Het Zoniënwoud

A difference of nearly 23°C — between Brussels city centre and the Sonian Forest, just a few kilometres away, on the same day: the urban heat island effect in action. 

As an omega block weather system drives a severe heatwave across Western and Central Europe, Copernicus Sentinel-3 data captured on 24 June 2026 shows land surface temperatures exceeding 47°C on sealed urban surfaces in Brussels, against 24.5°C beneath the tree canopy of the nearby Sonian Forest. Belgium issued a national heatwave alert the same day, with air temperatures forecast to reach up to 38°C later in the week.

The contrast is consistent with the urban heat island effect: dense buildings and sealed surfaces retain and radiate heat, while vegetation provides measurable cooling. Land surface temperature measures the heat of soil and built surfaces — it differs from air temperature, but directly affects the thermal environment experienced by people in densely built areas.

EEA assessments show that urban populations across Europe face disproportionate heat exposure — and that reducing it requires sustained investment in green infrastructure and planning that puts people's health at the centre.

Pascal Brackman