Elevation-dependent classification of outdoor thermal stress in a tropical Andean country: a case study of Colombia.

Outdoor thermal comfort in tropical mountain countries is shaped by air temperature, humidity, wind, solar radiation and topography, with elevation strongly controlling thermal stress (TS). This study evaluates how elevation modulates outdoor TS in Colombia and proposes an elevation-based thermal stress level (TSL) classification. Typical Meteorological Year (TMY) datasets from the National Solar Radiation Database (NSRDB) were used to compute four thermal indices: Universal Thermal Climate Index (UTCI), Effective Temperature (ETv), Thermo-Hygrometric Index (THI) and the national heat index (IDEAM-HI) for 69,874 locations. For each index, linear and second-degree polynomial regressions between elevation and the annual maximum, mean and minimum values were fitted and assessed using RMSE and R². Polynomial models generally outperformed linear models, particularly for UTCI, explaining up to 96% of the variance in the annual maximum values. The resulting functions were used to derive elevation ranges for different TSL and to produce a TS classification and map. Large lowland areas are predominantly classified within warm and very hot stress conditions, whereas thermally comfortable conditions are mostly restricted to intermediate elevations between 1,080 and 2,050 m.a.s.l. In addition, an exploratory hourly UTCI assessment was performed for four cities using TMY data for representative urban configurations. The urban results qualitatively confirm the elevation-based TS classes and show how diurnal cycles and intra-urban differences modulate TS within each elevation band. The proposed framework provides a climatological, screening-level multi-scale reference for typical outdoor TS in Colombia, enabling baseline spatial prioritization for heat-health risk assessment, climate-resilient planning and comparative assessment planning in tropical Andean regions.