The study deals with an assessment and interpretation of the bioclimatic conditions in Vranje (southern Serbia). The study aims at temporal distributions of bioclimatic conditions focussing on extreme thermal stress based on the Universal Thermal Climate Index (UTCI). The meteorological data required for the calculation of UTCI concern hourly (7 and 14 CET) weather data collected for the period 2000-2017. The frequency of very strong heat stress (VSHS), very strong cold stress (VSCS) and extreme cold stress (ECS) for both morning and midday hours. Furthermore, the daily difference of the UTCI hourly values (diurnal UTCI change) are specified, giving the daily variance of heat and cold stress. The results revealed the frequency of days in which thermal stress prevails for the studied period. The obtained results show an increase in extreme heat biothermal conditions, while extreme cold biothermal conditions are in decline, especially in the last 10 years. However, the frequency (the number of days) of very strong heat stress (VSHS) increased since 2007. A spectacular increase in heat stress was observed in the month of September, particularly in 2015.
