It would be more effective at cooling cities than ‘green roofs’ with plants. The advantage of painting roofs white is greater than you might think. Researchers at UCL (University College London) have confirmed this. The research was published in Geophysical Research Letters.
They used a climate model. They tested the thermal effects of different passive and active urban heat management systems. This included painted “cool roofs,” rooftop solar panels, green roofs, ground-level tree vegetation, and air conditioning.
Cooling London
What if they were widely adopted across London (the field of study)? Cool roofs could reduce outside temperatures across the city by 1.2 degrees C. Extensive street-level vegetation or solar panels would provide a minor cooling effect. Air conditioning would warm the outside urban environment by up to 1 degree C in dense central London.
The authors said in a statement: “We tested multiple methods that cities like London could use. We found that cool roofs were the best way to keep temperatures down during summer days.”
Cities typically trap heat, inducing the urban heat island effect. Finding ways to adapt to or mitigate this rise in temperatures is a high priority. The study comprehensively compared several common passive and active heat mitigation actions. By reflecting heat rather than absorbing it, cool roofs have a dual benefit. They not only cool the outside urban environment, but also the interior of buildings.
Other proposals
The advantage of painting roofs white is demonstrated in this study. The researchers found that its effect on temperature varied significantly over the course of the day. During the warmest hours of the day, it reduced urban temperatures by an average of 0.5 degrees C. This would be offset at night. This is because the thermal mass of roofs would retain daytime heat, releasing it when the sun went down. It would increase nighttime temperatures by about the same amount.
Converting the city’s green spaces from grass to deciduous tree cover would cool temperatures at night. But at best it would have mixed net effects during the day. It would likely increase the amount of water vapor in the air. This would effectively increase air humidity and could affect residents’ thermal comfort.
