The Impact of Heat Waves on Diurnal Variability and Spatial Structure of Atmospheric and Surface Urban Heat Islands in Kraków, Poland

The increasing number of heat wave (HW) days, combined with the urban heat island (UHI) phenomenon, poses a threat to the health and comfort of city residents. This study investigates the impact of HWs on the diurnal cycles of intensity and spatial structure of the atmospheric UHI (AUHI) and surface UHI (SUHI). A comparative analysis is conducted on the simultaneous night–day variability of AUHI and SUHI intensities in Kraków in two 24 h summer periods: one representing normal summer conditions (Period W) and the other HW conditions (Period H). Evaluating sub-daily UHI patterns based on integrated in situ and satellite data is a relatively novel approach. This study utilizes (1) air temperature from 21 measurement points located in different local climate zones and vertical (altitude) zones; and (2) land surface temperature from six NOAA/AVHRR satellite images. The findings indicate that AUHI and SUHI intensities in Kraków were generally up to 3 °C higher at night and up to 3 °C lower during the daytime in Period H compared to Period W, particularly in the valley floor. These results provide valuable insights into the increased heat load risk due to the co-occurrence of UHI and HW, with implications for sustainable urban planning strategies.