Urban heat on the move: How functional zoning and shifting populations intensify day–night exposure hotspots

Highlights

• Dynamic heat exposure assessed using UFZs, hourly temps, and mobile signalling.
• Exposure hotspots shift from commercial cores by day to housing belts at night.
• Youth dominate total exposure; elderly exposure concentrates in few zones.
• Strong fine-scale clustering of HERI revealed by high Moran’s I values.
• SHAP identifies key HERI correlates in impervious, pervious, and road patterns.

Abstract

Urban heat exposure in subtropical megacities emerges from the joint effects of urban form, land use and population rhythms, yet most assessments still rely on static census units and single-time temperature fields. This study develops a dynamic exposure framework for six central districts of Guangzhou, China, by combining point-of-interest and morphology-based urban functional zones (UFZs), hourly near-surface air temperature (Ta) fields reconstructed at 250 m for July 2022, and anonymized mobile phone signaling data disaggregated by sex, broad age group and activity purpose. We define a dimensionless Heat Exposure-Related Index (HERI) that combines grid-level thermal intensity with UFZ-level population-share and dwell-time terms, yielding a mixed-resolution exposure surface. 

Results reveal strong spatial and group-wise heterogeneity. Dense commercial cores and business belts combine the highest daytime temperatures with the largest shares of daytime HERI, while nighttime exposure shifts toward high-density residential belts (HDRA) and transport corridors (TA). Water belts (WA) remain cool by day but can show higher night-to-day zone-level HERI shares, consistent with slower evening cooling in these environments. Within this operator-derived mobility dataset, younger groups account for the largest citywide HERI shares, whereas seniors have smaller totals but more spatially concentrated exposure. Grid-scale HERI is strongly clustered (Moran’s I up to 0.8–0.9). XGBoost–SHAP analysis identifies impervious surfaces and building footprint as dominant daytime correlates, and pervious cover and road corridors as key nighttime modifiers. 

The framework provides a methodological reference for group-specific heat-exposure assessments in rapidly urbanizing, warm–humid cities.