Background Information

When heatwaves hit, cities are particularly affected. In addition to high daytime temperatures, urban areas often experience more tropical nights, during which temperatures do not fall below 20°C.

To show how pronounced the effect can be, the German Weather Service (DWD) compared the occurrence of hot days (>30°C) and tropical nights at weather stations inside and directly outside the city of Berlin over a period of 35 years. The comparison shows that the difference is particularly strong at night, with tropical nights occurring considerably more often in the city.

Why does the UHI effect occur?

The Urban Heat Island (UHI) Effect describes the phenomenon whereby urban areas are considerably hotter than their surrounding suburban and rural areas. It is a growing urban climate challenge driven by climate change and rapid urbanization. 

The effect was particularly visible in Brussels during the June 2026 heatwave, when land surface temperatures went above 47°C in the city, compared to a drastic difference of around 24.5°C in the nearby Sonian Forest. 

Moreover, buildings, roads and other concrete surfaces contribute significantly to the extended time heat stays in the city owing to the absorption of heat throughout the day and the slow release that happens until the late hours. At the same time, cities lack vegetation and natural soil which would otherwise support cooling through evaporation.

Consequently, urban areas cool down much more slowly than their surrounding areas. It is, however, important to notice that the intensity varies between and within cities, depending on factors such as the size, inner structure, surface materials, vegetation and the release of heat from human activities.

What are the implications of the UHI effect?

Extreme heat poses particular risks to vulernable groups including outdoor workers, people working in hot indoor environments, children, older adults and people with pre-existing health conditions. However, the effects of urban heat are not limited to urban residents. People who commute from outside areas because of work or other activities are also exposed to the Urban Heat Island Effect that potentially increases the temperature by over 10°C compared to its suburban or rural areas.

This effect is generally strongest during cloudless, low-wind nights, when heat is “stored” during the day and only slowly release when the sun stops shining. More tropical nights make cities feel sleepless. Research indicates that higher night-time temperatures are associated with insufficient and poorer-quality sleep. The effects can be particularly pronounced among older adults and people with lower incomes, who may have fewer resources to adapt their homes or living conditions to high temperatures.

CLIMACARE and the UHI effect

If cities are to sleep better, they need to cool down more effectively. Through CLIMACARE, we are working with cities to reduce heat exposure and create healthier, more climate-resilient urban environments. This includes urban greening, cooling and reflective surfaces, sustainable water management, as well as shading solutions and climate-resilient public spaces.