Thermal sensation of humans is often calculated by human energy balance models, which are defined for steady state conditions, where the human thermo regulation system has been exposed to the same climatic conditions for a considerable period of time. However, real live conditions in urban environments are often determined by frequent changes of the thermal environmental conditions and short-term thermal reaction and adaptation processes are required to maintain vital body functions. This presented research project examines the influence of short-term human thermal reaction and adaptation processes which are typical for urban environments with focus on extremely hot and dry conditions.

A pilot study has been carried out in the Eilat region (Kibbutz Yotvata, Israel) in July 2000 with temperatures ranging up to 44°C and humidity below 10%. Four climatic stations were set up at different locations in the area of the kibbutz. 36 students from Eilat College were examined during alternating exposure to hot outside and comfortable indoor conditions to compare the calculated and observed thermal stress levels. The preliminary findings were as follows:

The calculations of human thermal stress according to steady state energy balance models seem to be inadequate to evaluate human thermal perception in extremely hot conditions and for short term variations of thermal environmental conditions. A relatively poor overall correlation was found between calculated and observed thermal perception of the participants. The PMV values according to the adapted Fanger model were generally overestimating the actual heat stress levels whereas the DISC Index according to Gagge underestimated the stress levels. Those findings were generally valid for all examined locations, in unsheltered places with direct sun radiation as well as in the shade of buildings or plants.

A detailed analysis of the data led to the determination of a significant regularity in the diurnal development of the calculated differences. The two factors "short-term thermal adaptation" and "thermal expectation" serve to explain the differences qualitatively (figure).

The presented pilot study revealed some new aspects regarding the thermal perception of humans in a hot and arid urban environment, where the impact of the frequent short term changes of the thermal environmental conditions cannot be calculated adequately by the steady state human energy balance models. Future research projects are under preparation to further examine these aspects.

Figure: Schematic course of thermal sensation factors during the experiment