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The weather conditions also influence the spatial variations of roadsurface temperature. The thermal fingerprint of minimum road surface temperature is most clearly developed on clear, calm nights, typical of anticyclonic (high-pressure) weather systems, when temperature inversions are most marked. Rain and wind tend to dampen the fingerprints as discussed below.

(b) Extreme thermal fingerprints

Road surfaces on anticyclonic nights are often dry but there is the danger of frost formation: frost which may be compacted into ice by the pressure of traffic. Hoar frost will be formed on a road surface when a parcel of air containing water vapour comes into contact with a road surface whose temperature is below 0°C and below the frost point of the water vapour. Calm, clear nights produce the maximum variation in road-surface temperature. Cold-air pooling in frost hollows can lead to low road temperatures in valleys.

(c) Damped thermal fingerprints

If roads are wet, then the amplitude of the thermal fingerprint is reduced-literally and metaphorically damped. Overcast, rainy and windy nights, which are typically associated with cyclonic weather, give

5 1 smaller variations in road-surface temperature. High wind speeds promote mixing of air layers, preventing the formation of inversions.

Cloud cover reflects, absorbs and re-emits long-wave terrestrial radiation emitted by the road surface, reducing the amount of radiational cooling at the road surface, and hence keeping surface temperatures higher than under clear sky conditions.

Depressions are not always related to ice formation in winter.

However, one important scenario occurs after the passage of a cold front, producing rain, followed by clearing skies. All the wet roads in an area will start off at a similar temperature. The rate of evaporation and cooling will depend upon the topography, but the stretches of road that fall to 0°C most quickly will be the same stretches of road that are cold on clear nights.

(d) Intermediate thermal fingerprints

Roads at higher altitudes will be colder than roads at low altitudes on clear but windy nights when cold-air drainage or inversions do not occur. Due to the environmental lapse rate we normally expect upland roads to be colder, but upland areas are more susceptible to the prolonged effects of cloud and rain than lowland areas, so the coldest site on average in a region may be at low elevation. On windy nights, however, the coldest sites are usually those at highest elevation.

Most nights give rise to intermediate thermal fingerprints which are somewhere between the extreme and damped thermal fingerprints depending upon the amount of wind and cloud.

(e) Other fingerprints

Obviously, other factors have to be taken into account on nights when the weather conditions change. Patchy fog results in local variations in the thermal map, and freezing fog will lower road surface temperatures.

Similarly, broken, stationary cloud will limit profile development and cause local variations.

(f) Summary of fingerprint development

The fingerprint is most developed on clear nights with little wind; under other weather situations it is less pronounced. On any given night the maintenance engineer will wish to have sufficient suitably sited sensors, or weather information, to give him an impression of the sort of thermal fingerprint that has developed. However, the size of temperature variations along the stretch of road—i.e. the amplitude and shape of the thermal fingerprint—depends upon the geographical make-up of the road network.

Road microclimate

Thermal mapping and road-weather systems 5 2

(g) Forecast thermal maps

The thermal fingerprints can be related to a map of the road network, and on individual nights a map of expected minimum road surface temperature can be produced. Such a forecast thermal map is issued by combining the forecast minimum road-surface temperature from a weather office with the appropriate thermal map.

Owing to the geography of a region, zonal climates can be identified such that a different thermal map can be chosen for different zones on a given night. In the county of Cheshire in England, three regional climates have been identified, each with three thermal map types. For example, on a given night there are 27 (3×3×3) possible combinations of thermal map for the county. The larger the region, and the more varied the geography, the greater the variations. Variable climatic domains can be presented according to the weather conditions.

This has considerable significance for the maintenance engineer. The likely spatial severity of weather conditions can be related to roadsurface conditions and presented to the engineer in advance of their occurrence, enabling resources to be mobilized more effectively.

Recent developments include the use of aerial (Beaumont et al. 1987) and satellite survey techniques (McClatchy et al. 1987), but both methods are expensive and are restricted to cloud-free operation.

In both Sweden (Olafsson 1985) and the United Kingdom (Thornes 1985) thermal maps of road surface temperature are produced using vehicle-mounted infra-red thermometers. Bogren (1990) and Gustavsson (1990) have quantified many of the relationships discussed above. Figure 3.7a shows the counties that have carried out thermal-mapping surveys in Great Britain and Figure 3.7b shows the countries that have carried out thermal mapping up to the end of 1990.

(h) The use of thermal maps

Thermal mapping is of most use in climates where the minimum road temperatures in winter are close to 0°C. Forecast thermal maps can help to identify which parts of a road network need winter maintenance treatment first. In an ideal world, thermal maps can be used to redesign ‘gritting’ routes so that only ‘cold’ routes are treated on marginal nights, or so that cold spots, such as bridges, are treated first.

Thermal mapping is also used to locate permanent road sensors, and to reduce the number of road sensors required in a region. In the future it may be possible for gritting lorries to put more de-icing chemical down on cold stretches of a route, the chemical spread rate being controlled by the thermal map stored in an on-board computer.

Figure 3.7 (a) Kilometres of thermal mapping in the UK and (b) Kilometres of thermal mapping by country.

Thermal mapping and road-weather systems 5 4

(i) Limitations of thermal mapping

Thermal mapping only produces relative minimum temperature differences between stretches of road. It can be used to measure actual road surface temperature accurately, only to within ±2°C. This is because the emissivity of roads is not known accurately. If the emissivities of the road surfaces in an area are different then care must be taken in interpreting the results. Also, a dry road will have a different emissivity from a wet or snow-covered road. A difference in emissivity of 1% can give a temperature difference of 0.5°C. Again, care must be taken to maintain the infra-red camera within a constant temperature range Table 3.2 Possible errors in vehicle-based thermal mapping

Note: Systematic errors can be eliminated by careful preparation (e.g. 2, 4, 5, 7, 9) but the others may be random between surveys and different instruments.

5 5 during observations, and the infra-red cameras must be regularly calibrated. Table 3.2 lists the likely sources of error for infra-red measurements. There is a UK Department of Transport specification for thermal mapping which sets out minimum standards for contractors (Department of Transport 1988b).