This study measured the amount of hoar-frost on the road surface with the help of copper plates. The amounts were compared to data collected at a station in the Road Weather information System (RWiS) together with additional measurements of the temperature and humidity profile above the road surface. The data were used to specify the weather parameters when hoar-frost forms on the road surface and relate the measured amount of hoar-frost and resulting friction to measured weather parameters. The aim was also to see whether there was an increased possibility of predicting the amount of hoar-frost with additional measurements of the temperature and humidity profiles. The results of this study confirm that the amount of hoar-frost deposited on the road surface increases with increasing average wind speed, increasing maximum difference between dew point (Td) and road surface temperature (Tr) and increased duration when Tr < Td, measured at the RWiS station. However, the difference between Td and Tr failed to indicate hoar-frost on several occasions due to the location and poor accuracy of the measuring equipment. Measurements of temperature and humidity in a profile up to 2.5 m above the road surface showed that the air layer up to 0.4 m reflected conditions at the road surface whereas the surrounding areas influenced the measurements at higher levels. A forward multiple regression performed on the data showed that measurements of temperature and humidity at lower levels improved predictions of deposition of hoar-frost on the road surface. Measurements of friction on the road surface when hoar-frost was present showed that friction decreased with increasing difference between Td and Tr and it also increased with the duration of Tr < Td. There was no correlation between the amount of hoar-frost and friction at the road surface. The friction is probably influenced not only by the amount of hoar-frost but also by the structure of the hoar-frost, which has not been determined in this study.