Denmark’s urban heat mapped as dense city areas run 2.5°C hotter at night

New data from the Danish Meteorological Institute (DMI) shows the warmest spots in Copenhagen, Aarhus, Aalborg and Odense, with dense urban districts running up to 2.5°C hotter at night than greener areas, DR reports.

Researchers at DMI’s National Centre for Climate Research have produced the first detailed heat maps of the four largest Danish cities. The maps reveal that built-up areas with asphalt, roofs and brick retain solar energy, raising temperatures compared with parks, forests and waterfronts.

In Copenhagen, the compact north-western neighbourhoods are notably warmer than districts further south. On the hottest days, night-time readings can differ by 2–2.5°C between the coolest spots near Amager Fælled and the warmest on Nørrebro, according to Hjalte Jomo Danielsen Sørup, one of the researchers behind the mapping.

“Two degrees actually matters for the health risks linked to higher temperatures,” he said.

The phenomenon—known as urban heat islands—occurs because hard surfaces absorb the sun’s energy and re-emit it as heat, whereas vegetation uses energy for growth and evaporation. The effect is most pronounced at night, when built-up areas cool more slowly.

Sørup hopes city authorities will use the maps to target vulnerable groups. Measures can include planting trees to provide shade and reduce heat radiation from walls and ground.