On Monday 18th July 2022, London experienced a heatwave with temperatures of 38°C. As an experiment I decided to measure the temperature of my neighbours flat roof below our bedroom window. It was 49°C. Not only that, it’s about 8m2 - so on one of the hottest days ever recorded in London we had the benefit of a enormous radiator making the environment even hotter. This is the Urban Heat Island Effect in action.
What is the Urban Heat Island Effect?
The urban heat island (UHI) effect is a phenomenon describing the elevated temperatures felt in towns and cities compared to rural surroundings as the heat retained by artificial surfaces is slowly released, keeping temperatures higher than in the countryside Cities are covered with human-made materials such as roads and buildings which absorb and retain heat better than natural surfaces like grass or woodland. Albedo describes how reflective a surface is. High albedo surfaces, such as white roofs, are reflective and absorb less heat than low albedo surfaces such as asphalt roads. Vegetation cools the air around it through the evaporation of water. Vegetation cover and albedo are two of the most important factors which determine the strength of the urban heat island effect.
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What does the impact of car paint colour have on urban heat islands?
As cities in Europe grapple to deal with increasing heat and the Urban Heat Island effect - the colour and materials of vehicles could soon become part of cities strategies to dealing with heat. A research paper, "The underestimated impact of parked cars in urban warming", showed that in Lisbon at 36°C, a black painted car raised air temperatures nearby by as much as 3.8°C compared to the asphalt beside it. A white car raised air temperatures by 1°C less.
What are the most common vehicle colours in the UK?
The Top 5 most common car colours in the UK (src: DVLA, 2020) are:
In London, TfL data (2023) suggests there are 2.6 Million registered vehicles which would equate to about 520K black vehicles, enough to fill 730 football pitches.
What can we do about it?
There are a number of levers we can pull to mitigate the impact:
1. Make the surface area of vehicle surfaces smaller.
Micromobility (e.g. E-Bikes) have a much smaller relative surface area and can serve many of the "mobility jobs" which larger vehicles are making in urban centres (75% of all car trips are less than 10km). Microcars (L6e / L7e) such as the Toyota FT-Me have inherently smaller surface area due to its smaller proportions.
2. Use paint colours with a high albedo (e.g. white)
In many Middle Eastern countries such as Saudi Arabia, white is the preference of many vehicle owners due to the improvement in reflecting sunlight and keeping the vehicle interior slightly cooler.Bura
3. Green-roofs
In 2013, Spanish landscape artist Marc Grañén and green roofer Alex Puig developed a green-roof bus using a light hydroponic foam.
For urban-centric vehicles, in theory it would be possible to have sedum roofs irrigated automatically by Hydrogen Fuel Cell EVs (the waste product from FCEVs is water). Vegetation has a cooling impact on the environment and can thus reduce urban temperatures.
4. Incorporate more solar panelled roofs
Solar panelled surfaces do have a higher albedo than black but still lower than light coloured surfaces. Having said that, they have the additional benefit of providing cable-free charging. The solar panel in this concept is estimated to generate 20-30km per day of energy - which is roughly 35% to 50% of the daily distance travelled by cars in Europe.
5. Less vehicles in cities
Robotaxis and car sharing services could reduce the number of vehicles required by improving asset utilisation.The problem isn’t going away and so to misquote Henry Ford - “[Vehicles can be] any color the customer wants, as long as it’s white.”
