University of Manchester study proposes rooftop rainwater spraying to combat urban heat

Researchers at the University of Manchester have developed a model for using collected rainwater from rooftops to mitigate urban heatwaves and reduce air conditioning reliance. According to a study published in the journal Earth's Future, an automated system could store rainwater and spray it onto buildings during periods of extreme heat.

Using Tokyo as a case study, the research indicates that cooling rooftops reduces heat transfer into buildings, which in turn lowers energy consumption. This reduction in air conditioning use also creates a positive feedback loop by decreasing the amount of residual heat released back into the urban environment. This process could help lower overall urban temperatures and reduce the frequency and intensity of heatwave episodes.

The study highlights that the timing of the spraying—triggered by specific roof temperatures—is more critical to efficiency than the total volume of water or the size of the storage tank. Researchers noted that excessively large reservoirs offer only modest additional benefits, as excess water may remain on the roof rather than evaporating, which limits the cooling effect.