The aim of this paper is to describe how infrared thermography is used to analyse variation in road surface temperature during clear calm nights. Having knowledge of surface temperature variations along road stretches makes it possible to give an accurate survey of the risk of local slipperiness. The infrared equipment used is an Agema 870-system arranged on various kinds of platforms, stationary, car-and helicopter-mounted. Several factors are of importance in causing surface temperature variations, such as screening from the sun, variation in traffic intensity and construction materials of the road. This article reviews the influence of these factors on road surface temperature and how various platforms could be used to conduct detailed recordings. Sun screening by road rock cuts causes shadow patterns resulting in surface temperatures which are low when compared with sun exposed areas. Height and orientation of the screening object determine the intensity of the surface temperature difference. A maximum surface temperature variation after sunset of approximately 5'0°C was recorded. Road surface temperature differences also occur due to variation in traffic intensity. An intense traffic flow was found to affect the temperature by approximately I·O°C. Owing to different topographical surroundings and construction, bridges have been shown to differ in temperature when compared with adjoining roads. Variation in road bed material has also been proved to have a great effect on road surface temperature.
This paper is part of an ongoing project dealing with the modelling of local climate for predicting temperature variations and risks of road slipperiness under various synoptic conditions. Temperature recordings from mobile measurements taken along road stretches have been analysed to determine the influence of valleys on the variation in air and road surface temperatures. During clear, calm nights, the variation in air temperature between valley bottoms and surrounding areas can be related to geometric properties of valleys, such as depth and width. It is seen that temperature differences between valleys and surrounding areas increase with increasing depth and width. The prevailing wind speed has been found to be a major factor controlling the variation in temperature. The variation in air temperature is totally smoothed out in situations in which there is an ambient wind speed exceeding 3 ms-'. In the interval 2-3 ms-', the degree of wind exposure in valleys is of importance for the magnitude of the air temperature differences. Counter-radiation from clouds has been found to significantly reduce the intensity of cold air pools. Under partly cloudy situations (3-5 octas) with no wind, the temperature difference between valley bottoms and surrounding areas differs by approximately 6°C in comparison with clear, calm situations. The relationship between differences in air and road surface temperature has also been studied. In general, an increase in cold air pool intensity of 1°C lowers the road surface temperature by 0~4°C.
KEY WORDS Topoclimate Valley Road surface temperature Cold air pool
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