were identified in some probability levels. Our results show a positive correlation between the frequency of frost and elevation, and a significant negative correlation between the duration of frost-free period and elevation. Frost is more frequent in higher areas. The shortest and the longest frost-free periods occur on top of high mountains and on low altitude areas, respectively