Introduction

A climatic season is a part of the year that is distinguished from other parts due to the regular recurrence of astronomical or climatic phenomena. Seasons are generally defined using two main methods: a) the astronomical or calendar-based method, which is based on the apparent movement of the sun. According to this method, spring and autumn begin at the equinoxes (March 21 and September 21), summer at the summer solstice (June 22), and winter at the winter solstice (December 22). b) The climatic method, which defines seasons based on atmospheric conditions and temperature patterns in each region, is particularly applicable in subtropical areas such as Iran. This approach uses temperature values and their trends to determine the onset and end of the seasons.

Materials and Methods

In this study, temperature data from 35 synoptic stations across Iran from 1992 to 2022 were used. The identification of seasons was based on daily temperatures and the slope of their changes relative to the long-term averages. By reviewing the most current criteria for distinguishing the climatic seasons starting and ending, a new approach was designed for determining the season as follows:

1. Start of spring: daily temperature rises above the long-term minimum and average but remains below the maximum.

2. Start of summer: daily temperature rises above all long-term averages (minimum, average and maximum).

3. Start of autumn: daily temperature falls below the long-term maximum but remains above the minimum and average.

4. Start of winter: The daily temperature fell below all long-term averages and exhibited a downward trend.

Results and Discussion

In the south, southwest, southeast, and central regions of Iran, spring begins in mid-February, whereas in the north, northwest, Alborz foothills, and west, it starts from early to mid-April. The shortest springs (54–64 days) were observed in areas with rapid warming or persistent high-pressure systems. Spring in the south starts with temperatures between 14 and 21°C, and in the northwest and Alborz, it begins with temperatures between 4 and 10°C. Summer in the south, southeast, and central regions starts from early to late May, whereas in the northwest and west, it begins from early June to late July. The shortest summers (60–75 days) are observed in the mountainous areas of the west and northwest, and the longest summers (120–196 days) are observed in the south and east of the country. The starting temperature of summer ranged from 19 to 31°C. The warmest starts were recorded in the south and southwest, and the coolest in the northwest and east of the country (19–22°C). Autumn started earlier in the northwest, west, northeast, and mountains (mid to late September) and later in the south and southwest (until mid-November). The length of autumn in most regions is 74–97 d. The shortest autumns (45–62 days) were observed in Khuzestan, Hormozgan, and Mazandaran owing to the influence of the sea. Autumn in the south begins with temperatures between 25 and 31°C, and in the north and east, it begins with temperatures between 6 and 22°C. Winter in the south, southeast, and southwest starts later (January to mid-February), whereas in the northwest, west, Alborz, and northeast, it starts earlier (late November to mid-December). The shortest winters (35–45 days) are found in the southern and central regions, influenced by the subtropical high-pressure ridge, and the longest winters (84–121 days) occur in the north and west of the country. Winter in the northwest and northeast begins with lower temperatures (3–5°C), whereas in the southwest and south, it begins with higher temperatures (18–21°C).

Conclusion

Spring in Iran begins in mid-February with temperatures ranging from 14 to 21°C in the southern and southwestern regions and eventually starts in mid-April with temperatures between 4 and 10°C in the northwestern and mountainous regions. Summer begins in early May in the southern and southwestern parts of the country with temperatures between 27 and 31°C, and then from mid-July, it starts in the northwest and western mountainous areas of Iran with temperatures ranging from 19 to 22°C. Autumn begins in mid-September in the northwestern, western, northeastern, and southwestern highlands of the country, with an average temperature of 16°C, and in the southern and southwestern regions, it begins in mid-November with temperatures between 25 and 31°C. Winter starts in late November to mid-December in the northwestern, western, Alborz foothills, and northeastern regions with temperatures ranging from 3 to 5°C, and from early January to mid-February, it starts in the southern, southwestern, and southeastern regions with temperatures between 18 and 21°C. Overall, in most parts of Iran, spring and summer begin earlier, autumn starts later, and the beginning of winter remains unchanged. Additionally, the duration of the transitional seasons has decreased and the length of summer has increased.