City thermal comfort under the heatwave conditions.

Every city is characterised by a specific climate. Depending on the type of land use, the characteristics of the land cover, such as colour and the permeability of the surface, or the construction materials used in the urban space, there are locally large horizontal and vertical differences in air temperature in the city, defined by the local energy balance of the surface area. The varieties are represented by the topoclimatic units. Each of the topoclimatic types can be characterised by a specific sensitivity to the occurrence of high air temperature, which has its direct impact on the parameters of thermal bioregulation of an individual while in the urban space. The thermal stress impact on health and living comfort is well recognised and defined, but results are presented mostly for big city agglomerations. As European settlement structure is slightly less concentrated, we are willing to examine if medium-sized European cities are sensitive to heatwave stress.
We modelled the response for the conditions of high and extremely high air temperature for four towns in Poland, Central Europe: Wołomin, Pruszków, Wieliczka, Żory, in a resolution of 30 to 30 metres. We demonstrate the relationship between topoclimate and human thermal stress under outdoor conditions of high and extremely high air temperature (30°C and 35°C). The impact of the air temperature on humans is presented as the UTCI index (perceived temperature). Results prove that high, very high and extremely high thermal stress is a significant and important problem in medium-sized cities (40 000-70 000 inhabitants); spatially, thermal stress is strongly related to the density of the urbanised fabric. The most resilient are the topoclimate units containing green and blue infrastructure. Results suggest that targeted actions in urban space – reshaping topoclimates to resilient structures – could play the key role in mitigating the effects of heat waves. These measures are of considerable importance in the context of adaptation to forecast climate change and health protection. Results suggest that high-resolution spatial modelling of human thermal stress could be one of the key parameters in spatial planning as a part of health risk management.