A study by the University of Lisbon found that black cars parked in the sun can raise the surrounding air temperature by up to 3.8ºC on hot summer days, compared with nearby asphalt. White cars, by contrast, had a much smaller effect.

“You know when you walk past a parked car on a hot day and feel the heat radiating off it? That’s real, it’s not your imagination,” lead researcher Márcia Matias told New Scientist.

The difference is due to reflectivity. White paint reflects 75-85% of incoming sunlight, while black reflects only 5-10%, absorbing most of the energy instead. Thin car bodies, often made of steel or aluminum, heat up faster than asphalt and release more warmth into the air.

“When you imagine thousands of cars parked across a city, each one becomes a little heat source or shield,” Matias explained.

The researchers calculated that repainting dark vehicles in lighter shades could help reduce Lisbon’s heat stress. In parts of the city where parked cars cover more than 10% of street surfaces, such a change could nearly double street-level reflectance from around 20% to nearly 40% on clear summer days. This shift would significantly reduce the intensity of urban heat islands.

As Portugal faces hotter summers and more frequent heatwaves, the findings suggest that something as simple as car colour could play a role in cooling its cities.