In this study, the trends of frost indices were investigated using the Mann-Kendall nonparametric test at 90% probability level. Frost indices, namely number of frost days (nFDs), number of frost-free days (nFFD), last spring frost (LSF), first fall frost (FFF), and growth season length (GSL), were obtained using minimum temperatures for 12 synoptic stations in the Caspian Sea coast, northern Iran. The statistical period of data ranged from 2000 to the end of 2018. Results showed an upward trend line for the nFDs index in western parts of the study area (Guilan province and part of Mazandaran province, with a significant downtrend at Rasht station as well). The nFDs index had a downward trend line slope in the eastern parts (Golestan province and part of Mazandaran province). Results of nFDs index at Gorgan station revealed a significant upward trend for the nFDs index. The nFFD index results were contrary to those of the nFDs index. The LSF index also presented a downtrend in the western part (Guilan province). Positive trend slopes were observed for the LSF index in the eastern regions (Golestan province and most of Mazandaran province). This index showed a significant uptrend at Astara station. Thus, the results of the LSF index indicate that LSF will occur earlier in the western regions and later in the eastern regions than previous periods. The FFF index also had a downtrend in the western part (the whole Guilan province and most of Mazandaran province), and the eastern part (Golestan province and a small part of Mazandaran province) experienced an upward slope. It should be noted that the FFF index had no significant trends in the study area. The uptrend and downtrend of this index indicate later and earlier occurrence, respectively, of FFF than normal situations. Finally, the GSL index, which represents the length of plant growth period, had a downtrend in the middle areas (Mazandaran province). The western (Guilan province) and the eastern (Mazandaran province) regions also had upward slopes. Also, no significant trends were observed for this index at all the stations.

在这项研究中，使用Mann-Kendall非参数检验以90％的概率水平研究了霜冻指数的趋势。霜冻指数，即霜冻天数（nFDs），无霜冻天数（nFFD），去年春季霜冻（LSF），初秋霜冻（FFF）和生长季节长度（GSL），是使用最低温度伊朗北部里海沿岸的12个天气站。数据的统计周期从2000年到2018年底。结果显示，研究区域西部（桂兰省和Mazandaran省的一部分，nash的nFDs指数呈上升趋势线，Rasht站的下降趋势也很明显） 。nFDs指数在东部地区（Golestan省和Mazandaran省的一部分）的趋势线斜率下降。Gorgan站的nFDs指数结果显示nFDs指数显着上升。nFFD指数结果与nFDs指数相反。LSF指数在西部（桂兰省）也呈下降趋势。东部地区（Golestan省和大多数Mazandaran省）的LSF指数观察到正趋势斜率。该指数显示了Astara站的明显上升趋势。因此，LSF指数的结果表明，LSF发生在西部地区，比东部时期更早。FFF指数在西部（整个桂兰省和大部分Mazandaran省）也呈下降趋势，而东部（Golestan省和Mazandaran的一小部分）则呈上升趋势。应该注意的是，FFF指数在研究领域没有显着趋势。该指数的上升趋势和下降趋势分别表明与正常情况相比，FFF的发生时间更晚。最后，代表植物生长期的GSL指数在中部地区（马赞丹省）呈下降趋势。西部（桂兰省）和东部（马赞丹省）地区也有向上的斜坡。同样，在所有台站都没有观察到该指数的明显趋势。西部（桂兰省）和东部（马赞丹省）地区也有向上的斜坡。同样，在所有台站都没有观察到该指数的明显趋势。西部（桂兰省）和东部（马赞丹省）地区也有向上的斜坡。同样，在所有台站都没有观察到该指数的明显趋势。