[Article] Urban heat islands, heat waves, and heat stress in Brussels: What field measurements reveal

A city’s temperature does not tell us how heat is experienced across its neighborhoods. And the heat does not necessarily disappear when the sun goes down.

Heat waves are often described through a single temperature reported for an entire city. But people experience heat locally: on their street, around their home, and in neighborhoods with different building densities, vegetation, and opportunities to cool down.

In our study, we investigated how urban heat islands, heat waves, and outdoor heat stress interact in Brussels, using field measurements collected during summer 2022.

Together with @Mohamed Elhadi Matallah, @Aissa Boulkaibet, @Hicham Fawzi Arrar, @Tianyi Wang, @Deepak Amaripadath, @Atef Ahriz, and @Mohamed Amer, we analyzed measurements from four urban sites and one suburban reference site, covering 60 summer days.

The challenge is not only how hot a city becomes. It is also how much heat remains at night.

During the monitoring period, three heat waves occurred. Compared with non-heat-wave days, heat waves amplified urban heat island intensity by up to 2°C at night and 1°C during the day.

This matters because nighttime cooling provides an opportunity for people and buildings to release accumulated heat. When urban neighborhoods remain warmer, that opportunity becomes more limited.

We also assessed outdoor heat stress using Humidex, which combines air temperature and humidity. The analysis identified up to 38 hours of intense discomfort at a monitored site during the study period.

Measuring temperature is essential. Understanding its implications for people makes those measurements useful.

The results also revealed periods when urban sites were cooler than the suburban reference during daytime. This urban cool island effect reminds us that urban thermal conditions vary with location and time. A neighborhood cannot be understood through a single maximum temperature.

For urban planning, the message is clear: heat adaptation needs local evidence and attention to both daytime exposure and nighttime recovery.

Our findings support identifying priority areas for vegetation, cool roofs, and heat warning systems, particularly where vulnerable populations may face greater exposure. These interventions were proposed as applications of the findings; their cooling performance was not tested in this study.

The study covers five locations and one summer monitoring period. Wider monitoring and more detailed analysis of urban form, materials, and radiation are needed before generalizing the results across Brussels.

But the direction is important:

Urban climate data should inform where we intervene, whom we protect, and how we evaluate progress.

Should cities assess neighborhood nighttime cooling as systematically as they assess daytime heat extremes?

Matallah, M. E., Boulkaibet, A., Arrar, H. F., Wang, T., Amaripadath, D., Ahriz, A., Amer, M., & Attia, S. (2025). Data-driven characterization of urban heat island intensity, heat wave, and heat stress interactions in Brussels: A field measurement study. Sustainable Cities and Society, 135, 106974.

📘 Full article: https://doi.org/10.1016/j.scs.2025.106974

📚 Learn more about our research: https://www.sbd.uliege.be/

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