Heat wave: why is it hotter in cities?

While a heat wave is currently spreading in France, temperatures will skyrocket particularly in cities. This phenomenon, called “urban heat island”, is notably related to the absence of vegetation.

In rural areas, vegetation stores heat and releases it as evaporation at night. Thanks to this evapotranspiration, soils do not accumulate the heat received during the day, he explains. Meteo France.

In cities, however, heat is stored in building materials. The stones, concrete and glass will not only return heat like mirrors, but the facades will accumulate much of it. This trapped heat is released back into the atmosphere at night, keeping temperatures high.

In general, the urban heat island begins to grow in the late afternoon and increases in the evening, reaching its maximum in the middle of the night. Then a kind of “heat bubble” is created over the city. For an agglomeration like Paris, we can observe a difference of up to 10 ° C between the city center and the outskirts during a heat wave, says Météo France.

Finally, human activity also does not help to cool cities. Cars, public transport but also air conditioning produce heat on a daily basis. If the air conditioning cools offices and homes, its operation implies the rejection of hot air to the outside.

Note that to cope with the heat in the capital, the Urban Planning Workshop (Apur) has published a map with all the places where it is possible to cool off. Indicating the location of the usual fountains and nebulizers, free swimming areas and swimming pools, through museums, libraries and places of worship, it also classifies green spaces according to their exposure to the sun.

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This Sunday, August 15, five departments in southeastern France were placed under orange heat wave surveillance by Méteo France. These are Ardèche, Drôme, Var, Alpes-de-Haute-Provence and Alpes-Maritimes.

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