Some cities stay hotter at night due to their layout

Researchers at The National Center for Scientific Research (CNRS) have investigated why it is hotter in some cities after the sun goes down compared to others. They found that more organized cities with straight and perpendicular streets trap more heat than those with less structure and organization.

When the air temperature is higher in the city than its surrounding rural areas, urban heat islands (UHIs) are created.

Around 80 percent of the urban population in the United States is affected by heat islands, which have a detrimental effect on health, worsen air pollution, intensify energy consumption, and generally lower the quality of life. The environmental and economic costs of UHIs are barely being offset by any current strategies.

The research team examined the major factors that drive warmer urban temperatures, such as the thermal mass of buildings. The experts wanted to gain insight into how much of the heat being absorbed during the day is being released or retained at night and why.

The researchers documented the footprints of buildings and the temperatures recorded in urban and rural areas over the course of several years. By studying more than 50 major cities, the research team was able to investigate the link between urban geometry and the effects of UHIs at night.

The study revealed that buildings release heat depending on their level of spatial organization. The experts demonstrated that a high level of urban organization, which is found in most North American cities, corresponds with more pronounced UHI effects and greater heat retention. The opposite is true for less organized cities, which release more heat at night.

The findings of the research could be used to explore new urban planning methods that could help to achieve optimal energy management.

The study is published in the journal Physical Review Letters.

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By Chrissy Sexton, Earth.com Staff Writer