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The objective of the paper was the determination of the circulation conditions of occurrence of strong and very strong frost in Central Europe. A frost day was defined as a day with the minimum temperature lower than 0 °C and maximum temperature higher than 0 °C. Moreover, a division of frost was performed in terms of value of minimum temperature, resulting in the designation of mild frost (up to − 2.0 °C), moderate frost (from − 2.1 to − 4 °C), strong frost (from − 4.1 to − 6 °C) and very strong frost (< − 6 °C). The study was based on data from the years 1966–2015 from the Institute of Meteorology and Water Management—National Research Institute, Deutscher Wetterdienst and National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR). The analysis of baric conditions employed values of pressure at sea level, height of isobaric surface of 500 hPa and air temperature at a level of 850 hPa, as well as their anomalies. The spatial analysis showed that the number of frost days in spring and autumn increased from the west to the east of Central Europe. A decrease in the number of frost days, however, is observed over the prevailing area. Average conditions favouring the occurrence of strong and very strong frost both in spring and autumn were related to higher than average pressure at sea level over the prevailing area of the Euro-Atlantic sector. Such baric conditions caused advections of cool air masses from the northern sector. [ABSTRACT FROM AUTHOR]

Future anthropogenic climate change is likely to increase the air temperature ( T) across Europe and increase the frequency, duration and magnitude of severe heat stress events. Heat stress events are generally associated with clear-sky conditions and high T, which give rise to high radiant heat load, i.e. mean radiant temperature ( T). In urban environments, T is strongly influenced by urban geometry. The present study examines the effect of urban geometry on daytime heat stress in three European cities (Gothenburg in Sweden, Frankfurt in Germany and Porto in Portugal) under present and future climates, using T as an indicator of heat stress. It is found that severe heat stress occurs in all three cities. Similar maximum daytime T is found in open areas in all three cities despite of the latitudinal differences in average daytime T. In contrast, dense urban structures like narrow street canyons are able to mitigate heat stress in the summer, without causing substantial changes in T in the winter. Although the T averages are similar for the north-south and east-west street canyons in each city, the number of hours when T exceeds the threshold values of 55.5 and 59.4 °C-used as indicators of moderate and severe heat stress-in the north-south canyons is much higher than that in the east-west canyons. Using statistically downscaled data from a regional climate model, it is found that the study sites were generally warmer in the future scenario, especially Porto, which would further exacerbate heat stress in urban areas. However, a decrease in solar radiation in Gothenburg and Frankfurt reduces T in the spring, while the reduction in T is somewhat offset by increasing T in other seasons. It suggests that changes in the T under the future scenario are dominated by variations in T. Nonetheless, the intra-urban differences remain relatively stable in the future. These findings suggest that dense urban structure can reduce daytime heat stress since it reduces the number of hours of high T in the summer and does not cause substantial changes in average and minimum T in the winter. In dense urban settings, a more diverse urban thermal environment is also preferred to compensate for reduced solar access in the winter. The extent to which the urban geometry can be optimized for the future climate is also influenced by local urban characteristics. [ABSTRACT FROM AUTHOR]