Invisible, the heat? It all depends on how you look. These images, taken with an infrared camera in different places in Montreal, show how our urban development can make heat waves even more unbearable or, on the contrary, reduce them.
A question of reflection
The first phenomenon to consider is albedo, which is the reflectivity of surfaces. Dark surfaces absorb solar radiation (low albedo) and light ones reflect it (high albedo).
When heated, the materials themselves emit radiation. In the temperature range that interests us, it is infrared radiation. The measurement of this radiation by the camera enables conclusions to be drawn about the temperature of the objects.
However, these objects can also reflect infrared light from the environment. As in the optical sector, smooth surfaces such as glass, metal or mirrors reflect particularly well.
Influenced by these parasitic reflections, the temperature measurements of infrared cameras are therefore not perfect. Still, they can give a good idea of where the heat is hiding.
A question of the material
It was about 25°C in Montreal when this picture was taken. Compared to the pavement, the white lines were several degrees cooler. By putting your hand on the floor, you could easily feel the difference.
Parking lots are a major contributor to urban heat islands. Black bitumen, a very low albedo material that makes up part of asphalt, can reach 80°C in summer.
“The best way to avoid heat islands is to prevent heat from penetrating the materials,” explains Michel Vaillancourt, professor of civil engineering at ETS, who is interested in the thermal properties of coatings.
Choosing aggregate in the asphalt or applying white paint may fade the pavement. Some foundation materials can improve heat dissipation into the basement.
Although promising, because of the cost, these solutions ‘are not being applied on a large scale but rather on an ad hoc basis’, warns Mr Vaillancourt.
A matter of convection
If we don’t block the absorption of solar radiation, the materials start to retain heat. They then release it during the night, leaving no time for the roost to cool down.
People perceive ground heat in different ways. First they bathe in the air heated by the ground (convection). Then they absorb infrared radiation from the surface (radiation).
Our perception of heat would depend roughly equally on convection and radiation.
In addition to allowing us to feel the warmth of the ground, convection and radiation, along with evaporation, are the pathways through which urban environments shed their thermal energy.
According to a study published in the journal Nature in 2014, convection would also play a particularly important role in the formation of heat islands.
In fact, it increases when the wind rubs against the unevenness of the ground and becomes turbulent. The lack of harshness in cities – compared to forests, for example – would largely explain the concentration of heat in cities in temperate climates.
A question of human activity
Heat accumulation is also caused by human activities.
“Every car generates heat in the city,” explains Céline Campagna, researcher on climate issues at the National Institute of Public Health of Quebec (INSPQ).
As can be seen in this picture, vehicle exhausts are hot. The wheels, heated by mechanics and friction on the ground, are also very hot.
According to a study conducted in Manchester, car traffic in the city generates a third of man-made heat. The rest comes mostly from buildings.
A comprehensive shift towards active and public transport has therefore made it possible to reduce parking needs, but also to reduce heat emissions from vehicles, argues Ms. Campagna.
Air conditioners also radiate heat into the neighborhood. At night they can increase the outside temperature by 1°C. Experts therefore advise against installing them en masse to adapt to the climate crisis.
A matter of vegetation
When there’s a heat wave, not all neighborhoods are the same.
We see in this photo the Boulevard de l’Acadie that separates Parc-Extension, one of Montreal’s most deprived neighborhoods, from the affluent city of Mount Royal.
In thermal terms, the contrast between the two sectors is evident. The many trees and greenery of the lush residential area lower the ambient temperature by several degrees.
The vegetation works in two ways. First, the evaporation of water from the leaves “consumes” heat. Then their shade protects the ground from the sun.
Céline Campagna believes that cities need to be massively greened. This has to happen in all neighborhoods, otherwise the better off will flock to cooler areas and “green gentrification” will ensue.
In order to keep apartments cool, other strategies – such as using roller shutters on windows or improving insulation – can be advantageously combined with greening.
“There are all sorts of possibilities,” says the INSPQ researcher. Government simply needs to be more direct in its requirements, rather than leaving it to the goodwill of project promoters. »