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Cooler city? Greener city!

Cooler city? Greener city!

Have you noticed that temperatures can rise when going from the countryside to the city? This is known as the ‘heat island effect'.

5 minutes
17 August 2020

You might have already noticed that temperatures can rise considerably when going from the countryside to the city. And especially at night or when we’re suffering under a heat wave. Science has dubbed this phenomenon the ‘heat island effect’. KU Leuven researchers are taking a closer look at the heat island effect in Leuven and have already come up with some refreshing insights. Based on their research, the city hopes to bring the mercury down a bit. Even at this early stage, the researchers emphasise that more green spaces can work wonders.

The fact that it’s generally warmer in the city than in the surrounding rural areas is partly due to the heavy presence of asphalt and concrete, says researcher Eva Beele of the Division of Forest, Nature and Landscape.

Visit flanders3 web
© Visit Flanders

‘Those construction materials absorb sunlight easily. They are often dark in colour, meaning they don’t reflect much of the incoming solar radiation. They also emit more infrared radiation and have a high heat capacity. So they absorb a lot more heat than, say, a lawn or public square with trees. High and densely packed buildings also play a role. In a densely built-up city, solar radiation can’t easily escape, and because the wind speed is lower, less heat is dissipated.’

Since last year, Beele and her colleagues have been conducting research into the heat island effect in Leuven and its districts under the supervision of Professor Ben Somers. For this they joined forces with the City of Leuven, Leuven2030 and KMI (Royal Meteorological Institute). The researchers will determine, among other things, exactly how high the rise in temperature is and what role green spaces can play.

Weather stations

The researchers installed more than 90 small weather stations in Leuven and the districts, both at public locations in the city centre and in private gardens. For the latter, they called on volunteers who wanted to lend their garden for the project. ‘We eventually installed these sorts of weather station in 64 gardens. The others are located at points round the city including at Martelarenplein, Ladeuzeplein and at Museum M.’

The researchers also mapped green spaces using satellite images. ‘We’ve also already inventoried about 180 of them and described them in detail,’ says Beele. ‘Which tree species do you see? Which plants? Which parts of gardens are devoted to terraces or a lawn, and how much space do they take up?

The station in the Museum M courtyard
The station in the Museum M courtyard
© M. Reyniers

In addition, we divided 'greater Leuven' into twelve zones based on the density and height of buildings and the percentage of vegetation present. The intention was to distribute the weather stations over those zones, in order to get the most complete picture possible.’

The weather stations collect data all day long – every sixteen seconds, to be exact. ‘The weather station measures air temperature, which is most important to us, but also UV radiation, precipitation, wind speed and direction, and relative humidity,’ says Beele. ‘This data is sent to our server and converted into data blocks of five minutes. Then we can really get started.’

Heat builds up during the day, but at night it “escapes” less quickly in the city than, say, in the countryside.

Cool green

For her master's thesis, Beele – then still a final-year student of bio-engineering sciences – analysed the first data that the research yielded. She discovered that the average temperature at night during a heat wave is five to six degrees higher in Leuven and Kessel-Lo compared to the other municipalities. Differences during the day were smaller. ‘This is due once again to the denser buildings,’ says Beele. ‘Heat builds up during the day, but at night it “escapes” less quickly in the city than, say, in the countryside.’

Beele was also able to demonstrate that the presence of vegetation has a local cooling effect. The amount of green space as well as its shape and location can have an effect on the air temperature in the city centre. ‘A high percentage of green space made up of gardens can help, but the most efficient are parks and large areas of green space with a complex shape and consequently a large perimeter.’

In addition, ‘high green spaces’ and ‘low green spaces’ have a different influence on the temperature, says Beele. ‘Trees mainly help to lower temperature by a number of degrees during the day, while a lawn or low plants are more efficient at night. Low green spaces reflect a good portion of solar radiation and store very little heat. In turn, trees provide shade and cool the environment through "evapostranspiration” – evaporation through the stomata of leaves. The tree extracts energy or heat from the environment, which lowers the air temperature.’

Heat stress

The researchers hope to collect even more data this summer to map the relationship between green spaces and temperature reductions. They also want to learn more about ‘heat stress’. ‘We want to assess people's well-being at extremely high temperatures and their general comfort in the city centre,’ says Beele.

‘We know, for example, that young children are sensitive to extreme heat, or that the death rates in the elderly are higher during a heat wave. The city and Leuven2030 are also interested in the effect of heat on vulnerable groups. That’s why we’ve installed weather stations at social housing locales, day care centres, residential care centres, etc.’

The intention is that the City of Leuven will soon begin processing the research results. ‘Based on our data, the city wants to update its plans for heat and drought, among other weather phenomena. For each location, we want to be able to say what amount of green space and what type of vegetation is needed to lower the temperature as efficiently as possible. Further research will be needed to answer those questions, but one thing is certain: adding more green spaces is never a bad idea.’

Curious about the heat island effect in Leuven? Follow the weather stations at