Boston Mayor Michelle Wu declared a heat emergency from Monday through Wednesday after forecasters said the city will endure its third heat wave of the summer. The National Weather Service expects daytime temperatures across inland Massachusetts to stay in the low-to-mid 90s Fahrenheit, with heat-index readings climbing into the upper 90s on Wednesday before a slow-moving cold front triggers scattered thunderstorms late Wednesday into Thursday. Relief is expected by Friday, when drier air arrives. The city has opened cooling centres at Boston Centers for Youth & Families, public libraries and other facilities, and is extending hours for splash pads and pools. Officials urged residents to stay hydrated, limit outdoor activity, and check on vulnerable neighbours. Boston Emergency Medical Services typically reports a 10%–15% rise in calls during prolonged heat, and the mayor warned that "extreme heat can be dangerous to health, and even fatal." Across the Atlantic, London faces a parallel spell of hot weather. The UK Health Security Agency issued an amber heat-health alert for the capital until Wednesday evening as temperatures are forecast to top 30°C, possibly reaching 34°C. City Hall simultaneously triggered a high air-pollution warning, citing expected spikes in ground-level ozone driven by strong sunshine and imported continental air masses. London authorities are advising residents—particularly older adults, children and those with respiratory conditions—to avoid strenuous activity during the hottest part of the day, curb engine idling and refrain from burning wood or garden waste. Both cities are monitoring conditions closely as climate-fuelled extreme heat episodes become more frequent.