The urban heat island phenomenon

Our climate is changing, and this will result in new challenges. Among the long list of these challenges is the phenomenon of urban heat islands (UHI).
Urban centres are known to accumulate heat during the day and release it at night, creating areas that are abnormally hot compared to more vegetated areas.

Although spring 2024 was the second least sunny in the history of meteorological measurements in Luxembourg (since 1947, source: Meteolux), this does not represent the situation on a global scale. Indeed, the Copernicus programme’s readings are unequivocal. The year 2024 will in all likelihood be the hottest year in the history of meteorological measurements.
As engineers within our LSC Engineering Group design office, we are particularly aware of the growing impact of this phenomenon on urban environments. The 1.5°C limit advocated by the Paris Agreement (COP 21) has already been exceeded, only eight years after the agreement was signed. The consequences for our latitudes are already predictable: an increase in average temperatures, longer heatwaves and droughts, torrential rains, etc.

At the urban level, the heat island phenomenon will intensify. This refers to a localised increase in heat in urban centres, particularly at night.

Several factors contribute to this phenomenon: anthropogenic factors (vehicles, air conditioning, etc.), solar radiation, convective flow (wind), latent flow (evaporation and evapotranspiration linked to vegetation) and conductive flow (the ability of building materials to transfer energy to the ground rather than to the atmosphere).

While human influence on solar radiation is low, the same cannot be said for all the other parameters:

• Emissions of hot air into the atmosphere by industry, transport and air conditioning systems
• Densification of high-rise buildings, limiting fresh air flows and wind corridors
• Removal or reduction of vegetation and therefore evapotranspiration
• Choice of building materials (buildings and outdoor facilities) with low capacity to diffuse thermal energy into the ground and/or with high potential for energy accumulation and nocturnal diffusion into the atmosphere

For all these reasons, the phenomenon of urban heat islands, as its name suggests, is naturally concentrated in dense urban areas.

We have developed powerful tools to assess and anticipate the effects of UHI. It is possible to model this phenomenon at different scales (territory, city, neighbourhood) and with different levels of detail. Simplified models focus on ground temperature and do not take into account anthropogenic or convective flows. They offer a simplified and rapid approach and are a useful tool for communication and educational purposes in particular.

In principle, ground temperatures are calculated for a theoretical heatwave day with maximum sunshine (summer solstice, 21 June). Certain surfaces, such as asphalt, can reach temperatures of nearly 50°C under these conditions. The colour of materials also plays a significant role. Naturally, vegetated and shaded areas are areas where temperatures are significantly lower. These areas provide refuge for both wildlife and users.

Our expertise also enables us to design detailed models that incorporate complex parameters, such as wind corridors and building facades, to provide accurate and tailored recommendations. The results obtained are more relevant in that they refer to the perceived temperature rather than the ground temperature. This is known as PET (Physiological Equivalent Temperature).

By incorporating advanced bioclimatic approaches into our studies, we aim to develop innovative solutions to mitigate the effects of UHI.

These models enable us to map the existing situation and make recommendations for the projected situation in order to improve the UHI phenomenon. These improvement measures can also be modelled.

Furthermore, when planning a new neighbourhood, it is possible to use its 3D model and data on infrastructure development to model the UHI phenomenon and take action very early on in the planning process to prevent thermal issues from arising.

Thanks to our multidisciplinary expertise, including landscape architects, botanists, hydrologists, environmentalists, urban planners, architects, and civil engineers, we are able to coordinate the efforts necessary to anticipate and respond effectively to climate challenges.

Only through coordinated work and addressing the issue very early on will it be possible to design the neighbourhoods of tomorrow so that they are resilient to the climate challenges that await them.

Rainwater is not intended to be transported to a conventional treatment plant due to its volume and variability in flow rates. Older sewerage systems mix wastewater and rainwater, leading to polluting overflows when it rains. Newer networks separate these two types of water, storing rainwater before discharge, but without systematic treatment.

However, there are simple and robust solutions for passively treating rainwater before discharge, and Luxplan has been working and taking action in this area for over ten years.