Why Cities Stay So Hot at Night
Cities amplify local temperatures, creating heat islands that affect residents' health and comfort, a major challenge as the climate warms.
During a heatwave, city centres can be significantly hotter than surrounding areas, with the greatest difference occurring at night, precisely when the body needs to recover. This phenomenon is known as the urban heat-island effect.
The city as a heat-storage machine
Three mechanisms combine:
- Thermal storage. Concrete and asphalt absorb heat during the day and release it at night.
- Lack of vegetation. Cities lose the natural cooling effect of evapotranspiration.
- Human-generated heat. Traffic, air conditioning and industry add heat to dense urban areas where buildings obstruct air circulation.
A public-health issue
Hot nights prevent the body from recovering, significantly increasing mortality during heatwaves, especially among older and isolated people. The urban heat island therefore amplifies every heatwave for city residents, just as these events become more frequent.
Cities can be cooled
The solutions are known and already being introduced in many places: extensive planting through street trees, parks and green roofs; bringing water back through fountains and permeable surfaces; using light, reflective materials; and redesigning cities to create natural ventilation corridors.
These investments make urban areas more liveable summer after summer.
- The city-countryside temperature gap is greatest at night, when the body needs to recover.
- Dark materials, limited vegetation and human-generated heat are the three main causes.
- Vegetation and permeable surfaces are among the most effective measures.