Say no to black cars? How your vehicle's colour could be heating up your city

On a hot summer day, most people are familiar with the stifling sensation of walking past a parked car that seems to radiate intense heat.

According to a recent study by researchers at the University of Lisbon, the colour of a vehicle can significantly influence the temperature of the air around it.

The findings shed light on a previously underappreciated contributor to the urban heat island (UHI) effect, a phenomenon where cities are consistently warmer than nearby rural areas.

The research indicates that the heat emitted by thousands of stationary vehicles, especially those painted in darker shades, could be pushing city temperatures higher than previously estimated.

Understanding the Urban Heat Island (UHI) effect

The urban heat island effect occurs when urban landscapes absorb and trap heat due to human activity and the prevalence of heat-retaining materials like asphalt, concrete, and steel.

These materials warm up quickly under direct sunlight and release heat slowly, causing elevated temperatures that persist well into the night.

According to Copernicus, the European Union’s Earth observation programme, cities can remain up to 10°C hotter at night compared to surrounding rural areas. This difference is particularly striking during the summer months, when the sun’s intensity is at its peak.

Daytime heat is compounded by limited airflow in densely built environments, while human activity — such as vehicle traffic, industrial operations, and the widespread use of air conditioning — adds additional heat to the urban atmosphere.

At night, instead of cooling off, city streets, rooftops, and infrastructure release stored warmth, making recovery from the day’s heat difficult.

This effect is especially dangerous during heatwaves. In Europe, where nearly 70 per cent of the population resides in cities, the UHI phenomenon has become a growing public health concern.

In recent years, European cities have endured record-breaking temperatures, with several locations surpassing 40°C during summer heatwaves.

Prolonged exposure to extreme heat can lead to heat stress, which not only causes discomfort but also accelerates biological ageing, worsens mental health conditions, and increases vulnerability to dehydration and respiratory illnesses.

Children, older adults, and individuals with pre-existing medical conditions are especially at risk.

In cities like London and Paris, where night-time temperatures can stay 4°C above surrounding rural areas, the lack of relief after sunset only compounds the dangers.

How cars contribute to heat in urban areas

While factors like buildings and roads are well-documented contributors to the UHI effect, the new Lisbon study highlights a less obvious culprit: parked vehicles.

The research, led by Márcia Matias and her team at the University of Lisbon, explored how vehicles interact with their environment under intense sunlight.

The study found that when cars are parked in open spaces, their surfaces can drastically influence local temperatures — particularly in crowded urban centres where cars occupy a large percentage of road space.

“Now picture thousands of cars parked across a city, each one acting like a little heat source or a heat shield,” says Matias. “Their colour can actually shift how hot the streets feel.”

To understand this effect, the researchers parked two cars — one black and one white — under a clear summer sky with an ambient temperature of 36°C. After five hours of exposure to direct sunlight, they measured the air temperature around both vehicles.

The results were striking. The black car raised nearby air temperatures by as much as 3.8°C more than the adjacent asphalt road, while the white car produced a far smaller increase.

The difference stems from how colours interact with sunlight.

Light-coloured vehicles, like white or silver, reflect 75 per cent to 85 per cent of incoming sunlight, limiting heat absorption.

Dark-coloured vehicles, such as black or deep blue, reflect only 5 per cent to 10 per cent of sunlight, absorbing the rest and heating up significantly faster.

Unlike asphalt, which has greater thickness and warms more slowly, a car’s thin metal body heats rapidly. Once hot, it radiates that heat into the surrounding air, creating small but intense zones of elevated temperature.

For pedestrians, this can translate into noticeable discomfort — the “blast” of heat one feels while walking past a parked car on a sunny day.

“You know when you walk past a parked car on a hot day and feel the heat radiating off it?” Matias says. “That’s real! It’s not your imagination.”

When scaled up across an entire city, where thousands of vehicles line streets and parking lots, these individual heat sources can collectively amplify urban temperatures, especially in already heat-stressed areas.

The researchers calculated that in central Lisbon, parked cars can cover up to 10 per cent of road surfaces. This has a measurable impact on the overall reflectivity of streets.

Repainting dark-coloured cars to lighter shades could double the street-level reflectivity from 20 per cent to nearly 40 per cent, significantly cooling the air immediately above the surface during sunny, low-wind conditions.

Why this is a public health disaster

The findings have important implications for public health, especially during heatwaves.

When cars radiate extra heat, they worsen conditions for pedestrians and cyclists, who are already exposed to higher temperatures due to the UHI effect.

The additional heat can contribute to heat exhaustion, dehydration, and other heat-related illnesses, particularly for vulnerable populations like children and the elderly.

Moreover, densely packed urban environments often lack sufficient green spaces or shaded areas, leaving residents with few options for cooling down.

“Vehicles, especially fleets of dark-coloured ones, may be a more important factor in this heat dynamic than previously thought,” says Matias.

Sarah Berk, a researcher at the University of North Carolina, highlights the novelty of this approach. “Harnessing light-coloured vehicles as a mitigation strategy for urban heat is particularly novel,” she says.

Previous studies have focused primarily on modifying rooftops and pavement to increase reflectivity, but this research suggests that cars themselves can be part of the solution.

What solutions can be implemented

The researchers recommend several measures that city planners and governments could consider:

• Repainting fleets of vehicles: Large groups of vehicles, such as municipal service trucks, delivery vans, and taxis, are especially visible in urban areas. Repainting these vehicles in lighter, heat-reflective colours could provide immediate benefits by reducing localised heat.

• Integrating shade structures in parking areas: Installing shaded canopies or tree cover in parking lots and along streets can significantly limit the amount of direct sunlight cars absorb.

• Encouraging light-coloured vehicle production: Automakers could play a role by prioritising lighter paint options and incorporating heat-reflective paints into vehicle design.

• Promoting electric vehicles: While primarily aimed at reducing emissions, electric vehicles also produce less waste heat compared to conventional internal combustion engines.

Cities across Europe are already taking steps to adapt to rising temperatures and combat the UHI effect.

• Barcelona, Spain: The city has established “climate shelters” — public buildings like libraries, schools, and museums that remain open during heatwaves to provide residents with cool, safe spaces.

• Breda, Netherlands: Local authorities have transformed riversides into gardens and replaced large areas of concrete with greenery. Today, 60 per cent of the city consists of green space, and by 2030, Breda aims to be one of Europe’s most nature-rich urban areas.

While these projects are essential, they require significant investment and time to implement.

In contrast, interventions involving vehicles — such as repainting and shading — are cost-effective, fast to deploy, and highly visible.

Although this study focused on Lisbon, its implications are global.

Many cities around the world face similar conditions: high vehicle density, limited greenery, and rising temperatures due to climate change.

With inputs from agencies