Human thermal climate in the Hungarian lowland is investigated by using a clothing resistance-operative temperature model performing individual, local, and long-term concurrent observations of weather and human thermal perception. Human thermal load is characterized in terms of clothing resistance r(cl) and operative temperature T-o. The model is also used as a tool to analyse the relationship between r(cl) and the structural parameters of the body (relative fat mass index (fatBMI) and relative muscle mass index (muscleBMI)). This analysis works with data collected on more than 1,000 occasions of weather and thermal perception observation and uses body structure data of more than 3,000 adults and children. By analysing the data, the following human thermal climate characteristics have been established. (a) T-o of about 80 degrees C can be considered as the upper limit of heat stress in the Hungarian lowland, similarly, T-o of about -35 to -37 degrees C can be taken as the lower limit of cold stress. (b) Interpersonal thermal load variations increase with increasing of cold stress; these variations are the smallest within the "thermoneutral" zone. (c) In cold stress situations, there is a significant sexual dimorphism in the relationship between r(cl) and fatBMI. (d) In night time period of the day, T-o can vary between 25 degrees C and the lower limit of cold stress. In this range of T-o, thermal perception types "neutral," "cool," "cold," and "very cold" occurred. Based on the results, it can be seen that interpersonal thermal load variations cannot be neglected in extreme cold weather conditions, and that interpersonal variations of fatBMI index are determinant in the formation of individual clothing resistance.