Spatiotemporal Analysis of Daytime and Nighttime Urban Heatwave Dynamics: A Comparative Study of Riyadh and Perth (2003–2024)

Abstract

This report examines the daytime and nighttime urban heatwave dynamics in Riyadh, Saudi Arabia (hot desert, BWh) and Perth, Western Australia (Mediterranean, Csa) from 2003 to 2024. The daily maximum (Tmax) and minimum (Tmin) temperatures were retrieved from the ERA5-Land hourly reanalysis data within the urban boundaries of each city. Heatwaves were defined as exceeding the 90th percentile of Tmax and Tmin based on an 11-year baseline period (2003–2013), and must include at least three days of exceedance. The frequency, duration and intensity of the heatwaves were computed annually for both day and night time events and long-term trends were evaluated by Mann–Kendall and Theil–Sen slope estimator. The frequency of heatwaves is statistically significant increasing in both cities between 2003 and 2024. The frequency trend during the day is slightly more pronounced than during the night, but both are highly significant, as would be expected in an arid climate with low atmospheric humidity which means that overnight cooling is limited, which leads to amplification of heat extremes. The sea breeze effect of the Fremantle Doctor is also evident in Perth, with the magnitude of the trend similar but of lesser magnitude than that of Riyadh during day and night. Spatially, heatwave hotspots are more intense in the dense central districts of Riyadh, and lower intensity along the coast and higher intensity in the eastern suburbs (inland) of Perth. These results show that both cities are experiencing increased heat exposure and offer a climatological baseline at the urban scale for adaptation planning to heat. These findings carry direct implications for urban heat adaptation: the near similarity of daytime and nighttime heatwave trends in Riyadh signals limited nocturnal recovery for outdoor and informal workers, while the persistence of a warming trend in Perth despite the moderating Fremantle Doctor suggests that coastal sea-breeze buffering may not be sufficient to offset background warming in the longer term. The spatial intensity patterns identified at the 9 km grid scale provide an evidence base for prioritising heat-adaptation planning (e.g., shading, cool-roof, and urban greening programmes) toward the central Riyadh and inland eastern Perth districts identified as hotspots, although finer-resolution data would be required to target interventions at the street or building scale.