How heat makes our cities sweat at night

Beautiful data visualization in newspaper De Standaard today, who show-case the newest conclusions from our citizen science project. Here, I provide a shortened English summary of the longread, but for the full beauty of the visuals, you MUST check out the original story here!

Text by Ine Renson, translated largely by

Where we can cool our gardens with greenery and shade during the day, we are pretty defenseless against urban fever at night. The 5,000 lawn clouds of CuriousNoses in the Garden provide a unique insight into that dynamic.

It took some getting used to after that cool spring, when in mid-June the ventilators had to be brought out after all. Especially for residents of large cities, ‘urban fever’ set in. During the night of June 16 to 17, many were tossing and turning at temperatures that hovered between 20 and 25 degrees around midnight. In the surrounding countryside, it was often much cooler.

We know that the heat island effect, in which cities are significantly warmer than the surrounding area, exposes city dwellers to heat stress at higher rates. But much remains to be discovered about its exact dynamics. With their temperature data recorded every 15 minutes, the 5,000 lawn clouds from CuriousNoses in the Garden provide a unique insight into that process.

To study the heat island effect, we look at the air sensor at 12 centimeters above the ground. The temperature there is similar to the temperature we ourselves feel in our garden. The heat island effect can be observed most clearly at night: buildings, asphalt and concrete absorb heat during the day and give it off again in the evening.

It’s harder to sleep in the city

On that warm night of June 16, it was on average two degrees warmer at midnight in Flemish cities than in the countryside. At the coolest time of the night, the difference was three degrees. That doesn’t seem like much, but then it is the average temperature of all urban gardens versus the average temperature of all rural gardens. If you know that local weather conditions, soil type or relief also leave a strong mark and that our countryside is highly urbanized, those few degrees of difference are very significant, says Jonas Lembrechts, ecologist at the UAntwerpen and scientific supervisor of the project. ‘Despite all the variation, the fingerprint of the heat island effect remains. Compare it to global warming: two degrees doesn’t seem like much, but behind that average there are huge differences.’

Comparison between temperatuur (at 12 cm height) in the city and the countryside during a warm, clear night (16-17 june).

The Flemish average masks large local differences between cities and their surrounding countryside. In Antwerp, at midnight, you had peaks of up to 23 degrees and more in the center, while in the surrounding countryside it often stayed below 17 degrees. Across Flanders, the contrast is even greater: between the warmest and coolest gardens, there was a difference of over 15 degrees at midnight.

That makes a huge difference when you’re sleeping, says Lembrechts. Up to 18 degrees you sleep comfortably, but above that temperature it becomes more difficult for many people. The warmer it is outside, the more difficult it is to get rid of the heat that’s hanging in the house. In the countryside it usually cools off at night, so you can ventilate. In the city, this is then no longer possible. The only thing left is energy-hungry air conditioners. That drives up your electricity bill and, on top of that, those air conditioners heat up the outside air even more.’

The 5,000 lawn clouds beautifully illustrate that rhythm through the night. Consider this animation of the night of June 7-8: at 8 p.m. it is still about the same temperature everywhere (orange and red dots). But as the evening and the night progress, you can see big differences. While it gradually cools down in the countryside (blue to dark blue), the heat lingers in several cities (orange and light blue dots).

This spring the temperatures were not extreme yet in the region. But this pattern clearly shows what we can expect during the next heat wave. As climate change makes our summers hotter, the health risks associated with heat stress will also increase.

There is no such thing as ‘the’ heat island effect

An interesting observation: there is no such thing as “the” heat island effect. Every city has its own dynamics. The larger and more densely built up, the greater the heat island effect. As the largest city, Antwerp stands out. But even in Ghent, Mechelen or Leuven the heat lingers until the early morning.

Cities like Genk or Kortrijk stand out less on the maps. ‘A lot depends on how compact a city is built,’ says Lembrechts. ‘Genk, for example, has a less densely built-up city center, but rather a wide-spread development. More compact cities have urban planning advantages, but must consider adequate cooling in their urban planning.’

Regional differences also stand out. For example, in the Kempen region and in Limburg there are often temperature peaks on hot days. The sandy soil heats up faster and there is no cooling sea breeze that brings relief. But during the night the Kempen and Limburg gardens cool down well. ‘Sandy soil heats up more strongly in the sun, but also quickly loses its heat again,’ Lembrechts explains. ‘So the difference between day and night is usually greater in the Kempen than at the coast.’

