Complex approach for classification of winter severity in Bulgaria Available to Purchase

A method for evaluating of winter severity in Bulgaria through the use of a specific index that integrates various characteristics of the severe winter weather according to their socio-economic significance is presented. The cumulative effect of a few climatic indicators determines the relative severity of a winter compared to a statistically typical winter season on the basis of data from the meteorological network of the National Institute of Meteorology and Hydrology for the cold half-year in the period 1931-2011. Stations located in the non-mountainous parts of the country with continuous daily data for average, maximum and minimum air temperature, snowfall amount, snow cover presence and wind speed for at least 50 years were selected. The average air temperature from November to March is an important feature of winter severity and, particularly, determines seasonal energy consumption. The damages and economic losses can be considerable during persistent heavy snowfall, furthermore, many serious injury and deaths are attributed to icy and snowy pavements that emphasizes the importance of the snowfall amount as winter severity indicator. Also the number of ice days, in which the maximum temperature is below 0°C, as well as the number of very cold days, defined as days with minimum temperatures below -10°C, have been included as parameters in the winter severity index since they are related to a sharply increase energy consumption, damages in agriculture sector and negative impact on many public activities. The severity of thermal conditions that directly affect the human health and outdoor activity is assessed by the number of days with the Bodman’s weather severity index values over 3. All winter seasons in the period 1931-2017 have been examined and classified in the context of the developed winter severity index, which appears to depend on the continentality and latitude, despite the small territory of the country, reaching lower values in the coastal and southernmost areas. In the most severe winters, the higher index values are the result of lower seasonal temperatures, prolonged cold spells and excessive snowfall totals. The relationship between the winter severity index and the atmospheric circulation has been analyzed with the emphasis on the circulation weather types and their combinations.