Why it’s so hot at night in some cities

Urban Heat Island

It is hotter in the city than in the countryside or in the suburbs, especially at night. But this phenomenon varies depending on the city, where it can be more or less hot. Researchers from the MSE International Joint Unit (CNRS / MIT) and the Interdisciplinary Nanoscience Center of Marseille (CNRS / Aix-Marseille University) in France have shown that the organization of cities is at the origin of this phenomenon: more a city is organized, like most North American cities with very straight and perpendicular streets, the more it traps heat. Conversely, the more disorganized a city, like some of the historic cities of Europe, the more heat is easily evacuated. These results, published on March 9, 2018 in the journal Physical Review Letters, open up new avenues for optimized urban planning and energy management.

Urban heat islands (UHI), as those urban heat concentrations are called, result from increased air temperatures in cities, compared to rural and peri-urban areas. In the United States, this phenomenon affects over 80% of the population living in urban areas. These islets may imply an increase in energy demand (for air conditioning for example), air pollution, and the deterioration of living comfort and health. Strategies for reducing these heat islands have been put in place in some cities, such as the establishment of green spaces, but the environmental and economic impacts of these heat islands, on a national or even regional scale, remain to date little quantified.

Researchers have therefore studied some major parameters of temperature rise, such as the thermal inertia of buildings and their ability to radiate the energy absorbed during the day during the night. For this purpose, they used the temperatures recorded in cities or in the countryside over several years as well as information on the spatial footprints of urban constructions, combined with a heat dissipation model. This approach has made it possible to demonstrate, for the fifty cities studied, such as New York, Chicago and Boston, that the effects of the nocturnal heat islands vary according to the geometry of the urban fabric.

Buildings can indeed exchange energy, more or less easily according to their degree of spatial organization. Researchers measured this parameter using physics tools that reduce the complexity of the city to a statistical description, that is, to “batches” of relevant buildings. They showed that the more a city is organized, like most North American cities, the greater the effect of urban heat islands (UHI) and the more heat is trapped, and vice versa for “disorganized” cities.

In countries with warm or temperate climates, the “UHI” effect significantly increases the energy bill. On the other hand, for regions with cold climates, it can potentially reduce energy demand. In fact, in view of the urban growth forecasts, it becomes possible to identify the countries that have the greatest opportunity to use the “UHI” and thus help policy makers to optimize the energy demand of buildings and reduce afterwards their carbon footprint on the scales of cities, but also regions, or even states.

Shakes Gilles

Editor of The Talking Democrat. He enjoys bike riding, kayaking and playing soccer. On a slow weekend, you'll find him with a book by the lake.