Asymmetry, Bimodality and Design Values of Daily Air Temperature

Understanding how daily air temperature characteristics are expected to evolve in the future is critical for all countries, as air temperature influences numerous societal sectors. Reliable estimates of future extreme air temperatures are essential for public health, agriculture, hydrology, engineering, and economic planning.

In this study, the long-term trends of selected daily air temperature indices—average, minimum, and maximum—were analyzed for the Hurbanovo and Liptovský Hrádok climatological stations in Slovakia over the period 1871–2020. Furthermore, empirical histograms of daily minimum (TN), mean (Td), and maximum (TX) air temperatures were constructed using data from five climatological stations at different altitudes in the High Tatra Mountains, covering the period 1961–2010. The causes of asymmetry and bimodality observed in these temperature characteristics were investigated.

The findings indicate that the asymmetric bimodal shape of air temperature histograms in Central Europe is primarily caused by the latent heat of freezing, as thermal coupling occurs between snow/ice surfaces and the air. The asymmetry in these histograms is attributed to the lower specific heat capacity of cold air below 0°C compared to the heat capacity of air above 0°C. The energy-intensive phase transitions of ice and water near the freezing point result in a higher frequency of ground-layer air temperatures around 0°C, producing the observed local maximum. This has significant implications for the calculation of annual mean air temperatures at climatological stations, where negative temperatures should be assigned less weight than positive ones, while temperatures in the range of 0–6°C should receive higher weight.

Finally, theoretical probability distributions were developed for individual temperature indices to estimate T-year temperatures at the Hurbanovo station. The distribution functions were analyzed for three periods: 1901–1960, 1961–2020, and 1991–2020. For TN,min, the 100-year temperature increased from –35.75°C in 1901–1960 to –28.69°C in 1961–2020 and further to –26.52°C in 1991–2020. For TX,max, the 100-year temperature increased from 39.4°C in 1901–1960 to 39.63°C in 1961–2020. The most notable changes were observed for TN,min, where an increase of up to 9.23°C was recorded.

Acknowledgment: This research was supported by the project APVV-20-0374, “Regional detection, attribution and projection of impacts of climate variability and climate change on runoff regimes in Slovakia”