Urban Thermal Stress Dynamics in a Tropical Coastal City: Insights from Remote Sensing Analysis in Padang City, Indonesia

Abstract

Introduction. The Urban Heat Island (UHI) phenomenon has emerged as a critical global environmental challenge, significantly undermining the sustainability of urban ecosystems and the well-being of city dwellers. UHI directly impacts human thermal comfort, increases energy demands for cooling, and escalates public health risks. In tropical regions, these challenges are particularly acute due to high baseline temperatures, intense solar radiation, and rapid urbanization. In tropical coastal settings, the complexity of thermal dynamics is further heightened by unique geomorphological features and the degradation of natural buffer zones-such as mangroves and riparian vegetation-and massive land cover transformations, thereby altering the microclimatic balance. This study focuses on Padang City, a tropical coastal city in Indonesia, which has undergone intensive land-use transformation over the past decade. This rapid development has converted extensive non-vegetated areas and agricultural lands into dense residential and commercial zones, leading to the emergence of localized thermal hotspots that threaten the city’s long-term ecological resilience.

Data and Method. This research utilized multitemporal Landsat 8 OLI/TIRS satellite imagery (2015–2024) to analyze the spatio-temporal dynamics of the urban environment. The analysis focused on coastal lowlands (built-up and cultivation zones), excluding upland forests to eliminate topographic thermal bias. The Normalized Difference Vegetation Index (NDVI) was employed to quantify vegetation density, while Land Surface Temperature (LST) was retrieved using the thermal infrared bands through a rigorous series of calibration steps, including top-of-atmosphere (TOA) radiance conversion and surface emissivity estimation based on land cover proportions. To evaluate thermal stress, the Urban Thermal Field Variance Index (UTFVI) was modeled, classifying ecological quality into six categories to provide a precise spatial metric for urban environmental health.

Result and Discussion. The findings indicate that the relationship between NDVI and LST in Padang City lowlands consistently exhibits a negative correlation, underscoring the role of vegetation as a key thermal regulator. However, the strength of this relationship has progressively weakened over the study period, with the coefficient of determination declining by 2024. This trend suggests a reduction in the cooling capacity of existing urban vegetation amidst the expansion of impervious surfaces. Interestingly, during the COVID-19 pandemic in 2020, average surface temperatures declined to 28.18°C due to reduced anthropogenic heat emissions. Nevertheless, UTFVI analysis reveals that extreme thermal stress classes, specifically the ‘Strongest-Worst’ category, continued to expand structurally, particularly in the urban core. This expansion intensified in the post-pandemic period (2022–2024), where the ‘Worst’ ecological zone reached its peak spatial extent. The persistence of these hotspots reinforces the argument regarding the high thermal inertia of concrete and asphalt materials. This study finds that the thermal ecological quality of Padang City is undergoing structural degradation. Future urban planning must prioritize the integration of blue-green infrastructure with surface material albedo control and optimized morphological designs to enhance the thermal resilience against the escalating pressures of climate change and urbanization.