First Heatwave of the Year: Five Urban Cooling Tips

Stuttgart, Germany, June 19, 2026. Scorching hot roads, stuffy air, no respite at night: Germany is in the midst of its first heatwave of the year. Cities are hit hardest by heat. Sealed surfaces and the lack of shade reinforce the urban heat island (UHI) effect: asphalt and concrete store heat during the day and releases it at night – urban temperatures can be up to ten degrees (or more) higher than in the surrounding areas. “In tropical nights, when temperatures do not fall below 20 degrees, even night-time ventilation hardly provides any relief from the heat,” warns Gregor Grassl, Associate Partner and Head of Green Urban Development at Drees & Sommer SE. The consulting firm specializes in construction, real estate and infrastructure. “Only passive cooling measures efficiently reduce urban temperatures: sufficient shade, greening and the right materials are key. Air-conditioning is counterproductive: air cons transport exhaust heat to the outside and thus worsen the heat island effect,” he continues. Gregor Grassl has five recommendations for cities, towns and villages, including a fast solution and a long-term strategy.

1. Provide shade

Shadow takes effect immediately and does not cost very much. Trees are the best choice for this: not only do they provide shade, they also cool the surrounding air by evaporation. Where trees cannot be planted on impermeable surfaces, new solutions are needed. The city of Stuttgart shows how it is done: two ecotriis have recently been installed at Marienplatz – a Stuttgart-based startup approach provides up to 18 square meters of shade, using climbing plants on canopies made of wire netting. The system is solar-driven and controllable via an app. And the 3D-printed concrete base is designed as a sitting area.[i] Gregor Grassel emphasizes: “It is not the same as a tree, but it solves a problem where trees cannot grow.” In addition, bus-stop shelters, awnings and covered spaces are useful in all environments where people are exposed to heat: schoolyards, playgrounds and highly frequented squares.

2. Use light-colored surfaces, open up sealed ground areas 

“People who wear black in the sun sweat more. The same is true for cities,” says the expert. Dark asphalt stores the heat; bright surfaces reflect it. Many municipalities still make little use of the Albedo effect, although the Albedo measurement is an uncomplicated and economic approach. Cities and towns can lighten the colors of roofs, squares and sidewalks at a later stage, without having to re-lay them. Natural stone paving or light-colored concrete measurably reduce temperatures more than dark asphalt. “Many people underestimate how important the choice of material is in climate adaptation, though the immediate effect is obvious,” adds Gregor Grassl. The surface also plays a major role: open spaces and paths do not need to be entirely covered in asphalt. “Gravel surfaces and turf brick stones reduce the heat effect, and they are often cheaper than asphalt. Another point in favor: in the event of heavy rainfall, water soaks into the ground, instead of running off the surface. The city of Dormagen shows how this works in practice: impervious surfaces have been removed and drinking fountains installed in highly frequented public squares. These measures help to reduce the risk of heat stress.[i]

3. Passively cool buildings

What many people do not know: air-conditioning systems actually reinforce the urban heat island effect. Gregor Gassl explains: “Air-conditioning systems such as split design coolers are particularly bad for the outdoor air because they run at the times when the air is hottest. Although they cool the interior of the buildings, the exhaust heat emitted at the same time heats up the outdoor air. This means that even more cooling is needed. It is a vicious circle.“ He suggests the use of low-tech systems for buildings. This involves installing a large heat storage mass in the building which can be cooled by the outdoor air at night. The doors and windows are kept closed by day. “But this principle does not work when it is too warm outside at night. So in the future development, even buildings which are actually energy-saving and sustainable might need to be refurbished because of climate change,” he continues. As an alternative, underfloor heating can relatively easily be used as underfloor cooling in summer. One option is to cool the water circulation system at night and transport the heat of the interior rooms to the outside. It is also possible to use ceilings as cooling surfaces.

4. Further develop the blue-green infrastructure

Single trees provide shade, but only interconnected expanses of greenery can cool entire cities. Trees, green roofs and façades, parks and water surfaces create a blue-green infrastructure: They provide shade, evaporate water, cool down the temperature of the air and enhance the air quality.

“The implementation of greening measures takes time, but pays off several times over. Greening cools the air, improves the air quality, helps biodiversity, and creates spaces where people enjoy spending some time,” Gregor Grassl points out. Here are two examples: in Rastatt, around 1,000 new trees were planted in 2020 as part of the city’s climate adaptation plan, developed by Drees & Sommer together with the city administration.[i] The Lake Phoenix development in the German city of Dortmund shows how blue-green infrastructure can be integrated into the planning work from the outset: a 24-hectare lake with a surrounding park was created on an ex-industrial brownfield site, formerly home to the PHOENIX steelworks. The lake collects rainwater, and water evaporates during hot weather. It also offers a high quality of stay. [ii]

5. Building compactly, making smart use of vertical space

Gregor Gassl says: “Tall buildings provide mutual shade and protect apartments from overheating. If this is to work, the window surfaces must not represent more than 40 percent of the outer façade. Glass palaces are expensive in their consumption of energy, both in summer and in winter, because glass has a poor insulation value.” Another plus point: skyscrapers create turbulence and updrafts. This improves the overall air flow in the whole district. If used systematically, tower buildings can improve cooling and have a comparable effect to natural landscape features such as rivers which offer cooling with the water, act as a conduit for fresh air and provide a natural ventilation zone because of the movement of the water.

Taking precautions against heat is mandatory, not optional

When the Climate Adaptation Act (Klimaanpassungsgesetz) came into force in 2024, it enshrined precautionary measures against heat in federal law for the first time. As a result, the German government, the federal states and local authorities have a responsibility to take action. A current survey shows that 41 percent of all municipalities have already implemented measures. About the same percentage plans the adoption of specific initiatives. However, only twelve percent of them has a formal climate adaption plan.[i] There continues to be a significant discrepancy between the design of individual steps and the adoption of a strategic planning process. This gap increases with each heatwave. “Heat is no longer a weather event that you simply wait out. It is absolutely essential for local authorities to include it in their planning,” Gregor Grassl sums up.