Explainer: Urban heat islands and how to cool them
Have you ever noticed how the evenings cool down more – and faster – in rural areas than in cities? Urban areas also tend to get hotter during the day than any nearby green areas. In fact, during the day, urban locations in the United States run between 0.5° and 3.9° Celsius (1° to 7° Fahrenheit) warmer than outside areas. Even at night, cities get about 1.1 to 2.8°C (2 to 5°F) warmer. It is a phenomenon known as the urban heat island effect.
It is caused by the difference in materials that cover the ground in urban areas and in rural areas.
In the country, evaporation of water from the soil and the leaves of plants helps to cool the air. However, cities have covered much of the land and once green areas with roads, sidewalks, and buildings. With fewer plants, these areas also have less evaporative cooling. Dark roads, roofs and other building materials also have a lower albedo, so they reflect less sunlight. This increases even more how much solar heat a city absorbs.
Cities also have a harder time getting rid of heat, notes James Voogt. He is an urban climatologist in Canada at Western University in London, Ontario. Cities have concrete and asphalt instead of plants and soil. “The buildings in urban areas are usually composed of materials that can store heat better,” he says. Tall buildings also prevent warm air from rising up and out.
Finally, while those cars, buildings use heating and air conditioning equipment, dump cities’ waste heat directly into the atmosphere, “Voogt explains. That waste heat contributes to the solar energy stored in the asphalt and concrete and retained by the tall buildings.
Satellites can image heat islands from space and record how their temperatures rise and fall. They show that cities get hotter during the day. But at night the temperature differs most between city and country, says Voogt. A city will have to lose more heat. And, he adds, “the city is also getting rid of the heat more slowly.”
But cities don’t have to be so hot. They can increase their albedo, for example by painting roofs white or other light colors. Or they can add reflective coatings to dark roofing materials.
Planting more greenery also helps. Alberto Martilli is a climate scientist. He works at the Center for Environment, Technology and Energy in Madrid, Spain. He is part of a team that has shown how trees and greenery help cool cities.
“On a summer’s day on the street you can feel the heat,” explains Martilli. “This is due to the fact that the surface heats the air.” Those rising waves of energy are called sensible heat – because you “feel” it or feel it. By adding more trees and parks, he says, urban areas could change the way we experience that heat. By this he means that they can change sensible heat into another type: latent heat.
Plants trap sensible heat and convert it into latent heat — the energy that turns liquid water into water vapor. That vapor escapes from plant leaves, adding moisture to the air. But it doesn’t heat the air. Vegetable spaces and cool colors in a city convert more sensible heat into latent heat, say Martilli and his colleagues.
The scientists analyzed how many green spaces and cool roofs – with a highly reflective surface – could influence the temperatures in Barcelona, Spain. They used a computer model of a heat wave that hit the city in 2015. More cool roofs, it turned out, could have dampened the heat in the nearby area by 0.67 °C (1.2 °F) during the day. Such roofs had little effect at night.
That may not sound like much. But add a little water to the green and it becomes more intense. Green spaces, the model showed, would have cooled their areas by an average of 0.15 °C (0.27 °F) during the day and 0.17 °C (0.32 °F) at night. Plants release some of the extra water through their leaves. This pumps up the cooling capacity of the plants. Combine bleach-painted roofs with spraying greenery and the affected areas should now be about 1.26°C (2.27°F) less hot in the middle of the day.
That is cool.
Martilli and his colleagues their findings published in May 2021 urban climate.