BERN. On the night from which the new map was drawn, the sensors recorded 29 degrees in the Langstrasse district well after midnight, while the woods of the Uetliberg, fifteen minutes away by tram, had cooled to 19. Zurich now knows, street by street and courtyard by courtyard, exactly where its summer heat lives.
The map is the product of an unusually dense measurement campaign. Around 150 low-cost sensors, distributed across the city and read every few minutes, were combined with mobile transects on bicycles and cars and with thermal imagery from satellites, yielding a temperature picture resolved to individual building blocks. The raw data are published openly, and school classes have already used them for matura projects.
The science it visualises is well established, but never before at this granularity for a Swiss city. Sealed surfaces store the day's heat, street canyons trap it, engines and air conditioners add their own, and the result is released slowly through the night. It is the nights that matter for health: the body recovers from a hot day only if the temperature falls, and in the densest districts it increasingly does not.
MeteoSwiss data supply the trend line. Tropical nights, when the temperature does not fall below 20 degrees, were once rare enough in Zurich to be remarked upon; they are now a routine feature of July and August, and the city's climate projections show them multiplying through mid-century. Last summer the city counted its longest unbroken run of tropical nights since records began in the nineteenth century.
The policy response has been accumulating for years, and the map is its targeting instrument. The city's heat action plan already prescribes green roofs on new builds, lighter and more permeable surfaces, and the protection of mature trees; school grounds are being depaved, and the celebrated network of drinking fountains, some 1,200 of them, is being extended into the districts the map shows hottest.
The politics are fought at district level, tree by tree and parking space by parking space. Removing asphalt for canopy means removing parking, and the neighbourhood votes have been close; the map has shifted those arguments, because a sensor reading is harder to dispute than a planner's diagram. In two districts, voters approved tree-lined redesigns only after the map showed their own street among the city's hottest.
The costs are modest against the alternative. The planting and irrigation programme runs to millions of francs a year, while the health service's bill for a single severe heatwave, measured in admissions, lost work and, in the worst years, lives, is far larger. The summer of 2003, linked to roughly a thousand excess deaths across Switzerland, remains the reference point in every budget discussion.
“The map turns an argument into a measurement; nobody can now credibly claim their district is unaffected,” said a climate adaptation specialist at the city's civil engineering office. The office reports a queue of requests for the underlying data, from insurers to school architects.
The vulnerable are mapped as carefully as the heat. Top-floor flats in unrenovated blocks, care homes and hospital wards are cross-referenced with the temperature grid, and during heatwaves the city's outreach, from cool public rooms to welfare calls, is dispatched accordingly.
What happens next is institutional. The map will be updated annually, the sensor network is being expanded into the agglomeration, and from next year building applications will be checked against the heat layer as routinely as they are checked against zoning. Cantonal authorities are studying whether the method should be extended to Winterthur and the lakeside towns.
Zurich is not alone. Basel and Geneva run comparable programmes, alpine cities from Innsbruck to Grenoble face the same valley-trapped heat, and the methods, sensors and all, are being traded between European city administrations as a form of municipal open source.
A city cannot change its latitude, and it will not change its summers. What it can change is what it knows about them, and then, stone by stone and tree by tree, what it does with that knowledge.