Dependence of mean radiant temperature within urban structures on three-dimensional radiation flux densities

According to the state-of-the art in human-biometeorology the assessment of the thermal environment is based on thermo-physiological indices, which are derived from the human energy balance. Worldwide, various thermo-physiological indices are available like OUT_SET*, perceived temperature or physiologically equivalent temperature. Most thermo-physiological assessment concepts are related to a collective of people, which is represented by a standardised standing person.

In order to calculate the assessment indices, information on the atmospheric environment and human features is necessary. The mean radiant temperature as meteorological input variable comes important particularly during hot weather for urban environments, which are not under the influence of local or regional ventilation systems.

As the mean radiant temperature is governed by the short- and longwave radiation flux densities from the three-dimensional surroundings of the reference person, it cannot be measured directly. Therefore, it must be derived from appropriate measurements of the three-dimensional short- and longwave radiation flux densities or it has to be simulated by suitable models. Particularly in urban environments, the necessity exists to validate simulated values of mean radiant temperature by those achieved from measured radiation flux densities.

Within the scope of different projects on the human-biometeorological assessment of the thermal environment within urban structures in the city of Freiburg (southwest Germany) conducted against the background of a future increase of extreme heat in Central Europe and the following demand to develop mitigation strategies, which can be applied in urban planning, a specific experimental method was implemented to determine the level of human thermal comfort during hot weather. It included measurements of the short- and longwave radiation flux densities from the three-dimensional surroundings of the standardised reference person. The analyses of data from measurement campaigns within different urban structures enable the investigation on the time-dependent significance of each radiation flux density for the mean radiant temperature. Due to the standardisation that the reference person is standing, horizontal radiation flux densities stronger influence the mean radiant temperature than the vertical ones. Hence, a reduction of mean radiant temperature, which is aimed during hot weather, can be achieved by suitable changes of material and surface type of vertical walls within urban structures. The data analyses also point out the physical reasons of differences in the mean radiant temperature under the canopy of street trees (shaded conditions) compared to completely sunny conditions.