Identifying the Rainy Season in Iran

Document Type : Research Article

Authors

1 Department of Geography, Faculty of Human Sciences, University of Zanjan, Zanjan, Iran.

2 Department of Physical Geography, Faculty of Geography and Environmental Planning, University of Sistan and Baluchestan, Sistan and Baluchestan, Iran.

Abstract

The rainy season represents a key manifestation of the climate system’s response to atmospheric general circulation, synoptic-scale interactions, and land–atmosphere thermodynamic conditions. The timing of the rainy season—its onset, cessation, and duration—reflects the trajectories of cyclones, the intensity and positioning of westerlies, and variations in moisture flux convergence. Iran, situated within the arid and semi-arid subtropical belt, exhibits high spatiotemporal variability in its rainy season due to its location at the intersection of Mediterranean, Sudanese, Caspian, and Monsoon influences. Consequently, identifying the rainy season using a physically based approach with daily precipitation data is essential for understanding the mechanisms governing regional precipitation. This study aims to extract and analyze the temporal and spatial characteristics of Iran’s rainy season and interpret them within the framework of the governing atmospheric processes.
Daily precipitation data from 72 synoptic stations across Iran were analyzed for the period 1960–2023. Stations were selected to ensure appropriate spatial coverage, topographic diversity, and minimal data gaps. The rainy season was identified using a 30-day moving average applied to daily precipitation series, with the long-term mean over 366 days serving as the reference threshold. Any period of at least 30 consecutive days in which the moving average exceeded the threshold was classified as a rainy season. Transition points from dry to wet and vice versa defined the onset and cessation, respectively. Based on this method, indicators including the onset, cessation, duration of the rainy season, and the proportion of seasonal precipitation relative to annual totals, were extracted. To evaluate the influence of spatial factors on rainy season variability, multivariate regression and Pearson correlation analyses were conducted between geographical variables (latitude, longitude, and altitude) and rainy season characteristics. The analysis revealed that Iran exhibits both a general and a regional rainy season regime. The general rainy season extends across the country, whereas the regional rainy season occurs in the northwest, parts of the Caspian Sea coast, and the southeast of Iran.
The general rainy season initiates in areas along the Caspian Sea coast and gradually extends westward, northwestward, eastward, and southeastward. This pattern aligns with the expansion of the Siberian high, weakening of the subtropical high-pressure system, and Mediterranean cyclone activity. Cessation occurs first along the Caspian Sea coast, followed by the central and southern regions. Maximum rainfall duration is observed in the northwest, while the central and southeastern regions experience the shortest durations. The length of the general rainy season ranges from approximately four to over seven months. The ratio of seasonal to annual precipitation indicates a high concentration of cold-season rainfall (>85%) along the southwest to eastern regions, consistent with large-scale cyclonic dominance. In contrast, the Caspian Sea coast and northwestern coastal areas exhibit greater temporal dispersion of precipitation due to the interaction of synoptic and convective systems.
The regional rainy season is primarily influenced by mesoscale processes in the northwest and monsoon flows in the southeast. Its duration exceeds two months in the northwest but is less than one month in the southeast. Statistical analyses indicate that altitude strongly influences the spatial variability of rainy season onset and cessation, particularly in northern and southern regions, whereas latitude plays a significant role in the east. These findings highlight the combined control of topography and geographic location on precipitation responses to general atmospheric circulation.
This study demonstrates that the rainy season in Iran reflects a structural spatiotemporal organization. Variations in onset, cessation, and duration correspond with Mediterranean cyclone trajectories, moisture flux convergence, the subtropical jet stream, and regional sea surface temperatures. Overall, the rainy season in Iran is not only a climatic phenomenon but also a manifestation of underlying physical changes in the regional atmosphere–ocean system.

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References

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Journal of the Earth and Space Physics
Volume 52, Issue 1
online publication date: 30 May 2026
May 2026
Pages 179-194

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