In the past few decades, severe heat waves were representative of the weather conditions in various parts of the world. They had distinctly negative impacts on humans, plants, animals and different economic sectors. Particularly in cities, where most humans are living, their well‐being, efficiency and health were affected, which even caused a relatively high mortality rate. This is why a large interest still exists to analyse heat waves in the past by use of statistical methods. As an investigation on heat waves is not available for the territory of Ukraine up to now, a retrospective analysis was conducted. Its main aims were (1) to determine the frequency of heat wave episodes for this country and (2) to analyse their spatiotemporal distribution, duration and intensity. On the basis of 13 selected stations of the meteorological network of the Ukrainian Hydrometeorological Centre, daily values of maximum air temperature (Ta,max) in the summer months June to August were used to determine heat wave episodes according to the definition recommended by the IPCC. For all stations, complete and reliable Ta,max time series were available for the period 1951–2011. With respect to some stations, the Ta,max time series already started some years earlier, e.g. in 1936 for five stations and even in 1911 for four stations. However, the Ta,max time series of almost all stations had gaps in the summer months before 1951, particularly in the 1940s of the 20th century. As they could not be filled in a satisfying way, this limitation was considered in the data analysis. The results indicate that most of the heat wave episodes occurred at stations located in Eastern Ukraine. In contrast to other decades, the number of heat wave episodes was highest for almost all stations in the decade 2001–2010. The fewest heat wave episodes were determined for the two decades 1961 to 1980. The longest heat wave duration varied between 7 d in Henichesk (southern seashore) and 24 d in Lugansk (Eastern Ukraine). For many stations, the longest heat wave duration occurred in the first two decades of August 2010, i.e. in the period of the extremely severe heat wave in Western Russia. The intensity of heat waves was analysed by use of the cumulative Ta,max excess. The results show that heat waves of the longest duration mostly represented the strongest heat waves. The 24‐d heat wave 2010 in Lugansk was the strongest in the investigation period quantified by a cumulative Ta,max excess of 128 °C.

The human-biometeorological conditions in Kyiv (Ukraine) and changes in the frequency of heat stress during the summer period due to recent climate trends were analyzed. The evaluation based on physiologically equivalent temperature (PET). The results revealed the highest probability of thermal comfortable conditions in Kyiv is from the last 10-day period of April to the end of June and from the last 10-day period of August to the end of September. The probability of heat stress reached nearly 90% during the second and third 10-day periods of July. A pronounced increase in thermal stress during the studied heat wave cases (HW), as well as increasing amount of days with heat stress in the period 1991-2015, were found.

The features of the manifestation of climate change in the cities of Ukraine are investigated. It has been established that over the past decades mean annual air temperature in big cities has increased on 0.7–1.2°С, compare to the normal climatic period (1961–1990). The most significant increase in the mean monthly air temperature in the modern period in Ukrainian cities was recorded in January and in the summer months. An increase in the recurrence of tropical nights (when the air temperature did not drop below 20.0°C) and heat waves (HW) was revealed. In all studied cities located in different regions of Ukraine (with the exception of Kharkiv and Kyiv), the number of HW cases in the modern period (1991–2015) has increased two or more times compared with 1961–1990. The most vulnerable in the modern period to the effects of HW are the cities of the southern regions of Ukraine. No HW cases were recorded in Odessa during the period 1961–1990, and 14 cases have been identified in the modern period; the frequency of HW in Zaporizhzhya and Kherson increased from 3 to 22 cases. A change was found in the distribution of precipitation over the seasons and the nature of their precipitation: with a decrease in the number of cases of precipitation, their intensity increases. Based on RCP-scenarios, projections of air temperature for the studied cities were made until the end of the 21st century. According to the calculations for all scenarios, the average annual air temperature will increase gradually in all regions and cities of Ukraine and will increase on average from 2.0–2.5°C in the low-end scenario (RCP 4.5) to 5.0–6.0°C in the high-end scenario (RCP 8.5). Thus, the results of the study indicate the need to analyze the vulnerability of Ukrainian cities to climate change and can be used both to carry out such assessment and to develop measures and plans for adaptation to climate change.

During the last decades, the number of heatwaves (HWs) has increased worldwide, as well as in Ukraine. It is very important to determine the duration, intensity, and other HW parameters, in particular under climate change condition. For this purpose, various heatwave indices and characteristics are used. The aims of this study were (1) to investigate heatwave indices and their characteristics over the territory of Ukraine for the reference period 1981–2010, as well as to examine the extreme heatwave event of 2010 and (2) to make a comparison and establish a statistical relationship between the HW indices and their characteristics and to assess their suitability and sensitivity to changes in the modern climate of Ukraine. On the basis of 49 selected stations, daily values of maximum air temperature (Tmax) in the summer months June to August of 1981–2010 were used to determine two heatwave indices (HWMI (heatwave magnitude index) and HWMId (heatwave magnitude index daily)) and five heatwave characteristics (HWM (heatwave mean), HWA (heatwave amplitude), HWN (heatwave number), HWD (heatwave duration), HWF (heatwave day frequency)). The calculated indices of heatwaves appeared to be sufficiently sensitive to minor changes in the daily maximum air temperature. HWMId was found to be more sensitive to temperature changes than HWMI. The heatwave characteristics of the HWN, HWF, HWM, and the HWMId climate index proved to be the most informative in the study of heatwaves in Ukraine.

The human-biometeorological conditions in Ukraine during two mega-heat waves were analyzed. The evaluation is based on physiologically equivalent temperature (PET). The calculation of PET is performed utilizing the RayMan model. The results revealed these two mega-heat waves produced strenuous human-biometeorological conditions on the territory of Ukraine. During the summer 2010 mega-heat wave, strong and extreme heat stress prevailed at about midday at the stations where this atmospheric phenomenon was observed. The mega-heat wave of August 2015 was characterized by a lower heat load. The diurnal variation of PET values during the researched mega-HW was similar to that of the diurnal variation of air temperature with minimum values in the early morning and maximum values in the afternoon. On the territory where mega-heat waves were observed, the number of days during which heat stress occurred for 9 h amounted to 97.6% for the period from 31 July to 12 August 2010 and 77.1% for the mega-heat wave of August 2015.