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Winter hoar frost conditions on Swedish roads in a warming climate

Journal article
Authors Yumei Hu
Tinghai Ou
Jianbin Huang
Torbjörn Gustavsson
Jörgen Bogren
Published in International Journal of Climatology
Volume 38
Issue 12
Pages 4345-4354
ISSN 0899-8418
Publication year 2018
Published at Department of Earth Sciences
Pages 4345-4354
Language en
Keywords climate change, hoar frost risk, North Atlantic oscillation, relative humidity, road surface temperature, Sweden, traffic accidents, air advections, expert-system, slipperiness, weather, variability, reanalysis, prediction, sweden, bridge, Meteorology & Atmospheric Sciences, kle es, 1990, journal of applied meteorology, v29, p727
Subject categories Earth and Related Environmental Sciences


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 degrees N), whilst increasing in central Sweden (approximately 59 degrees-65 degrees 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.

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