Public alleyways are being repaved with materials that are permeable to water. The white oak, the state tree of Illinois, has been banned from city planting lists, and swamp oaks and sweet gum trees from the South have been given new priority. Thermal radar is being used to map the city’s hottest spots, which are then targets for pavement removal and the addition of vegetation to roofs. And air-conditioners are being considered for all 750 public schools, which until now have been heated but rarely cooled.