It is tempting to treat trees as a thermostat. More green, less heat. The reality is messier, and that is what makes it interesting. Heat in a city is not evenly distributed, and neither is the greenery that mitigates it. To see where planting works and where it barely helps, you need to map a whole city at once.
Reading cities from above
That is what Monika Kuffer does. She is a professor and expert in remote sensing. She reads cities from above using satellite data. She measures the temperature of the city’s surfaces, its roofs, roads, and patches of green, and links it to the shape of the neighbourhood underneath.
The pattern is not random. Working across a string of European cities, a study supervised by Monika has shown that the form of a street shapes how hot its surfaces get. How tightly the buildings pack together, how much of the ground is dark and paved, how much green breaks it up: these decide how much heat a place holds. And the hottest, most paved, least green streets are rarely where the wealthy live.
This is why the leafy street wins. Trees do not just look cooler. They shade the paved surfaces that would otherwise soak up the sun, and they release moisture that pulls heat out of the air. Where a street has neither shade nor greenery, the asphalt and brick bank the day’s heat and hand it back long after sunset. Into the rooms where people are trying to sleep.
Heat has an address
This is where the heat stops being about comfort. Monika’s and ONEKANA colleagues’ recent work places low-cost sensors into slums to measure what people there actually live with. Under bare iron roofs, with no money for air conditioning and power too unreliable to run it, homes stay hot through the day and often through the night. Her colleagues report residents taking sleeping pills simply to get a few hours’ rest in rooms that were never built to cool down.
In one Indian city, the poorest and most tightly built areas sat in the hottest spots, yet the data could not prove which came first. But the reason is rarely a mystery. With no affordable housing, the urban poor are pushed onto the land nobody else wants, which is often the hottest and most exposed there is.
What a satellite cannot feel
There is one more catch, and Monika is quick to point it out. A satellite sees surfaces, not people. The temperature it reads is the heat coming off the asphalt and roof tiles, which is not the same as what your body feels. That also depends on humidity, on wind, and on whether you can reach shade.
So she does not stop at the satellite image. In places that have no weather stations at all, she, together with colleagues and local residents, set up low-cost sensors to measure the air people actually breathe. It is slow, local work, and it is the only way to capture what a map from orbit leaves out. “A satellite gives me the temperature of a roof. It cannot tell me about the night someone spent under it, unable to sleep. A heatwave passes for us. For the people under those roofs, it never really does”, she says.
So, more trees?
Yes, but plant them where they earn it. Greening cools a neighbourhood when there is enough of it, placed on the truly hot streets, not the ones that already feel fine. As a slogan, “plant more trees” scatters shade where it is needed least. With a map like Monika’s, it can put cool air where it changes lives, not just where it adds comfort. The streets that need it most are not the ones we notice during a bad week. They are the ones where the heat never leaves.
