Urban Heat

source: le Monde, Climat

author: Guillaume Krempp

translation: doxa-louise

Heat Wave: 'Between a rural area and the city, the temperature
spread can reach more than 10°

...Temperatures that are being amplified in the cities by hot spots
of urban heat. Erwan Cordeau, researcher on climate, air and energy for the
Institut d'aménagement et d'urbanism, gives us the causes as well as possible
measures to attenuate this phenomenon.

How can one explain these high temperatures in the city?

Erwan Cordeau: These higher temperatures are linked to urban hot spots.
These are the result of the omnipresence of mineral surfaces that store
luminous energy. The sidewalks, roads and buildings store solar energy
whereas vegetation tends to attenuate by cooling the air.

During the day, heat accumulates in these materials before being progressively
given back at night, as urban surfaces very slowly cool down. The true hot spot
phenomenon thus only happens at night, as urban surfaces are slow to cool.
The more the streets take on the form of urban canyons, that is narrow and with
high walls, the more heat has trouble dissipating. The city then has evermore
difficulty cooling down.

In rural spaces, the ground, vegetation and aquatic surfaces will give back the
water they hold through evaporation. in so doing, they are cooling the atmosphere.
Thus the great difference between urban and rural space.

What is the order of temperature differences created by hot spots?

Between a hot urban street and a green space, one might se a difference of 4 to
5° C. Between a rural space and this same street, the difference can be up to 10°
C, even more given how strong a heat wave is.

Does atmospheric pollution amplify the negative effects of heat waves?

Most certainly. The degradation of air quality is worse in a heat wave. A moment
of high heat, often accompanied by an absence of wind, stops the dispersion of pollutants.
Ozone in particular will stabilize, which raises major problems of public health
for city dwellers. In particular, one should not forget that certain constituents of
atmospheric pollution (fine particles, nitrogen dioxide, ozone...) create dangers
to health, especially for the young.

What measures should cities take if they want to attenuate urban hot spots?

There are many solutions but these are case specific. Each city and each climate will
require a different response. Upstream, it is possible to use lighter coverings for
façades and roofs. Thus luminous energy from the sun can be sent back outward.
As well, it is possible to use construction materials that store less heat. Construction
rock has the advantage of very slowly capturing heat but it gives back this heat with
the same slowness, which makes night temperatures higher.

Urbanism already offers some solutions. One must find water points in a city in order
to permit evaporation phenomena to better regulate ambient temperature. Trees are also
an efficient countermeasure to urban heat. They are at once a physical obstacle to light,
and they can act as powerful regulators of temperature as they can go deep underground
for water and emit water vapor in the atmosphere. As well, vegetation of roofs and
lawns can have a beneficial impact on temperature regulation. Although this effect is
very locality specific when compared to trees.

Are cities taking sufficient measures to fight against hot spots?

There are still many places where increased greenery would reduce hot spots. Cities
need to do everything possible in this regard.

During heat waves, opening public spaces at night would be an additional measure
helping citizens better deal with hot spots. Body temperature needs to go down after a hot day.
Another solution are shaders, physical protection against the rays of the sun. One is
starting to see these, covered with photovoltaic panels, on top of parking areas, thus
creating energy while producing shade. This technical solution could protect the façades most exposed to the rays of the sun
.
Finally, what works best are information campaigns to restore a certain common sense.

What are these reflexes to protect oneself from the heat?

Keeping the drapes drawn to keep one's interior cool. As well, one should use air
conditioning with parsimony because every degree taken from a room is that much
heat ejected into the city, which raises high temperatures all the more.

Personal hygiene is important to behaviour intimes of heat waves. Drink a good
deal of water, little alcohol and take lower calorie meals are all necessary measures
to get through heat waves.