Collecting rainwater from rooftops and using it to spray buildings during hot weather could help cities cut air conditioning use, lower urban temperatures and lessen the impact of heatwaves, according to new research from The University of Manchester. Cities around the world are facing rising temperatures, putting pressure on public health, infrastructure and energy systems. As people rely more on air conditioning to stay cool, energy demand increases and waste heat released from buildings can make urban areas even hotter. Urban watering technologies are becoming important ways for reducing extreme heat in cities, but their use is often limited by the availability of water. In the study, published in Earth’s Future, researchers used process-based numerical simulations and Artificial Intelligence (AI) to test a system that stores rainwater collected from rooftops and automatically sprays it onto buildings during hot weather. Using Tokyo as their case study, they found that cooling rooftops, the system reduced the amount of energy needed for air conditioning. The cooler roofs transferred less heat into buildings, while lower air conditioning use meant less waste heat was released into the city. Together, these effects helped reduce urban temperatures, cutting the number of heatwave days overall and lessening the intensity of extreme heat events. Lead author Dr Zhonghua Zheng, Co-Lead for Environmental Data Science & AI at Manchester Environmental Research Institute (MERI) and Senior Lecturer (Associate Professor) in Data Science and Environmental Analytics at The University of Manchester, said: “Cities around the world are facing growing challenges from extreme heat. Air conditioning can help keep people safe and comfortable, but it also consumes large amounts of energy and releases additional heat into the urban environment. “Our study shows that harvesting rainwater from roofs and using it strategically for cooling could provide a practical way to reduce both energy demand