Document Type : Research Article
Authors
1
Professor in Watershed Management, Sari Agricultural Sciences and Natural Resources University, Sari, Iran
2
PhD Candidate in Watershed Management, Sari Agricultural Sciences and Natural Resources University, Sari, Iran
Abstract
Given the significance of urban heat islands, this study examines Sari City's thermal comfort classification and the Urban Thermal Field Variance Index (UTFVI). After obtaining eleven years of Landsat 8 satellite imagery, the urban area's thermal variance index was extracted based on land surface temperature (LST) and mean temperature, and thermal comfort was classified accordingly. The results showed that the lowest recorded average surface temperature in Sari City was 30.1 °C on August 8, 2013, while the highest recorded average temperature was 40.62 °C on July 12, 2018. Therefore, it can be seen that areas with residential and commercial buildings and artificial surfaces exhibit the highest temperatures. At the pixel level, the lowest Urban Thermal Field Variance Index was 0.352 on August 1, 2022, while the highest value was 0.122 on August 7, 2016. The greatest extent of thermal comfort, covering 58.49% of the area, was recorded on July 10, 2023, for the excellent, good, and normal comfort classes. The lowest comfort level, covering 50.39% of the area, was recorded on August 8, 2023. On August 8, 2013, the bad, worse, and worst thermal comfort classes covered the largest area (49.61%), while on July 10, 2023, they covered the smallest area (41.51%). Based on these results, it can be concluded that the central areas of Sari and other parts of the city with limited green cover or unplanned construction conducted without regard to urban life standards fall into the lower comfort categories.
Extended Abstract
Introduction
Due to the fact that many people live in urban areas, it is critical to consider urban temperature when making decisions regarding urban management. Urban heat islands are a phenomenon caused by changes in the thermal energy balance of urban areas as a result of burning fossil fuels, high vehicle density, expanding impermeable surfaces, and a lack of proper vegetation cover. Therefore, it is essential to monitor land surface temperature (LST) and categorize it quantitatively and qualitatively using standard indices to measure thermal comfort. The Urban Thermal Field Variance Index (UTFVI), which is used to categorize LST in terms of thermal satisfaction in urban contexts, is one such criterion.
Material and Method
In this research, Landsat 8 satellite images of Sari City for 11 summer seasons (2013–2023) were downloaded from the USGS website. Then, ENVI 5.3 software was used to preprocess the images (geometric and radiometric corrections). Segment-by-segment calculations of spectral radiance, black body temperature, NDVI, and vegetation cover were carried out using the digitized data of each image and the urban area map. The land surface temperature and emissivity were estimated using the split-window approach. The Urban Thermal Field Variance Index was evaluated using ArcGIS 10.5 software, which took into account the LST of each pixel as well as the average LST of all pixels. A zoning map of this index was then created in six classes (excellent, good, normal, bad, worse, and worst) using the UTFVI.
Results and Discussion
The results showed a negative correlation between temperature and vegetation cover. The lowest recorded average surface temperature in Sari City was 30.1 °C on August 8, 2013, while the highest recorded average temperature was 40.62 °C on July 12, 2018. At the pixel scale, the lowest UTFVI was –0.352 on August 1, 2022, while the highest value was 0.122 on August 7, 2016. However, the average of this index over all years was nearly zero.
The highest level of thermal comfort, covering 58.49% of the area, was recorded on July 10, 2023, for the excellent, good, and normal classes; and the lowest level, covering 50.39% of the area, was recorded on August 8, 2023. On August 8, 2013, the bad, worse, and worst thermal comfort classes covered the largest area (49.61%), while on July 10, 2023, they covered the smallest area (41.51%).
Considering the overall upward temperature trend over the study period, it may seem paradoxical that thermal comfort zones in the normal, good, and excellent classes have expanded over the 11 years. This contradiction lies in the structure of the UTFVI equation. The average LST of the urban area and the pixel-level LST are the two key parameters in this index. Thermal comfort is classified as excellent, good, or normal when the pixel-level LST is lower than the urban average. Conversely, if pixel-level LST exceeds the urban average, thermal comfort falls into the bad, worse, or worst categories. As the difference between pixel-level and average LST increases, thermal comfort shifts from normal to worst.
Conclusion
Thermal comfort, heat islands, and variations in land surface temperature are all significant indicators for evaluating the sustainability of the urban environment. Given the general trend of rising urban temperatures and climate change, attention must be given to construction practices, land-use changes, and the development of green spaces such as parks and gardens—all of which can significantly reduce LST.
Other factors affecting temperature variation in urban areas include cloud cover, solar radiation intensity, vehicle density, refrigeration and air conditioning systems, and electric energy consumption. Addressing these elements within the context of urban management is key to achieving sustainable urban development.
Acknowledgements
This study was supported by funding from Sari Agricultural Sciences and Natural Resources University. We are grateful to all the scientific advisors who contributed to this research.
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