# Assessment of Frost Depth and Frost Heave in Romania
## 2 Winter Severity Parameters  
The present study is based on the minimum and maximum daily air temperatures measured at the meteorological stations at 2 m above the surface. Other databases used in the analysis were as follows:  
* The Copernicus Climate Change Service (ECMWF COPERNICUS REPORT, global bioclimatic indicators from 1979 to 2018 derived from reanalysis) [2];
* Geographical coordinates and altitudes for the 3186 administrative-territorial units, retrieved from Global Multi-Resolution Terrain Elevation Data 2010 (GMTED2010), [3];
* The Ensemble Version of the E-OBS Temperature and Precipitation Datasets [4];
* WorldClim version 2.1 climate data for 1970-2000;
* Monthly average values and annual averages for 111 cities published in the Official Gazette of Romania;
* Statistical indicators of the maximum negative and maximum positive annual temperatures for 128 meteorological stations. Due to recent climate instabilities, interest in the investigation of extreme weather and climate events has been increasing during the last few decades. The availability of information for the last 120 years made it possible to investigate the parameters of winter severity (Days with Tmin \(<\) -5 \({}^{\circ}\)C, Days with Tmin \(<\) -10 \({}^{\circ}\)C, Days with Tmax \(<\) 0 \({}^{\circ}\)C, Days with Tmax \(<\) -5 \({}^{\circ}\)C, Days with Tavg \(<\) 0 \({}^{\circ}\)C), taking into consideration the different time intervals of the catalogues of daily temperatures. Figure 1 presents the trends of the number of days per year with minimum temperatures of less than \(-\)10 \({}^{\circ}\)C and \(-\)5 \({}^{\circ}\)C during the 1990-2020 period. In Table 1, we present statistical indicators of other parameters used to evaluate the winter severity. From the four selected time intervals, we can identify the differences between the maximum recorded days per year in the period 1990-2020 and the other time intervals considered in the analysis. Another indicator of the winter severity is the length of the frost-free season (LFFS). The time of the occurrence of the last spring frost (LSF) and the first fall frost (FFF) determine the length of frost-free season (LFFS), which significantly impacts various aspects of human activity. In the last 20 years, at the Arad station, there has been an increase in the LFFS season in comparison with the period 1980-2000, Figure 2.  
The numbers presented in the Table 1 indicate a general decrease in the average number of days with extreme cold temperatures (e.g., Tmin \(<-5\)\({}^{\circ}\)C, Tmin \(<-10\)\({}^{\circ}\)C, Tmax \(<0\)\({}^{\circ}\)C, Tmax \(<-5\)\({}^{\circ}\)C, Tavg \(<0\)\({}^{\circ}\)C) as we move from earlier periods (1900-2020, 1950-2020, 1950-1990) to the most recent period (1990-2020). \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Investigated Period**} & \multicolumn{3}{c}{**Days with**} & \multicolumn{3}{c}{**Days with**} & \multicolumn{3}{c}{**Days with**} & \multicolumn{3}{c}{**Days with**} & \multicolumn{3}{c}{**Days with**} \\  & \multicolumn{3}{c}{**Tmin \(<-5\)\({}^{\circ}\)C**} & \multicolumn{3}{c}{**Tmin \(<-10\)\({}^{\circ}\)C**} & \multicolumn{3}{c}{**Tmax \(<0\)\({}^{\circ}\)C**} & \multicolumn{3}{c}{**Tmax \(<-5\)\({}^{\circ}\)C**} & \multicolumn{3}{c}{**Tavg \(<0\)\({}^{\circ}\)C**} \\ \cline{2-13}  & \multirow{2}{*}{**Avg**} & **St.** & \multirow{2}{*}{**Dev**} & **Max** & **Avg** & **St.** & \multirow{2}{*}{**Dev**} & **Max** & **Avg** & **St.** & \multirow{2}{*}{**Dev**} & **Max** & **Avg** & **St.** & \multirow{2}{*}{**Dev**} & **Max** \\ \cline{1-1} \cline{6-13}  & & & & & & & & & & & & & & & & & \\ \hline
1900–2020 & 29 & 14 & 67 & 11 & 9 & 47 & 20 & 12 & 57 & 6 & 6 & 24 & 42 & 18 & 87 \\
1950–2020 & 30 & 13 & 67 & 11 & 9 & 47 & 19 & 11 & 54 & 5 & 6 & 24 & 43 & 16 & 85 \\
1950–1990 & 32 & 13 & 67 & 12 & 10 & 47 & 20 & 12 & 54 & 7 & 8 & 24 & 45 & 16 & 85 \\
1990–2020 & 27 & 12 & 53 & 9 & 7 & 32 & 18 & 10 & 37 & 4 & 3 & 12 & 41 & 16 & 77 \\ \hline \hline \end{tabular}
\end{table}