HEATWAVES IN ROMANIA -FREQUENCY AND DURATION

. Heatwaves are recognized, although there is still no general acknowledged definition, as periods of unusually hot and dry or hot and humid weather, which occur gradually and cease in the same way, lasting at least 2 to 3 days with visible impact on human activities. Episodes characterized by heatwaves induce excessively hot weather compared to the local climatic specific features. The period with or without heatwaves is different from a region to another, depending on the particularities of each area. This climatological hazard can be described as an advection of tropical air mass which, compared to the climatological standards, leads to reaching large positive temperature deviations, that in some cases will set new thermal records. In the northern hemisphere, in the area of temperate latitudes, which includes Romania, the highest values of air temperature are generally recorded from mid-June to the end of August. Heatwaves in Romania are mainly generated by the advection of continental hot air masses from North Africa, leading to a stable stratification of the atmosphere from ground levels to more higher ones. The ridge of the North African anticyclone extends either over Central Europe or Central Eastern Europe, up to Romania, generating heatwaves for the first scenario in the western regions of our country, whereas or the second one outside the Carpathians, most frequently in the Romanian Plain. In Romania, the most frequent heatwaves, taken by duration in time are those between 2 and 5 days. while analysed in terms of intensity, stand out those during summer months (June to August).


INTRODUCTION
The Intergovernmental Panel on Climate Change (IPCC 2007), in the evaluations conducted after 2000, confirmed the persistence of meteorological phenomena attesting to the warming of the Earth's climate, including the increased frequency of heatwaves. These climatic extreme, such as heatwaves, have a negative impact on the environment, cause significant economic damage and even human deaths. Recent studies in the literature provide solid evidence that the problem of global warming is certainly a serious one. Although characterized by notable barriers, the connection between climate change adaptation and disaster risk mitigation is significant and its effects are beginning to emerge (McBean and Ajibade 2009).
Even if surface temperature values are rising globally, warming is not uniform across continents (Jones 2001, Jones and Moberg 2003, Solomon et al. 2007, Kerr 2009, Nick et al 2009, Lorentzen 2014. The global average temperature has risen by about 0.85 Celsius degrees from 1880 to 2012 and will continue to rise so that by the end of the 21 st century it will be 0.3 to 4.8 Celsius degrees higher, according to the IPCC's fifth assessment (Hartmann et al 2013). Regional and local particularities are crucial for regional and local variations in the climate system.
According to the guide published by the World Meteorological Organisation (WMO) in 2015, the heatwave or hot weather that continues in successive days (without the previous requirement of at least 5 days) represents a heatwave if it has the characteristics of a natural hazard, meaning that the excessively hot weather has negative effects on human life and way of living, on socio-economic activity in general, being possible to adversely affect even the infrastructure. Natural systems can also be severely affected by the impact of this phenomena, depending on the duration of the event.
Although there is still no universally valid definition of a heatwave, they are acknowledged as events of unusual hot and dry weather or hot and humid weather, which occur gradually and cease in the same way, with a duration of at least 2 to 3 consecutive days and a visible impact on human activities. Heatwaves depend on the climate of the analysed location, because the same weather conditions are perceived differently by the population living in one area or another. In some countries periods with certain temperature values can be characterized as severe weather events, while in others they are just normal weather pattern.
Therefore, heatwave events are periods of at least 2 to 3 consecutive days with excessively hot weather related to local climatic features. This natural hazard can be described as an advection of tropical air mass that leads to large positive temperature deviations and in some cases also to the establishment of new thermal records.
In the northern hemisphere, for the area of temperate latitudes, where Romania is included, the highest values of air temperature are generally recorded from mid-June to the end of August (Climate of Romania 2008).
Romania, because of its physical-geographical peculiarities, has a high degree of exposure and vulnerability in terms of the occurrence of various meteorological hazards. Due to the fact that the orographic feature of our country starts from the highest peaks of the Carpathians and ends at the lowest regions near the Black Sea, gradually passing through all landforms, a series of climatic characteristics develop (Dima et al 2016). Therefore, the mediated thermal regime for each region of the national territory is different. From another perspective, the way in which the weather changes or not from one day to the next one is influenced by the alternation, often at an accelerated pace, of the types of circulation and advection of specific air masses (such as tropical ones regarding heatwaves) and by changes in the thermo-baric field at the synoptic scale set-out by the distribution and dominance of one of the main centres, both at ground level and higher in the troposphere.
Heatwaves in Romania are usually associated with the presence of a high atmospheric pressure filed that lead to a stable stratification of the atmosphere levels and the advection of continental hot air masses from North Africa. Heatwaves appear in Romania in the western regions when the ridge of the North African anticyclone extends over Central Europe and outside the Carpathians, most frequently in the Romanian Plain, when it shifts over Central Eastern Europe.
Heatwaves in Romania are usually analysed according to the general warnings' thresholds: 32.5 °C, 35.0 °C, 37.5 °C and 39.0 °C (Croitoru et al 2018, Timu, 2010). Due to the complex physical-geographical characteristic of Romania, hot air masses responsible for the periods with unusual hot weather extend different from one region to another in Romania, and as a result, a heatwave event is more intense and has different impact in the north compared for example with the south of the country (Barbu et al 2014).

Methods
Data processing was conducted using Microsoft Excel. The main work sheet is a table with 9 columns: year, month, day, daily maximum temperature, the multiannual average of the daily maximum temperature (average between daily maximum temperature from 1961 to 2017), deviation (difference between the daily maximum temperature and its multiannual average), YES/NO (if the average is higher than 5 Celsius degrees appears an YES in the cell, otherwise NO), COUNT 1 (an if function searches for the YES cells and the first one encountered gets value 1 and in case of a second consecutive one value 2 appears and so on) and COUNT 2 (an if function searches for the maximum consecutives from COUNT1 and displays it, such as 1, 2, 3 or more). A pivot table was used in order to summarize the number of episodes with specific lengths and create the table behind each chart.
In conclusion, for each meteorological station, using the formulas presented above, it was possible to easily identify the number of episodes with excessive hot weather, starting from 1 day to 2, 3, 4 consecutive days and so on.

RESULTS AND DISCUSSIONS
Between 1961 and 2017 for Arad meteorological station the following were identified: -278 days when the daily maximum temperature exceeded the multiannual average of the daily maximum temperature with 5 °C; -heatwaves with a duration of 2 days (170 episodes

Fig. 22. -Frequency (%) of heatwaves episodes taken by duration
In fig. 22. the pie chart summarizes the frequency of heatwaves episodes taken by duration. From a total of 6547 heatwaves episodes with duration between 2 to 20 days, the most frequent ones are the following: 44.81% for 2 days events, 23.17% for 3 days events, 13.21% for 4 days events, 7.29% of 5 days events, 4.20% for 6 days events, 2.72% for 7 days events, 1.51% for 8 days events, 1.15% for 9 days events and under 1% for 1-to 20 days events.

CONCLUSIONS
Heatwaves in Romania generally occur as a result of the advection of tropical hot air masses of continental origin from the north of the African continent, when the ridge of the North African anticyclone extends towards Europe. They appear all over Romanian territory with different intensity from one region to another due to characteristic features of the local climate.
The most frequent heatwaves, as shown by the analyzed data, are those with a duration of 2 to 5 days, and there are some exceptional situations when a heatwave lasts up to 18-20 days.
Periods with extreme weather, such as intervals with heatwaves, most intense during the summer months, have a negative impact on human life and can disrupt socio-economic activities.
Heatwaves represent a meteorological hazard of unusually hot weather, that for Romania has a high probability of occurrence, more intense from June to August.