Higher temperatures in cities

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Our cities are particularly vulnerable to higher temperatures in a warmer climate. The urban heat island effect and mean radiant temperature affect cities.

Illustration showing how heat is reflected in hard surfaces in cities.
Cities are particularly vulnerable in a warmer climate. As a general rule, the hard surfaces in cities absorb more sunlight and store more heat than green spaces. This heat is given off at night, raising the air temperature.

Built-up areas create higher temperatures than their surroundings, and this has a number of different causes:

  • As a general rule, the hard surfaces in cities absorb more sunlight and store more heat than green spaces. This heat is given off at night, raising the air temperature.
  • During the day, the limited vegetation produces less cooling (through transpiration) compared with the surrounding countryside.
  • Heat radiation from hard surfaces, raising the perceived temperature for people in the city.
  • Waste heat from heating and cooling buildings, transport and various activities is another source of heat.

The increased temperature in built-up areas is called the urban heat island effect.

Graph showing the urban heat island
The air temperature is higher in densely built-up urban areas, and is lower in less built-up areas and natural areas such as parks and alongside water. Enlarge Image
Map showing average air temperature, 2 metres above ground during summer 2014 in Stockholm
Average air temperature, 2 metres above ground during summer 2014. The urban heat island effect in Stockholm results in temperatures that are up to 3°C higher than the surrounding countryside. The Baltic Sea has a cooling effect, while the warmer waters of lakes increase the average air temperature nearby. Enlarge Image

Mean radiant temperature

As well as the air temperature, people are also affected by heat radiation from surfaces.

Mean radiant temperature is a measurement describing the perceived effects of air temperature and radiation. Just as the wind chill effect is often used to describe how the temperature is perceived when the wind blows, mean radiant temperature is used to describe how the temperature is perceived with regard to solar and heat radiation.

The concept can be explained as an equivalent air temperature with the same effect on a person as the sum of the radiation components that the body is exposed to. Mean radiant temperature does not take wind, air humidity or cooling through evaporation into account, but is a good measure of perceived temperature during a heatwave when the wind speed is often low.

Mean radiant temperature varies significantly in a built-up environment, mainly due to shading conditions, but also as a result of the presence and type of hard surfaces. Mean radiant temperature can form part of useful data for climate planning.

City map showing Mean Radiant Temperature (MRT)
Mean radiant temperature (MRT) is reduced by 30°C in areas shaded by buildings and trees. The calculation results from the SOLWEIG model for Eskilstuna on 26 July 2014. Enlarge Image

Surface temperature calculated from infrared satellite images can provide a measure of how the heat radiation from the ground to which people are exposed varies in the urban environment. This provides detailed material for identifying larger areas with hard surfaces and a lack of shade in the existing urban environment.

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