Document Type : Research Article
Authors
1
Ph.D. Candidate in Climatology, University of Zanjan, Zanjan, Iran
2
Professor in Climatology, Department of Geography, University of Zanjan, Zanjan, Iran
3
Assistant Professor in Climatology, Department of Geography, University of Zanjan, Zanjan, Iran
Abstract
Lightning is a prominent atmospheric phenomenon, characterized by the transfer and discharge of electricity between clouds and the Earth's surface. This process poses significant risks to both human safety and the environment, thereby necessitating a detailed investigation of its climatological features. In addition to the electrical transfer, lightning is also accompanied by intense sound and light. Cloud formation typically begins at altitudes above the condensation level, where water droplets form below the zero-degree isotherm, and ice crystals develop above it. As these water droplets and ice crystals grow, they descend at an approximate rate of 8 meters per second. If the downward velocity of the particles is slower than the upward movement of the cloud, the droplets and crystals are subjected to continuous updrafts and downdrafts, resulting in repeated collisions. This process leads to the accumulation of larger particles and the separation of electrical charges, with negative charges concentrating at the base of the cloud and positive charges accumulating at the top. This study examines the statistical and geostatistical characteristics of lightning frequency across Iran from 2000 to 2022, using data from 382 synoptic stations. Descriptive statistics, including the mean and coefficient of variation, were computed for lightning events, and their correlations with geographic variables such as latitude, longitude, and altitude were assessed. Regression analysis was employed to model these relationships. The results reveal that the spatial distribution of lightning frequency in Iran follows a stochastic pattern, exhibiting significant annual variability. Among the spatial variables analyzed, latitude and longitude showed a stronger correlation with annual lightning frequency than altitude.
Introduction
Lightning is a natural phenomenon that can be hazardous, claiming the lives of over two thousand people globally each year. It occurs predominantly in regions with variable climates and involves the transfer and discharge of electricity between clouds and the Earth's surface. This phenomenon has far-reaching consequences for both the environment and human society. For instance, lightning can ignite fires, cause financial losses, and result in fatalities when it strikes tall structures such as trees and buildings. It ranks second among atmospheric hazards in terms of mortality.
The occurrence of lightning on Earth is closely linked to climatic conditions, particularly temporal and spatial variations in solar radiation. The highest global frequency of lightning is found in the tropical regions of the Americas, Africa, and the oceans. Approximately 78% of lightning events occur between latitudes 30°S and 30°N. Satellite data indicates that lightning frequency is generally higher in tropical and humid climates. In coastal areas, lightning events account for about 70% of all oceanic lightning occurrences, with a higher frequency observed on the eastern coasts of continents compared to the western coasts. Consequently, the spatial distribution of lightning demonstrates considerable variability.
Lightning frequency tends to peak in the morning and afternoon, with a notable reduction during nighttime hours. Seasonal patterns also influence lightning occurrences, with the highest frequency generally observed in July. Studies conducted in Iran have identified diverse temporal and spatial patterns of lightning, with the highest frequency recorded in the northern regions of Khuzestan Province and the southern regions of Lorestan Province. In contrast, the central and flat interior areas exhibit lower lightning frequencies.
Material and Methods
Data
Lightning frequency data from 2000 to 2022 were collected from 411 active synoptic stations across Iran. After reviewing the dataset, stations with insufficient records were excluded, leaving 382 stations with reliable data for analysis (Figure 1). Daily lightning event records were obtained from the Iranian Meteorological Organization (IRIMO) website (https://data.irimo.ir/withoutlogin/index.aspx)).
An examination of station altitudinal distribution (Table 1) revealed that around 56% of stations (212) were located between 500 and 2000 meters, while only about 3% were situated in low-lying areas (–22 to 0 meters). The number of stations decreased at higher altitudes, with only one station located above 3000 meters.
This study investigates the long-term (climatic) frequency of lightning and its relationship with geographic coordinates (longitude, latitude) and altitude. Descriptive statistics, including the annual mean and coefficient of variation, were computed to characterize lightning events, and their spatial distribution was mapped.
Results and Discussion
General Characteristics
The highest frequency of lightning was observed in northwestern Iran. Precipitation in this region is primarily driven by convective activity, orographic uplift, and local fronts. Additionally, temperature and pressure contrasts induced by topography contribute to the frequency of lightning events.
In southern Iran, which has a hot and humid climate, the annual lightning frequency was found to be similar to that in the western regions. Stations in northern regions also recorded relatively high frequencies, consistent with their temperate and humid climate, which is conducive to lightning activity. The mean coefficient of variation across stations was approximately 25.5%. A value below 25.5 indicates relatively stable lightning occurrence, while higher values suggest more irregular patterns and strong year-to-year variability.
Probability Analysis
The highest probability of lightning occurrence was observed in the high and mountainous regions of the Zagros and Alborz mountain ranges. Factors contributing to this include the mountainous terrain, the frequent passage of air masses, and the uneven distribution of solar radiation on slopes, which promote atmospheric instability and convection.
The western half of Iran, especially the northwest, lies along the path of various air masses (including westerlies, Mediterranean cyclones, and associated short waves) during the cold season, creating favorable conditions for thunderstorms. The northern regions also experience significant lightning activity due to the moisture from the Caspian Sea, considerable elevations, and the combined effects of migratory westerly systems and Siberian high-pressure systems. In addition to these regions, scattered areas across the country experience occasional lightning events.
Conclusion
Lightning is a natural phenomenon that occurs worldwide, especially in regions with variable climates such as Iran. Analyzing data from 2000–2022 reveals that geographic coordinates and elevation have a minimal linear effect on the distribution and frequency of lightning events. The coefficient of variation maps highlight irregular lightning occurrences and significant annual variations across the country.
Correlation analysis indicates weak linear relationships between geographic factors and lightning frequency. Latitude exhibits a slight positive correlation with lightning activity in the northern regions, while longitude shows a moderate negative correlation, with lightning frequency decreasing eastward. Overall, lightning in Iran follows a stochastic pattern, with no strong linear spatial association with geographic coordinates or altitude.
Satellite-based monitoring, such as the Lightning Imaging Sensor (LIS) on the TRMM satellite, offers near-real-time and accurate coverage, complementing station-based measurements. Future studies should incorporate satellite data to more effectively capture the temporal and spatial distribution of lightning, thereby providing a comprehensive understanding of this phenomenon across Iran's diverse climatic regions.
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