On a hot summer day in Lisbon, two cars—a black one and a white one—sat parked under the sun for hours. When geographer Márcia Matias measured the air around them, she found something surprising: the black car was heating the nearby air by as much as 3.8°C more than the asphalt beneath it. The white car 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?” said Matias, as per New Scientist. “That’s real! It’s not your imagination.”
In a new study published in City and Environment Interactions, Matias and her colleagues show that parked cars—especially dark-colored ones—can make city streets noticeably hotter. This effect, when multiplied across tens or hundreds of thousands of parked vehicles, could shift air temperatures in entire neighborhoods.
And because the solution may be as simple as changing the color of a vehicle, the researchers say it’s a small fix that could have big benefits.
Micro Heat Islands on Wheels
Urban heat islands form when dense buildings, dark pavement, and human activity raise city temperatures above those of the surrounding countryside. But this study adds a new culprit to the list: cars, especially dark ones.
In their Lisbon experiment, Matias and her team parked one black and one white vehicle on asphalt at 8 a.m. and left them under direct sunlight for over five hours. When they returned in the early afternoon to measure the surrounding air, they found a striking difference.
The air just above the black car’s roof was up to 3.8°C hotter than the nearby asphalt. The white car had a much smaller effect—sometimes even producing lower local air temperatures than the street itself.
The difference comes down to physics. Black paint reflects only about 5 to 10% of incoming sunlight. White reflects up to 85%. And, unlike asphalt, cars are made of thin sheets of steel or aluminum that heat up fast and radiate that heat outward into the surrounding air.
“Now picture thousands of cars parked across a city, each one acting like a little heat source or a heat shield,” said Matias. “Their color can actually shift how hot the streets feel.”
Parked Cars Vs. the Climate
That effect isn’t just about a few hot spots in a parking lot. In Lisbon, the researchers counted over 91,000 street parking spaces, plus more than 700,000 cars circulating through the city on a typical weekday. Many people park their vehicles in dense, sun-exposed areas during the hottest parts of the day.
By combining field measurements, traffic data, and land use maps, the researchers were able to extrapolate their findings to the city scale. They discovered that in parts of Lisbon where parked cars cover 10% or more of the road surface, switching from dark to light-colored vehicles could increase solar reflectivity by up to 19 percentage points with tangible cooling potential.
In these areas, replacing dark-colored cars with white ones would raise street-level albedo (a surface’s ability to reflect sunlight) from 20% to nearly 40%, significantly reducing the amount of solar energy absorbed and re-radiated as heat.
“Fleets of municipal vehicles, taxis and delivery trucks or vans are obvious candidates for getting light-colored makeovers,” Matias said.
Sarah Berk, a climate researcher at the University of North Carolina, noted that most urban heat strategies focus on rooftops and pavements. “Harnessing light-coloured vehicles as a mitigation strategy for urban heat is particularly novel,” she said, as per New Scientist.
Parked Cars, Unparked Problems
It is no secret that prior studies have shown how vehicle exhaust contributes to heat, especially in traffic jams. But those analyses often ignore what happens when vehicles stop moving.
Unlike thick, slow-heating materials like asphalt, car surfaces respond rapidly to sunlight. The metal skin heats up within minutes. And even modest coverage by dark-colored cars can shift a neighborhood’s heat signature.
In Lisbon’s most densely built areas—where roads cover nearly a third of the land surface—the researchers identified cars, not buildings, as the most dynamic factor. Their movement from suburbs into the city center by day, and out again at night, subtly changes the city’s thermal landscape over the course of a single day.
So Can We Cool Cities by Changing Car Colors?
The authors are careful to note that their study provides a snapshot—measurements taken during peak sunlight on a single summer day. It doesn’t account for seasonal shifts, cloud cover, or the growing presence of electric vehicles. But even so, the findings hint at a low-tech, relatively cheap way to reduce urban heat exposure.
From a policy perspective, Matias and her colleagues suggest several ideas. City planners could:
- Incentivize light-colored vehicles in heat-prone zones
- Install shade structures in parking areas
- Encourage use of reflective coatings on cars
- Consider color-based parking restrictions in critical areas
There’s even a case for embedding these strategies into the design of electric vehicles, which—though cooler-running—still have the same heat-radiating surfaces as their combustion-engine cousins.
More fundamentally, the study challenges a blind spot in how we model city temperatures. Buildings and streets are typically treated as permanent structures. Cars, meanwhile, are considered mobile sources of emissions. But this research shows that their physical presence—even when parked—can be just as impactful.
