Hoarfrost that forms on the road surface is one type of slipperiness that is hard to predict both considering the amount that will be formed and the resulting slipperiness. Gustavsson and Bogren (1990) proposed that the size of the temperature difference between Road surface temperature (RST) and frost point temperature would be proportional to the amount of hoarfrost deposited. The problem of hoarfrost on roads has been dealt with by others, for example Hewson and Gait (1992) made a study on the hoarfrost deposition on roads. They used physical reasoning and case studies to develop two methods to predict the formation of hoarfrost. Takle (1990) used a survey on frost occurrence from the highway maintenance people to describe the occasions when hoarfrost was formed.
Several of the above-mentioned studies showed the importance of preceding and present weather on the formation of hoarfrost. The temperature difference established between air and road surface is also mentioned as important for the formation of hoarfrost on the road surface. Earlier studies showed that large temperature differences occur in the air layer close to the road surface during certain weather occasions. When estimating the state of the road surface it is important to be aware of these temperature differences and the effects of them. Therefore it was decided to study the spatial temperature and humidity differences in the air layer above the road and its surroundings and how this could be related to the amount of hoarfrost that is formed on the road surface.
The aim of this study was:
i) to investigate the temperature and humidity fluctuations in the air layer close to the road surface during occasions when hoarfrost is formed.
ii)to measure the amount of hoarfrost produced during these occasions
The measuring took place at the Swedish National Road Administrations test station at road 45 in the county of Bohuslan. The test station is equipped with a monitoring system with sensors at 0.3, 0.9 and 2.0 m. At all these levels temperature, humidity, wind speed and wind direction are measured during the winter season. Surface temperature is measured with a probe in the road and sensors are also installed at 0.05, 0.10 and 0.5 m depth for measurement of heat flow in the roadbed. A denser measuring profile measured, for this project, the temperature at five levels up to 2.5 m and humidity at three levels. The amount of hoarfrost formed on the road surface was measured by weighing thin slices of asphalt when hoarfrost was formed on them.
Early results show that the temperature difference in the near surface layer is strongly correlated to the amount of hoarfrost formed at the road surface. Further results will be presented at the conference.
References
Gustavsson, T. & Bogren, J. (1990). Road slipperiness during warm air advection. Met. Mag.,119: 267-270
Hewson, T.D. & Gait, N.J. (1992). Hoarfrost deposition on roads. Met. Mag., 121: 1-21
Takle, E.S. (1990). Bridge and roadway frost occurrence and prediction by use of an expert system. J. Appl. Met., 29: 727-735