The aim of the study was to establish to what extent extreme thermal conditions have changed and how they affected mortality, and what conditions favor lower mortality rates or conversely, higher mortality rates. Heat/cold exposure was measured with the Universal Thermal Climate Index (UTCI). Daily mortality and meteorological data for 8 large Polish cities (Białystok, Gdańsk, Kraków, Lublin, Łódź, Poznań, Warszawa, and Wrocław) in the period 1975–2014 were analyzed. Generalized additive models were used to investigate the relationship between UTCI and mortality, and TBATS models were used for the evaluation of time series UTCI data. Most of the cities experienced a clear and statistically significant at p ≤ 0.05 decrease in cold stress days of 0.8–3.3 days/year and an increase in the frequency of thermal heat stress days of 0.3–0.6 days/year until 1992–1994. There was a clear difference as regards the dependence of mortality on UTCI between cities located in the “cooler” eastern part of Poland and the “warmer” central and western parts. “Cool” cities were characterized by a clear thermal optimum, approx. in the range of 5–30 °C UTCI, changing slightly depending on cause of death, age, or sex. For UTCI over 32 °C, in most of the cities except Gdańsk and Lublin, the relative risk of death (RR) rose by 10 to 20%; for UTCI over 38 °C, RR rose to 25–30% in central Poland. An increase in mortality on cold stress days was noted mainly in the “cool” cities: RR of total mortality increased even by 9–19% under extreme cold stress.

Urban heat island (UHI) is one of the most distinctive characteristics of urban climate. The objective of the presented study was to apply a statistical modelling of the nocturnal atmospheric UHI based on the relationship between observed air temperature from ground stations and remotely-sensed temperature of the urban surface. The goal of the approach was to limit input data for the developed modelling method in order to assure transferability of the methodology to different cities. Time series of surface temperature and NDVI were obtained from the MODIS instrument for a 10 year period (2008-2017). The air temperature was collected from the in-situ observational network of 21 stations. The study was conducted for different locations with gradual changes in urbanization in order to assess the impact of urbanization on the relationship between simultaneous air and surface UHI. The urbanization was described by commonly available land cover metrics. Results showed that the proposed approach provided satisfactory AUHI modelling results for the locations with the least degree of urbanization. The best results were obtained with a simple linear regression model with the iterative procedure to minimize the mean absolute gross error (MAGE). The lowest MAGE for modelled UHI was 1.18°C with 69% of the variance explained. The strongest linear relationship between simultaneous SUHI and AUHI was noted for those station pairs which their surroundings have the highest differences in urbanization, and the highest UHI intensities observed. The strength of the SUHI/AUHI linear relationship decreases gradually with the increasing urbanization of the stations' surroundings.

Significant changes in climate variables in the last decades resulted in changes of perceived climate conditions. However, there are only few studies discussing long-lasting changes in bioclimatic conditions. Thus, the purpose of this paper is to present the temporal and spatial distribution of hazardous heat and cold stress conditions in different regions of Poland. Its focus is on long-lasting changes in such conditions in the period 1951–2018. To assess changes in hazardous thermal stress conditions, the Universal Thermal Climate Index (UTCI) was used. UTCI values at 12 UTC hour (respectively 1 pm winter time, 2 pm summer time) were calculated daily based on air temperature, relative humidity, total cloud cover and wind speed at 24 stations representing the whole area of Poland. We found that the greatest changes were observed in minimum (1.33 °C/10 years) and average (0.52 °C/10 years) UTCI values as well as in cold stress frequency (− 4.00 days per 10 years). The changes vary seasonally and regionally. The greatest increase in UTCImin and decrease in cold stress days were noted from November to March and had the highest values in north-east and east Poland, and also in the foothills of the Carpathian Mountains. The trends in maximum UTCI are much smaller and not always positive. The spatially averaged trend in UTCImax for Poland as a whole was 0.35 °C/10 years and the increase in heat stress days was 0.80 days/10 years. The highest increases in UTCImax and heat stress days were noted in eastern and south-eastern Poland.