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As one of the most common reasons for slippery roads in wintertime, hoar frost can reduce surface friction and affect traffic safety. The risk of winter road hoar frost is subjected to changes in the warming climate. A better understanding of these changes could lead to improved forecasting of hoar frost risk and provide information to policymakers in making climate adaptation strategies. In this work, the decadal variation in winter road hoar frost risk between 2000 and 2016 in Sweden was examined by using in situ observations from 244 stations in the Swedish Road Weather Information System. Results show that hoar frost risks have decreased in the south of Sweden (south of 59°N), whilst increasing in central Sweden (approximately 59°–65°N). Hoar frost risk tends to increase (decrease) in regions where there is a relatively high (low) mean number of hoar frost risk days. Further analysis indicates that the strengthened winter North Atlantic Oscillation (NAO) over the last few decades, which resulted in warmer and wetter winters in Sweden, is the main cause of the changes. During strong positive NAO winters, the frequency of blocking anticyclones centred to the south‐west of Sweden significantly decreased and led to more warm and moist air from south‐west being transported to Sweden. The reduction in hoar frost risk in the southern part of Sweden is mainly due to an increase in road surface temperature, while the increasing hoar frost risk in central Sweden is dominated by the increase in relative humidity, which favours the occurrence of hoar frost. [ABSTRACT FROM AUTHOR]

Atmospheric moisture transport (AMT) is known to have an influence on the winter Arctic surface air temperature (SAT). However, a systematic investigation involving a verification of the physical linkages and spatial sensitivity within the Arctic, as well as a quantification of such contribution is still lacking. Our work first verifies the variation coherence between AMT and the SAT over the winters of 1979–2015, based on multiple data sets. Climate model projections reveal tendencies towards more frequent high values in both SAT and AMT from 2010 to 2065 relative to that from 1950 to 2005, as well as a high correlation coefficient between their detrended series. Then, a composite analysis is applied based on different AMT values. During high‐AMT episodes, enhanced AMT intrudes into the Arctic, with the Norwegian Sea as the gateway, and further induces unevenly distributed warming inside the Arctic. The greatest warming occurs in the Barents‐Kara Sea, which is tightly associated with the altered atmospheric moisture content and its subsequent changes of surface radiation balance. In addition, the intruding moisture can induce latent heat release and sea ice melting, which also influences the warming. Furthermore, by conducting numerical model experiments in the winter of 2012/2013, a total warming effect of 9.66 °C averaged across the Arctic for the original AMT is revealed. By setting different incremental AMT values in five cases, the leading role of AMT is found to switch in the 0.5MT case (the case wherein the AMT value is 0.5 times the original) from altering the atmospheric moisture content to producing precipitation. This finding suggests that evenly spaced AMTs result in the alleviation of SAT increases. This photo, taken by Mingju Hao in the Chukchi Sea, shows the Arctic atmospheric moisture (black regions in the sky). The Barents‐Kara Sea is where the surface air temperature is most sensitive to atmospheric moisture transport (AMT) variations, and the altered atmospheric moisture content and consequent changes in the surface radiation balance are the dominant linkages. When AMT evenly increases, the primary role of AMT switches from altering the atmospheric moisture content to producing precipitation. [ABSTRACT FROM AUTHOR]