Scientists explain the ‘heat sink’ effect that makes Paris feel like an oven

Concrete, air con and squares - scientists explain why Paris feels so much hotter than the surrounding areas when heatwaves strike.

Scientists explain the 'heat sink' effect that makes Paris feel like an oven
All those buildings are making Paris feel a lot hotter than it is. Photo: AFP

Researchers call it a heat sink – a passive exchanger designed to dissipate heat – but when the sink is an actual city, its concrete and asphalt sweltering in the heat, it feels more like an oven to those who live and work there.

The phenomenon where cities are hotter than the surrounding countryside is actually known as an urban heat island, and while the effect exists year-round, it is most acutely felt during a heatwave.


Urban microclimate

In the countryside, vegetation uses sunlight and water from the soil for photosynthesis which in addition to converting carbon dioxide into oxygen, also releases water into the air. 

This helps disperse solar energy and cool the surrounding area.

Meanwhile, in cities, there is not nearly as much vegetation to disperse heat.

Moreover, asphalt and cement absorb solar energy during the day and release it during the night.

The result is the city is hotter than the surrounding countryside, as buildings and streets act as a giant heat sink, and this is most noticeable during heatwaves.

France's national meteorological service has found an average annual difference between Paris and surrounding rural areas on the order of 2 to 3 degrees Celsius (4 – 5 degrees Fahrenheit).

Illustration photo: AFP

During a heatwave, the difference “can reach close to 10C”, said Meteo-France. 

During the 2003 heatwave, when the daytime temperature hit 40C it fell to between 23C and 26C during the night in the areas of the city that are the greenest or got the most breeze. But in the city centre it fell only to 28 degrees.

This urban microclimate “aggravates the effects felt, in particular during the night, a critical period when normally the human body recuperates,” said Aude Lemonsu, who heads up Meteo-France's research centre.

These urban heat islands thus magnify the effects of climate change which is expected to increase the number of heatwaves, researchers warn. 

Air conditioning

To the list of factors making cities feel like ovens, there is another one which must be added: air conditioning.

“The more you use air conditioning in buildings, the more you heat the outside air,” noted Lemonsu. 

The vicious circle of air conditioning is abetted by the design of major cities.

A study published in a March 2018 issue of Physical Review Letters found that the more a city is designed into a square grid pattern, the more it traps heat.

The orientation of buildings can also play a role – letting in more light lets in more heat. 

Cool down 

While redesigning cities would be difficult and prohibitively costly, there are things that can be done.

Planting more trees and plants can help cool the air locally.

“You can even create vegetal walls and roofs to reduce the heat created by buildings, said Amandine Crambes, an urban engineer and planner at France's environment and energy management agency, ADEME.

When buildings are built and renovated the choices made can have considerable consequences. 

If the outside temperature is 26C, the surface of a dark coloured roof can reach 80C, said Crambes.

Meanwhile, the surface of a light coloured roof will be around 45C as it reflects more sunlight and absorbs less heat, and a roof covered with plants won't rise above 29C, she said.

Such cool roofs are gaining traction, and some cities like Los Angeles have even experimented with painting streets white.

Another possibility that can be explored: water. 

We all know from experience that a shower provides relief from heat. 

The city of Paris has been experimenting in recent years to see if watering streets during extreme heat can help reduce temperatures

 “The question of urban resilience is being taken into account more and more,” said Crambes, noting that authorities face difficult choices constrained by costs and the interest of different stakeholders, including property owners.

A nap in the park

For cities, it is fast becoming imperative to make the most of the cool islands within the heat islands.

Paris frequently opens up public parks normally closed at night to allow residents to sleep in the open air when a heatwave makes it difficult to spend a restful night in an apartment without air conditioning. 

Paris and the central French city of Lyon have created maps showing where people can escape the heat, like air-conditioned museums and pools.

The city of light has also been working to adapt school courtyards to climate change. Out are stark asphalt squares and in are trees and water fountains.

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Scorching summer was France’s second hottest on record

Three heatwaves since June produced France's second-hottest summer since records began in 1900, the Météo France weather service said on Tuesday, warning that scorching temperatures will be increasingly common as the climate crisis intensifies.

Scorching summer was France's second hottest on record

With 33 days of extreme heat overall, average temperatures for June, July and August were 2.3C above normal for the period of 1991-2020.

It was surpassed only by the 2003 heatwave that caught much of France unprepared for prolonged scorching conditions, leading to nearly 15,000 heat-related deaths, mainly among the elderly.

Data is not yet available for heat-related deaths this summer, but it is likely to be significantly lower than 15,000 thanks to preventative measures taken by local and national authorities. 

Most experts attribute the rising temperatures to the climate crisis, with Météo France noting that over the past eight summers in France, six have been among the 10-hottest ever.

By 2050, “we expect that around half of summer seasons will be at comparable temperatures, if not higher,” even if greenhouse gas emissions are contained, the agency’s research director Samuel Morin said at a press conference.

The heat helped drive a series of wildfires across France this summer, in particular a huge blaze in the southwest that burned for more than a month and blackened 20,000 hectares. 

Unusually, wildfires also broke out even in the normally cooler north of the country, and in total an area five times the size of Paris burned over the summer. 

Adding to the misery was a record drought that required widespread limits on water use, with July the driest month since 1961 – many areas still have water restrictions in place.

MAP: Where in France are there water restrictions and what do they mean?

Forecasters have also warned that autumn storms around the Mediterranean – a regular event as air temperatures cool – will be unusually intense this year because of the very high summer temperatures. A storm that hit the island of Corsica in mid August claimed six lives. 

“The summer we’ve just been through is a powerful call to order,” Prime Minister Elisabeth Borne said on Monday, laying out her priorities for an “ecological planning” programme to guide France’s efforts against climate change.