Cooling the Hot City

Cities can run five degrees hotter than surrounding fields: asphalt stores the day’s heat and releases it all night. The fix is a patchwork. Paris paints roofs silver to reflect sunlight; Medellín strings green corridors of trees along concrete channels, cooling nearby streets by three degrees. Permeable pavements let rain sink instead of flood, while ponds and fountains chill the air through evaporation. Critics warn greening raises rents, pushing out the residents heat planning should protect. Planners answer with rules tying new parks to affordable housing. Cooling a city, they argue, is less an engineering project than a social contract written in shade. Heat is also distributed unevenly. Within a single city, temperatures can differ by seven degrees between a leafy avenue and a bare industrial yard, and the hottest districts are usually the poorest, with less tree cover and more traffic. Public health records mirror the geography: during heatwaves, emergency admissions rise fastest in those neighbourhoods. Remedies that treat shading as infrastructure rather than decoration reach them first, which is why some cities map heat block by block before deciding where to plant. Night-time temperatures matter most, because bodies need recovery; a district that stays warm after dark produces worse outcomes than one that merely peaks high at noon. Measuring the problem is the first step toward cooling it fairly.