Urban Heat Islands: Causes and Countermeasures

In short. An urban heat island forms because densely built-up, sealed surfaces store heat during the day and release it only slowly at night. As a result, city centres can be noticeably warmer than their surroundings, especially at night. The causes are well understood, as are the countermeasures: greenery, water, desealing, light-coloured surfaces. The challenge lies in long-term, area-specific implementation.

Why this matters for your municipality. The heat island effect hits hardest precisely where many people live and work. Night-time heat above 20 degrees, the tropical night, prevents the body from recovering and is a key factor in heat-related health impacts. Knowing the effect in your municipality allows you to take targeted countermeasures rather than acting across the board.

What causes the heat island effect?

Several factors work together. Sealed surfaces such as asphalt and concrete absorb a lot of solar energy and heat up strongly. Dense, tall development traps heat and hinders air exchange. A lack of greenery means a lack of evaporative cooling, since plants and moist soil cool through evaporation. In addition, there is waste heat from traffic, buildings and air conditioning systems. The UBA describes that this allows cities to remain several degrees warmer at night than their surroundings.

Why is the night particularly critical?

During the day, many people find heat unpleasant but can avoid it. At night, the body is supposed to recover. If it remains hot in the city, this recovery does not happen, and the strain accumulates over several days. This is why nocturnal heat exposure is often more meaningful for health than the daytime peak. Good heat maps make exactly this nocturnal component visible.

Which countermeasures work?

The most effective approaches address the causes. More greenery, especially large trees, provides shade and cools through evaporation. Desealing reduces the ground's heat storage. Light, reflective surfaces heat up less. Water features and evaporative elements cool the surrounding area. And preserving fresh air corridors ensures that cooler air can flow in from the surrounding countryside. Combining measures usually has more effect than a single one.

The honest limitation. The heat island effect can be reduced but not switched off. Large trees take years to decades, desealing is expensive and often difficult within existing structures, and fresh air corridors conflict with development pressure. Which measure has the greatest effect where depends on the local situation.

When KomAdapt is worthwhile. Before deciding on individual measures, you need a picture of where your heat islands are located and who is affected there. KomAdapt's risk and exposure analysis combines thermal exposure with development and land use, making visible where countermeasures bring the greatest benefit.

_How much warmer is a city compared to its surroundings?_ The difference varies depending on the size of the city, its development and the weather situation. The UBA describes that city centres can be several degrees warmer at night. Reliable figures for your municipality can only be obtained through a local analysis. _What is a tropical night?_ A night in which the temperature does not fall below 20 degrees. Such nights are particularly relevant for health because night-time recovery does not occur. They occur more frequently in urban heat islands. _Are fresh air corridors really important?_ Yes. Cool air from the surrounding countryside can flow into the city through open corridors and provide cooling there. If such corridors are built over, this effect is lost. _Do white roofs help against heat?_ Light, reflective surfaces absorb less solar energy. As a single measure its effect is limited, but combined with greening and desealing it contributes noticeably.

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