Abstract
This study investigates the long-term evolution of the urban thermal environment in Malang, Indonesia, from March 1944 to May 2026. A comparative analysis using Thom’s Discomfort Index (TDI), Humidex (HMDX), monthly minimum temperature (T_{min}), and daily diurnal temperature profiles was conducted to assess changes in thermal comfort. Results indicate a progressive deterioration of Malang’s urban bioclimatic conditions, transforming the city from a historically cool hill station into an increasingly thermally stressed urban environment. Uncomfortable thermal conditions now persist for 25-31 days per month, particularly during October-April. This trend is primarily associated with intensified nocturnal warming, reflected by rising T_{min} and an upward shift in the 24-hour temperature cycle, indicating enhanced heat retention within the urban canopy layer. The observed changes are linked to the accumulation of high thermal inertia materials, such as asphalt and concrete, which reduce nocturnal cooling, and are further amplified by anthropogenic heat emissions from transportation and buildings. These findings suggest that elevation alone can no longer offset the thermal impacts of rapid urbanization, highlighting the urgent need for climate-sensitive urban planning and nature-based mitigation strategies.