Abstract
<title>Abstract</title> <p>Urbanization alters land surface and atmospheric conditions, intensifying the surface urban heat island (SUHI) effect and increasing heat stress vulnerability. In Bangladesh, SUHII has been widely studied for Dhaka; however, long-term spatio-temporal variations of diurnal, seasonal, and annual land surface temperature (LST) and SUHII in Sylhet City Corporation (SCC) remain underexplored. This study integrates Landsat and MODIS data to evaluate land use and land cover (LULC) changes, analyze daytime and nighttime LST and SUHII from 2003 to 2021, and examine the biophysical controls of NDVI and NDBI on observed thermal patterns. A sensitivity analysis was conducted to objectively determine the optimal rural reference buffer distance for SUHII computation. The results show a 61% increase in built-up areas, while bare land and water bodies declined over the study period. Daytime LST anomalies peaked at 4.08°C during the 2021 monsoon, while nighttime anomalies reached 1.94°C in the pre-monsoon and 1.5°C in winter. The highest daytime SUHII reached 7.00°C during the 2021 monsoon, while the nighttime peak was 2.77°C in the pre-monsoon and winter of 2021. Mann–Kendall and Sen's slope analyses identified statistically significant upward trends in SUHII, with the annual maximum daytime increasing at 0.051°C yr⁻¹. Daytime trends were the most significant across all seasons at the 95% confidence level. Hotspots are persistently concentrated in the central SCC, with the most spatially extensive configuration observed during winter in the daytime and southward expansion during the monsoon and post-monsoon seasons at nighttime. NDBI exhibited a significant positive association with daytime SUHII (r = 0.45), while NDVI showed a stronger negative association with nighttime SUHII (r = − 0.52), confirming that impervious surface expansion and vegetation loss are the primary biophysical drivers of thermal intensification in SCC. Incorporating green infrastructure, reflective materials, and heat-resilient zoning can help reduce SUHI effects and support climate-adaptive urban planning in SCC.</p>