Urban Heat Stress: Rotterdam and Antwerp Under the Lens

This summer was characterised by heat and drought. Wildfires raged through Western Europe, rivers were almost running dry, and citizens in urban areas were challenged by heat stress. This summer was an excellent motivation for the work we do in UrbanAIR. Heat in cities is challenging for stakeholders such as city administrations, both now and in the future, when temperatures are expected to increase. What can they do to cool their city? And what temperatures should they prepare for?

In this post

• The Urban Heat Island (UHI) gap: The late-June 2026 heatwave pushed city-centre temperatures in Rotterdam and Antwerp up to 7 °C above the surrounding rural areas.

• Green infrastructure performance: Larger parks with water bodies — like Rotterdam's Kralingse Plas — reduce local heat below 38 °C vs. 41 °C in paved urban areas.

• Future Risk #1: Re-running the same heatwave under a future +3 °C climate (Pseudo-Global Warming method) shows city centres could run 3–4 °C hotter still, with parks providing less cooling than they do today.

• Future Risk #2: Both cities would see the daily heat arrive earlier and linger later into the evening, and a tropical night would become more extreme, with parts of the city staying above 26 °C during the night.

  
  

The heatwave of June 2026

The most severe heatwave in Western Europe this summer occurred at the end of June, lasting roughly from 17 to 30 June. A ridge of warm air from northern Africa reached into Europe, transporting exceptionally warm air northward. A so-called Omega block formed. This is a persistent atmospheric situation in which a high-pressure system is surrounded by lower-pressure systems to the southwest and southeast. This relatively stable large-scale weather pattern lasted for about two weeks, transporting warm air northward. It also prevented colder and wetter weather, as is often associated with low-pressure systems. This situation is also sometimes called a “heat dome” in the media.

Fig.1 Geopotential at mid-troposphere on 26 June 2026, showing high and low pressure areas and transport of cooler and warmer air. Source: adapted from Wetterzentrale.de.

Method

Many temperature records were broken, and many heatwave red alerts were issued. However, most official measurement stations are located outside cities. What effect does the heat actually have on cities? Two UrbanAIR partner cities, Rotterdam and Antwerp, were analysed. And not only the actual June heatwave was examined, but also how the same heatwave could develop in a future climate. This was done using a technique called Pseudo-Global Warming (PGW). With this technique, past weather events are re-simulated under future climate conditions to assess how global climate change might alter them. The future climate conditions are based on CMIP6 data, the latest generation of global climate model projections used internationally by climate scientists. In this case CMIP6 data assuming a global temperature increase of 3 °C was applied.

First, the heatwave was simulated using a computer-based weather model. The simulations were performed using the HARMONIE-AROME weather model, a high-resolution model widely used by multiple European weather services to simulate local-scale weather in detail. A control simulation (CTL) represents the present-day heatwave at 26 June. A pseudo-global warming simulation (PGW) represents the future version of the same heatwave at the same day. The weather was simulated at different horizontal resolutions at a domain covering (parts of) the Netherlands and Belgium. For the analysis on city-scale we use the highest resolution of 200m.

Fig.2 Model domains of 2500 m, 500 m and 200 m resolution

Second, after the present and future simulations are available, the data is analysed. To compare the city with the surrounding green areas, all grid points that belong to the built-up areas of Rotterdam and Antwerp were determined, together with the green areas in and around the city. The focus was on the cities themselves. Therefore, other urban areas nearby were excluded. The mean temperature at 2 m height was then calculated for the urban areas of each city and the surrounding green areas, for both the present (CTL) and future (PGW) simulations.

Fig.3 Built-up areas within Rotterdam and Antwerp, larger green areas inside or directly outside the cities, nearby urban areas, water and administrative boundaries

Rotterdam

In the Netherlands, the heatwave was at its warmest on June 26. Mean temperatures in Rotterdam reached over 39 °C, with local temperatures in the south of Rotterdam reaching over 41 °C. Larger green areas within the city clearly show lower temperatures. In particular, larger parks with water bodies, such as the Kralingse Plas in the northeast of Rotterdam, seem to provide some cooling for citizens, with temperatures below 38 °C.

Fig. 4: Temperature development for Rotterdam for present and future climate inside the urban area, urban green spaces and rural green

Fig. 5: Rotterdam in present and future climate at daytime peak and early morning. Colours show temperature; blue represents water bodies

Within the city, temperature differences of up to 4 °C occurred during the day. Outside the city, however, temperatures could drop below 35 °C, while the air was coolest over water bodies. In the future climate, the same heatwave day would look very different. A large part of Rotterdam would experience temperatures above 43 °C, while the cooler parks and surrounding countryside would still reach over 40 °C and 37 °C, respectively.

The air cooled rapidly from 16:00 onwards. Nevertheless, temperatures remained above 30 °C until 21:00. In the future climate, cooling starts later, around 17:00, and temperatures remain above 30 °C until about midnight. In both scenarios, there is a tropical night, with temperatures not dropping below 20 °C. The night is even more extreme in the future climate, with temperatures not dropping below 26 °C in parts of the city.

Antwerp

In Antwerp, the heatwave was even more intense. In the present climate, average temperatures reached up to 41 °C over large parts of the city and would exceed 45 °C in a future climate. Larger parks also provided less cooling than in Rotterdam. During the night, average temperatures dropped to similar levels as in Rotterdam, in present and future climates.

[Figure 6 placeholder: Temperature development for Antwerp for present and future climate.]

The surrounding area of Antwerp, however, cooled more than the surrounding area of Rotterdam during this particular night. There may be several reasons for this, such as differences in how the heatwave developed, or the different spatial configuration and sizes of green and urban areas surrounding the cities. Another possibility may be more sandy soil near Antwerp, giving higher temperatures during the day and cooling at night, in comparison to a more water-retaining clay soil around Rotterdam. This is subject to further analysis.

Also, both cities have several things in common. Temperatures are higher inside the cities, with differences of up to 7 °C compared with the surrounding areas. In the future climate, city centres will be 3–4 °C warmer than now, while parks will provide less cooling than they do today. During the day, the heat starts earlier in the morning and cooling starts later in the evening, giving a larger window for heat stress.

Fig. 7: Antwerp in present and future climate at daytime peak and early morning

Parks

Especially parks and other green spaces give a little relief to citizens who cannot easily go out of the city for a break. Parks are, in general, a little cooler, by up to a few degrees, than the urban areas. Larger parks and green areas seem to be cooler than smaller parks, and especially large water bodies cool the air very locally.

Fig. 8: Urban green or park size versus temperature during the warmest and coolest times of day for Rotterdam and Antwerp, present and future heatwave situations.

Summary

The late June heatwave already had a major impact on citizens. In the future, however, the same weather event could lead to substantially higher temperatures and much greater heat stress in both cities.

What can we learn from this case for urban policy and planning?

Urban Design: Expand the blue-green infrastructure. Parks, especially with water bodies, give relief to citizens, both during the day, where water cools the air locally and trees provide shade, and during the evening and night, since green areas cool faster than the nearby paved area. This is relevant both during current and future heatwaves.

Public health & infrastructure: National or local heat action plans should plan for higher temperatures and a longer high-heat window in future, e.g. for opening cooling shelters. Also nighttime recovery will be reduced, with more extreme tropical nights.

Local differences: Every city is different; therefore cooling strategies should be based on local analysis of each city’s building and green-space configuration.

This study is a first impression of what can be done with 200m Pseudo-Global-Warming data. Further analysis is needed to get a better understanding of the impact of heatwaves and future climate on cities.

Team behind it? The KNMI (Royal Netherlands Meteorological Institute) Team.