Evaluating the relationship between heat waves and urban heat islands in tropical cities: a case study of Kuala Lumpur and George Town.

Urban Heat Island (UHI) and heat stress pose significant threats to human health in humid tropical regions, where elevated temperatures and high moisture levels intensify thermal discomfort. This study examined UHI dynamics, heat wave (HW) events, and their combined influence in an inland (Kuala Lumpur, KL) and a coastal (George Town, GT) Malaysian city using ground-based meteorological data. Diurnal, annual, and seasonal UHI patterns were studied alongside UHI–HW synergies. A new metric, Cumulative Heat Exposure (CHE), was developed to quantify the excess hourly heat experienced by residents, and machine-learning techniques were applied to identify the dominant drivers and threshold conditions governing CHE occurrence. To enhance spatial representativeness and model transparency, satellite-derived land surface temperature (LST) was integrated for validation, and Random Forest interpretability was improved using SHapley Additive exPlanations (SHAP) alongside K-fold cross-validation. The study found mean annual UHI intensity peaking at 2.1 °C in KL and 2.9 °C in GT, with further amplification during heatwave events. However, these HW-induced amplifications were not uniform across seasons, highlighting the complex and city-specific nature of UHI–HW interactions. CHE analysis revealed that the residents of urban KL and GT experience an additional 0.53 °C and 0.35 °C of heat every hour in March and April, respectively. Random Forest results confirmed air temperature as the primary driver of CHE, while partial dependence plots revealed city-specific temperature thresholds between 26 and 30 °C for triggering CHE. These findings highlight the complex, non-linear behavior of urban heat in humid tropical climates and underscore the need for targeted, city-specific heat-mitigation strategies.