Thermal perception of 36 students has been calculated and observed in a case study on the 10th and 11th of July 2000 at Kibbutz Yotvata under extremely hot and and climatic conditions. Calculations of thermal sensations were done by energy balance models of Fanger [Thermal Comfort: Analysis and Applications in Environmental Engineering, McGraw-Hill, New York, 1972] and Gagge et al. [ASHRAE Trans. 92 (1986) 709]. Observed and calculated values differed considerably during the daytime under extremely hot conditions, whereas they corresponded well for warm night-time conditions. A temporal development of the differences lead to a theory, which explains the differences according to the factors "short-term thermal adaptation" and "thermal expectation". (C) 2002 Elsevier Science B.V. All rights reserved.