The heat island effect is also a dynamic phenomenon that can be experienced differently every night. ‘The effect is most pronounced during clear nights, warm or cold,’ says Lembrechts. But on cloudy or rainy nights, there is little sign of it. That’s when classic weather patterns take over, such as a rainstorm washing away the heat or a cold front moving across the country. That rain is often not distributed evenly across the region, so other patterns appear on the dot map.

The June RMI data show that it rained a lot less than average in the Kortrijk region, which meant that it was locally warmer at night than in other regions. These regional differences can influence the size of the heat island effect in each city from day to day.

An extreme example of the impact of weather phenomena can be seen on the night of June 20-21, when a heat storm rolled over Flanders, coming in from France. You can see clearly how a blue wave rolled from west to east, washing away all the heat.

A difference between day and night

But it gets really interesting when we compare day and night. Because during the day the heat island effect plays much less. The Curious Noses data already showed that in urban gardens you can cool down just as well in the shade of buildings and trees. How you design your garden has a big impact on how it feels during the day, but also on the warming of the soil, and thus how plants and soil life thrive in your garden.

This was evident when we looked at maximum temperatures in the garden soil. Contrary to expectations, there was no heat island effect on that dot map: urban garden soils are not necessarily warmer during the day than those in the countryside.

But on the urban fever at night and how we experience it when we sleep, we have less impact. The heat spreads through the city at night and lingers between the buildings like a warm blanket,’ says Jonas Lembrechts.

Whereas the soil temperature shows a diffuse picture during the day, you can see clear patterns in the map of the air temperature at night.

It is the first time that this fascinating difference has been mapped so accurately with a sensor network.

So are we defenseless against urban fever at night? ‘Not quite,’ says Lembrechts. ‘We still see big differences between gardens that are close to each other. You do have an influence as a gardener. This may have to do with the amount of greenery or paving in your garden, and how much heat your garden gives off in the evening. We want to analyze that further. In a garden with ten trees, you might have a cooler head start.’

And what if the neighbors also have ten trees? And the rest of the neighborhood too? Then together you have a small forest. That is the interesting question’, says Lembrechts: ‘We see our gardens as isolated places, but from a climate perspective they can form one big park. If we work together, perhaps we can provide a lever against the nocturnal heat island effect.’

This is then a matter of collective responsibility of neighbors, but also of urban planning and landscaping in which parks and natural areas close to the city can play a key role. ‘We’re going to use this beautiful dataset we are amassing to find out how big that collective effort has to be to have an effect,’ says Lembrechts.

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Exotic species enter through the city gates

3DLab-member Charly Géron’s second paper recently got featured on! Here is an English translation of the story they brought there:


Plants from warmer regions feel perfectly at home in our cities. From there, they can colonize the countryside.

Exotic animal and plant species that settle outside their natural range cause biodiversity loss. They can displace indigenous species. To be able to do tackle that issue, it is important that researchers gain insight into how this colonization process takes place.

For 24 exotic plant species that occur in Western Europe, scientists investigated from which climate zones they originated. They also looked at the habitats they tended to colonize in our country. This excercise showed that exotic species that are more often found in urban areas generally come from warmer and drier regions.

In cities it is often several degrees warmer than in the surrounding countryside. This is a result of the so-called heat island effect, caused by the large amounts of stone and concrete in the city. It is also often drier in cities because the large percentage of impervious surfaces prevent water from seeping into the ground. ‘This connection between urbanity and climate of origin is therefore not surprising, but it had never been thoroughly investigated before,’ explains ecologist and author Charly Géron (UAntwerpen and ULiège).

‘Cities can serve as an outpost from which exotic species can colonize other areas when it gets hotter and drier there as well,’ says Géron. ‘In addition to the direct negative impact of urbanization on nature, this favouring of non-native species provides an additional negative effect of cities.’ Better monitoring of which species are popping up in cities could help nip the advance of exotics in the bud at an early stage, the scientists suggest.

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It is possible!

I just received the beautiful lake-view above and, oh boy, is that an important picture!

It means that we made it happen, despite the ongoing pandemic and the kilos of extra administrative chaos it brought with it: we sent a research team to Abisko, northern Sweden!

Today, the first two members of the team arrived, later this month four more will dare international travel. What can and cannot be achieved in the field will remain a mystery for a while (the main bottleneck will be crossing the border to Norway), but at least several of our ongoing and new research ideas will see fulfillment.

