Due to results of regional climate simulations for Central Europe, the likelihood is very high that not only the near-surface air temperature is increasing but also an intensification of extreme heat waves in summer will occur. The change of the large-scale thermal background conditions is strengthened in cities by their specific urban micro-climates. Against this background, the demand for human-biometeorologically based concepts is continuously increasing in town planning, by which the stronger impairments of human thermal comfort for citizens in the future can be minimised. A comprehensive analysis of human thermal comfort within cities during large-scale heat conditions requires a coordinated combination of different methods: (i) experimental investigations and numerical simulations to calculate comfort-relevant thermal assessment indices, (ii) questionnaires with respect to the individual perception of the local thermal environment and (iii) monitoring of behaviour patterns of citizens in urban open spaces.

Within the scope of the joint research project KLIMES, which is conducted in Freiburg (south-west Germany), the warmest city in Germany, urbanistic concepts are developed to mitigate the impacts of extreme heat on citizens. In one of the four KLIMES subprojects, experimental investigations on human thermal comfort in different urban quarters in Freiburg are carried out on typical summer days. The internationally used physiologically equivalent temperature PET is applied as thermo-physiologically significant assessment index. The coordinated investigation design of KLIMES, the experimental approach to determine PET and exemplary PET results for different sites in Freiburg are presented and discussed. Related to the climatic background conditions of Freiburg, the mean radiant temperature Tmrt turns out to be the meteorological variable, which is strongest governing PET on typical summer days. According to the human-biometeorological methodology to assess human thermal comfort for citizens, who are outdoors in different urban thermal environments, the collective of citizens is represented by a standardized standing person. Therefore, Tmrt mainly depends on the absorbed long-wave radiation flux densities from the four horizontal cardinal directions. The direct significance of the absorbed short-wave radiation flux densities, however, is relatively low. Based on structural analyses of PET in different urban quarters, the maintenance of human thermal comfort even on summer days primarily requires the reduction of the absorbed long-wave radiation flux densities from the four horizontal cardinal directions, i.e. suited concepts of town planning are necessary.