Where air-conditioning is essential - Raising a red flag ten times - Weather station analysis of limits for passive cooling

This study examines conditions where passive cooling strategies may fail during heat waves and where access to active cooling could be critical for health and resilience. Using reanalysis of 16,582 globally distributed weather stations, the author extends previous work by focusing on hotter-than-normal years and the number of days exceeding thresholds of shaded wet bulb globe temperature (Wsd and Ysd WBGTs): white-flag-day (≥25.6 °C) and yellow-flag-day (≥29.4 °C). Results indicate that 27% of the sampled population resides in high-risk “red zones” with ≥10 yellow-flag days per year, where passive measures are unlikely to suffice. About 30% occupy “green-amber” zones with frequent white-flag-days but few yellow-flag-days, suggesting attention to vulnerability-dependent cooling needs. The 10-day exceedance criterion aligns with ASHRAE’s unmet-hour allowances, supporting its use as a design trigger. Findings highlight practical limits of passive strategies and the need for adaptive planning in regions approaching these thresholds. The caveat of this study is that if shaded breezy outdoor living areas would suffice to provide cool relief from a heat wave in a locality, then it is plausible that shaded and cross-ventilated buildings also could provide adequate indoor shelter from heat waves, whereas air-conditioning may be generally necessary elsewhere.