Much interest is shown nowadays, in the era of climate change, to information about the local impact of climate changes in every domain of our lives, from politics to public health and even science [1
]. It is already widely known that one of the main consequences of global air temperature rises is the increasing frequency of intense, extreme events [2
]. They lead, mostly during extreme seasons, to severe impact on different social and economic sectors, causing serious health problems to the population and disturbing agriculture, transportation, building industry and tourism activities. It is considered that the most affected climatic variables will be temperature, precipitation, humidity and wind, mostly because of their increase in extreme values, in terms of frequency, intensity and persistence [5
]. Furthermore, international specialists in territorial planning state that climate change also affects, in a negative way, life quality in urban areas, especially through heat waves that occur in urban climate islands (ICU), to an increase in the energy and water consumption, as well as to an increase in polluting substances. Additionally, there is a higher risk of infrastructure damage and winter tourism. For all these, the mitigation and adaptation strategies propose both general and specific measures such as the continuous information and education of the population, preservation of the natural resources, encouraging infrastructure and geo-system resilience processes, as well as expanding green spaces and parks, building green infrastructure, innovation and diversification of winter tourism through finding new solutions independent of snow [6
]. However, some of the effects of ICU under high temperature conditions (e.g., heat waves intensification) on human body are represented by the extreme thermal stress for the cardiovascular system and increase in the mortality rate, especially for children and elderly people [7
]. Therefore, some of the proposed solutions by the same institution are developing pilot projects for acclimatization, infrastructure and green spaces, as well as the development of local, regional and national climatic change adaptation strategies.
In this context, common people show a high interest for knowledge, forecasts and predictions about weather conditions [5
It has been believed since ancient times that weather plays an important role on the human body, but the latest scientific research has proved it. The sensitivity of the organs and the psycho-physiological reactions are increased by external atmospheric conditions. The adaptation ability to sudden changes in weather conditions differ from individual to individual and widely depends on genetic predisposition and specific characteristics [10
]. Over the last 20 years, a multitude of studies have been conducted in order to explain the relationships between humans and their environment, trying to investigate how thermal comfort or thermal stress in outdoor environment influences human behavior during daily activities. Two comprehensive reviews have been developed [11
], whereas some other research papers focused on specific issues. The great majority of these studies have focused on thermal sensations in urban areas [1
One of the most realistic and objective ways to assess thermal perception and stress for humans is based on using appropriate indices [1
]. Even though inside the meteorological and biometeorological community there are different opinions about the indices that should be used or not, in general it is considered that the more indices used, the better and more reliable the image of the changes is created [5
]. Indices, serving as tools for heat stress and thermal comfort analysis, have a major role in describing the combined effect of meteorological variables on humans in terms of thermal stress or comfort [28
In Romania, the number of biometeorology studies is quite small, most of them being focused on bioclimatology and more likely presenting theoretical aspects [29
]. In 2008, impressive work was done by Nicoleta Ionac and Sterie Ciulache that represented the Romanian Bioclimatic Atlas [36
]. The number of studies that integrated biometeorological indices is quite low. One of them focused on the analysis of some biometeorological indices in the southern Dobrogea, which is one of the most hot and dry regions of the country [37
]. Most recently, a bioclimatic analysis in the context of urban environment and tourism was developed, based on the temperature–humidity index, described by fractal Higuchi Dimension, and covering a period of 17 years (2001–2017) in a mid-sized city (Focșani). The study emphasized the increasing air temperature defined by this index [38
The main objectives of this study are: (i). to assess the general bioclimatic conditions, based on five bioclimatic indices in 10 of the largest cities of Romania, and (ii). to find out if there has been any change in the bioclimatic indices over a 56 year period (1961–2016) in terms of duration of their occurrence period (DOP) and frequency of occurrence considering the number of days for each class (FO). With this study, we intend to present regional differences in changes in bioclimatic conditions in Romania.
4. Discussion and Conclusions
With this study, we intended to present regional differences in changes in bioclimatic conditions in Romania. The detailed analysis for each index trend points out that, in general, both parameters considered in this study, the frequency and length of occurrence period, indicated a similar pattern for the majority of indices and their thermal conditions classes. Since the cities were randomly distributed across the country, we can conclude that, although aimed to develop a study on a local scale, the similar patterns identified lead to the conclusion that we can talk about a regional scale.
From 1961 until 2016, regardless of the calculation method, its purpose or the number of classes, the trend analysis for FO reveals a shift from cold stress conditions to warm and hot ones for each index. However, the most stressful conditions for hot extremes did not indicate significant change. Under these circumstances, one can say that the climate in the big cities of Romania became milder during the cold season and hotter during the warm periods of the year. Further, all indices, except for TE, indicated a general negative trend in the number of comfortable days in terms of thermal sensation. For the DOP, the conclusion was similar, with a longer occurrence interval during the year for comfortable or warm stress classes, except for the H index.
However, based on the FO analysis, comfort (H, UTCI and THI indices) or even cold stress (TE or TeK indices) conditions, were still dominant in the area. Thus, the increasing trends detected will intensify the hot stress, according to some indices, or on the contrary, they will lead to a more comfortable climate, based on other indices.
Due to the previously obtained results indicating significant changes in temperature, and especially in extreme hot temperatures and sunshine hours in Romania over a similar period with that considered for this study, temperature and sunshine seems to be the triggering factors for the identified changes in bioclimatic indices.
Since the great majority of the weather stations are inside the built areas of the selected cities, they are representative for peripheral urban areas with low-rise buildings and green spaces around them. The general assumption is that, for all the indices, under the impact of the urban heat islands, in the high-rise building or dense building areas specific to central areas and to big neighborhoods, the hot stress intensifies, especially during extreme high temperature conditions (such as heat waves), and the cold stress diminishes during the cold season. All considered cities for this study are mid-size and large cities: from more than 100,000 inhabitants to more than 2,000,000 inhabitans (Bucharest). However, most of them shelter between 200,000 and 350,000 people. In their central areas, we can expect that, during the warm season, under the high-rise building impact and rush hours traffic conditions [73
], the bioclimatic conditions considerably modify compared to those identified in the peripheral areas and become more stressful in terms of hot stress. For the assessment of the real bioclimatic conditions in all types of local climate zones of the cities, installing urban climate monitoring systems is of crucial importance. They will allow identification of the most critical areas in terms of thermal stress (hot spots) as wells as the most comfortable ones.
Moreover, the difference between the results of the indices considered for this study leads to the conclusion that a scientific validation by people’s perception of these indices on a national scale for the bioclimate conditions of Romania should become a research priority. After validation of thermal stress classes, the index identified as the most relevant should be used for general biometeorological forecasting and considered for implementation in the early warning system for extreme weather events. In the case that different indices are to be found appropriate for different regions, a regional approach should be considered and implemented by the Regional Weather Forecast Centers for a more efficient protection of the population. Under these circumstances, this study could become an extremely useful tool for local and regional authorities in order to adopt the best adaptation measures in terms of thermal stress in the urban areas considered.