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.
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.
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.