During heat waves, the temperature at ground level is sometimes different from one place to another. According to France Bakkar, head of climate activity at Egis, ” we can’t have the same temperatures and that’s normal “.
In June 2022, a heat wave put France to the test, with temperatures sometimes exceeding 40°C. This heat wave was characterized by its intensity, but above all its precocity. As we arrive at a time of the year when the days are the longest, the heat accumulated during the day has difficulty evacuating at night. As a result, the nights are heavy, especially in large cities. You must also have noticed that the heat on the ground is not always the same from one place to another. How is it possible ? What is this phenomenon?
The temperature change between soils, “ it’s normal »
“ Regarding the mode of occupation of the ground, we cannot have the same temperatures and that is normal. The natural soil (with vegetation) is going to be moist, have water, plants. This vegetation will create a kind of ‘evaporation’” explains France Bakkar, head of climate activity at Egis, a French consulting and engineering group. ” It is also a question of the property of the materials used in public spaces and buildings. When looking at these materials, there are two things. First, the albedo: the ability to reflect solar radiation (dark surfaces, asphalt, granite), which will raise the temperature like on the body of a car. Then there is thermal inertia: the fact of accumulating heat and releasing it at night, which will contribute to the fact that, even if there is no more sun, the heat will not go down. »
In cities, air, surface and ground temperatures are almost always higher than in rural areas. This phenomenon is known as urban heat island “, explains France Bakkar. “ The temperature difference can be significant because of the urban heat island phenomenon, sometimes even stronger at night. But, this is not the only reason for the temperature change between soils. ” That’s not all, there is another reason for this: the morphology of the territory where you are. Depending on the size of the building, the corridor and the winds, if the city is built in such a way that the winds do not circulate at all, or even if it is south facing with maximum sun radiation, it will cause a high heat sensation. »
“On an urban scale, this change results in differences in nighttime temperatures which can be 3 degrees in Paris and Rambouillet in normal times. If you are in a place where there is a lot of sun and little wind, the temperature can rise up to 10 degrees, as for the heat wave of 2003. The expert continues: But on a local scale, looking at a lawn surface and concrete, you can see a difference of up to 10 degrees. If we have a space with full earth and on the other side of the asphalt with black cars, the difference can go up to 30 degrees. »
What can be done to limit these extreme deviations?
What solutions are possible? “There are several points to keep in mind: the subject of revegetation is very important, because plants refresh the air. It would also be necessary to install fountains in the cities so that people can cool off, this will allow via evaporation to reduce the temperature of the grounds “, emphasizes France Bakkar.
The big cities, like Bordeaux or Paris, have already built large greening campaigns aimed at combating heat islands, and thus preventing them from turning into real ovens: “ it is also necessary to offer islands of freshness, as is the case in Paris. It is very important to have areas with shade, vegetation and water. »
” If we think about architecture, we also have to think about building design differently. Here again, the subject of ventilation is very important, because you need a thermal house, a through apartment, protection from the outside sun (with shutters). When you prevent the heat from entering, it’s already good. »
Pay attention to the air conditioning
Air conditioning devices release chemical refrigerants into the atmosphere that are harmful to the ozone layer, contributing directly to global warming. France Bakkar stresses that you have to be very careful about the use of air conditioning : “it is necessary to avoid the risk of bad adaptation, in particular air conditioning”. She adds that “ certainly, the air conditioning will bring freshness in the space in question where the device is installed. On the other hand, to create cool inside, it will send heat outside. If everyone chooses to install air conditioning at home, it will greatly increase the temperature in the street. »
Do our soil choices have an impact on pollution and global warming?
Today, intact soil that has grown naturally and is able to perform its essential ecological functions is invaluable. Not only can it provide drinking water, but it also serves as a natural habitat for countless organisms and is the basis for food production. On this subject, France Bakkar emphasizes: “Today, we put a lot of effort into exhaust gas, however we never worked on the subject of cars, even though that is what represents soil pollution on the Ile-de-France. -France. »
The choice of floors therefore has a very strong impact on the feeling of warmth felt in a given place. ” If we talk about global warming, the question of soil will rather concern sequestration capacities. We now know that artificial soil will not have the same sequestration capacities as natural soil. If we want to meet the challenge of global carbon neutrality, we know that we have to work on carbon neutrality in the soil. »