In Tokyo, a city famous for its steamy summers, researchers have found a surprisingly simple way to help urban areas stay cooler during heat waves. Scientists at the University of Manchester discovered that collecting rainwater from rooftops and spraying it on buildings during hot weather can meaningfully reduce how much energy cities need for air conditioning and lower overall temperatures.
The study, published in the journal Earth's Future, used computer simulations and artificial intelligence (AI) to test this approach. Their model showed that cooling rooftops with collected rainwater means buildings need less air conditioning. When people use less air conditioning, less waste heat enters the city. These combined effects help decrease urban temperatures, cut the number of heat wave days, and lower the intensity of extreme heat events.
Lead author Dr. Zhonghua Zheng, who works at the Manchester Environmental Research Institute, said cities face 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," he explained. "Our study shows that harvesting rainwater from roofs and using it strategically for cooling could provide a practical way to reduce both energy demand and urban temperatures."
Perhaps most importantly, Dr. Zheng noted that the benefits actually grow during hotter years. "This suggests this approach could become increasingly important as the climate continues to warm," he said.
One surprise from the research was that timing mattered more than size. The researchers found that when sprinklers were switched on had a greater impact than either the size of the water storage tank or the amount of water used. Very large tanks only provided modest additional benefits, and dumping extra water on rooftops did not always lead to more cooling, since some of the water simply stayed on the roof instead of evaporating.
PhD researcher Junjie Yu, who also contributed to the study, highlighted another advantage: these systems can help reduce dangerous flooding. "The rainwater tank also provides an additional co-benefit of reducing extreme urban runoff," he said. "This approach exemplifies a natural solution to natural challenges, in which rainwater serves as a natural resource to mitigate both thermal stress and hydrological extremes."
The researchers believe rooftop rainwater cooling could become one tool in a broader strategy to help cities adapt to rising temperatures and protect public health.