And the significance of that can simply not be overstated.

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Fieldwork preparations

Hot summer day in the heathlands of Kalmthout, north of Antwerp, last week. With a team of two PhD students and four master students, we had a trial fieldwork day for this summer in northern Sweden.

Trying out vegetation surveys, preparing practicalities and looking at plants. 75% of species we saw in this Flemish heathland matched with those we find above the polar circle!

So ready to make this summer happen for the students, despite the tons of extra chaos due to Covid!

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The hunt for Arctic aliens

Whilst they do not hunt for extra-terrestrial aliens that may or may not be hidden under the ice (as some on the more unbridled sections of the internet would rather they did), hunting for terrestrial aliens is exactly what they do. Ronja Wedegärtner and Jesamine Bartlett recall their team’s expedition in the high-Arctic Svalbard to monitor alien flora and publish their latest research which presents the most complete survey of alien vascular species in the archipelago to date.

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The hunt for aliens on Svalbard © Lawrence Hislop, Norsk Polarinstitutt

You can read their full story here. Special shout-out for this fantastic animated movie to explain the risks of bringing sneaky beasties to the Arctic!

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The hottest lawns are not always in the city

According to the first results of our citizen science project ‘CurieuzeNeuzen in de Tuin’, lawns in urban gardens can also be quite cool. This came as a bit of a surprise.

[English summary based on today’s discussion of our results in De Standaard by Ine Renson]

Today, we are launching something amazing: the daily updated temperature maps of our 4400 temperature sensors in our citizen science network. These maps have been highly anticipated, as we all were wondering about the patterns that would show up on them. Most importantly, one thing we wanted to see: are garden soils in cities warmer than those in the countryside due to the heat island effect?

It turns out that they are not. The map of soil temperatures shows a fairly diffuse picture, where cities do not immediately stand out. This is perhaps different from what we expected to see, but it is precisely why it provides interesting insights.

Our TOMST-built lawn daggers have three temperature sensors: one at a depth of 10 centimeters, one just at the ground surface and one at 12 centimeters above the ground. Those three curves show a different rhythm through the day. The air temperature at or just above the ground fluctuates strongly, while the soil temperature goes up and down in a gentle manner. So an earthworm feels a very different temperature than the one we experience ourselves in the garden.

Soil as a buffer: the average soil- and air temperature in Flemish lawns on April 13th. The two curves show the air temperature (green) and the soil temperature (brown). Soil Temperature remains rather stable, while air temperature fluctuates heavily. Soil warms much slower than the air, and reaches its maximum temperature late in the afternoon. Infographic as appeared in De Standaard newspaper.

We see that the soil buffers the temperature fluctuations, and that this pattern occurs consistently in every garden in Flanders. Here, our gardens nicely follow the rules of the soil physics textbooks. But when we compare soil and air temperatures, we encounter a remarkable phenomenon. When we plot the nighttime minima of air temperature on a map, the cities clearly stand out (see map below). In the early morning, city gardens are clearly warmer than the surrounding countryside. On the map of maximum soil temperatures, the pattern is a lot more subtle. There you see a lot more local variation across the whole urban-rural gradient. Very interesting: you’ll find most of the warm gardens there in the city outskirts, so halfway the urban-rural gradient. From the soils’ point of view, urban gardens are not necessarily warmer than average.

Heat islands in the coldest nights. This map shows the minimum air temperature (12 cm height) on April 13th, and visualizes the ground frost in early morning. Cities immediately pop out as warmer, with differences across the region of up to 15°C (-10 to +5°C)! Infographic as appeared in De Standaard newspaper.

We believe this goes to the heart of how the heat island effect works. Structures like buildings and roads absorb a lot more heat, so the air temperature close to the ground heats up quickly. Wind and air currents cause that warm air to spread, even to places in the shade. So the city as a whole heats up. Much of that heat is re-emitted in the evening and retained between the buildings. That’s why the heat island effect in the city is so obvious at night.

In the soil, things are different. It absorbs heat by radiation from the sun, but that heat is not transmitted laterally as much as in the air. A soil that is covered with plants, and not asphalt or concrete, will heat up less. Also, a soil that is shaded by buildings or by trees and shrubs stays cooler The fact that cities don’t really stand out on the soil temperature map might thus be because city gardens are often smaller and therefore catch extra shade, highlighting the critical aspect of shade for cool garden microclimates, even in the heart of the city.

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