The best way to keep a city cool is by making roofs white or reflective.

So-called "cool roofs" could reduce the average air temperature in cities like London more during a heatwave than green roofs, trees, or solar panels do.

Painting rooftops white or covering them with a reflective coating is the most effective way to reduce air temperature in a city like London during a heatwave. These “cool roofs” outperform solar panels, green roofs, or adding more trees at ground level.

Oscar Brousse at University College London and his colleagues conducted climate simulations to see how London's temperature during the two hottest days of summer in 2018 would have changed if the city had widely implemented cooling measures such as cool roofs, air conditioning, and solar panels. Temperatures that summer peaked at 35.6°C (96.1°F).

The research team found that cool roofs were the most effective, lowering average outdoor temperatures in the city over the two-day period by 1.2°C, and by as much as 2°C in certain locations. In comparison, additional tree cover only reduced air temperatures by about 0.3°C, while solar panels lowered temperatures by 0.5°C.

The study also revealed that widespread use of air conditioning might keep internal temperatures cool but would raise outdoor air temperatures by up to 1°C in parts of central London. Brousse emphasized that for London, cool roofs worked the best for reducing outdoor temperatures at a pedestrian level.