Systematic spatial variations (a) Altitude

Normally, the higher the altitude the lower the road minimum temperature expected. This is the result of the decrease in air temperature with height which occurs in a normal, unstable atmosphere.

The environmental lapse rate (the fall of air temperature with height above sea level) is usually about 6.5°C/1000m. Road-surface temperature could be expected to decline with altitude at a similar rate.

Figure 3.5 shows a comparison of daily minimum road surface temperature at two sites in Cheshire: Hassall Green on the M6 at 79m and the Cat and Fiddle at 514m. The mean temperature difference is 3,4°C for a height difference of 435m giving a lapse rate of 7.8°C/1000m.

However, frost hollows in valleys at low altitude can cause the lowest temperatures to be recorded in valley bottoms, especially on clear and calm nights due to either the formation of inversions or the pooling of cold air. Inversions occur under clear calm conditions. The ground cools the air immediately above it so that air temperature increases with height. Above the inversion layer the normal decrease of air temperature with height resumes.

Cold air, being denser than warm air, will tend to fall under the influence of gravity. This is called katabatic drainage. A frost hollow will occur where a hill slope is sufficiently steep for drainage to take place, resulting in lower road temperatures.

The size of the cold air pool is related to the length and steepness of the slope. A cold air pool at the bottom of a long, shallow slope will be greater in extent than one at the bottom of a short, steep slope because of the greater volume of cold air on the longer slope. However, the cold air at the bottom of the short, steep slope may experience colder temperatures due to the greater difference in height and therefore in temperatures between top and bottom of the slope.

In some circumstances, where there is sufficient relative relief, the normal lapse rate will give cold temperatures at the higher altitude and cold air drainage will also give low temperatures at lower altitudes. In these situations, the warmest temperatures are obtained at middle altitudes between the cold hilltops and the cold valley bottoms. The phenomenon of warmer temperatures at middle altitudes is referred to as the thermal belt.

Figure 3.6 shows the variation of minimum air temperature with altitude for twelve sensor sites in Hereford and Worcester under the

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three weather types as discussed below in section 3.1.1 (d). The coldest air temperatures occur in valley bottoms in calm clear conditions.

(b) Topography

Topography restricts radiational cooling of a road surface, limiting the amount of long-wave radiation that can escape from the road by controlling the sky-view factor. At night, a road surface cools by radiation loss.

Radiation loss to the environment is reduced by buildings, trees, cloud cover, traffic and cuttings, all of which reflect, absorb and re-emit back to the surface, thereby restricting radiation loss from the road surface and maintaining temperatures. Hence roads in cuttings, under bridges, or lined by trees and/or buildings will stay warmer at night than more exposed roads.

Conversely, it must be remembered that sheltered roads may warm more slowly than more exposed roads since the early morning solar radiation cannot reach the road surface. This can be important: consider a night of moderate overnight frost during which hoar frost has sublimed onto the road surface and surrounding fields. After sunrise, these hoar-frost deposits are melted or sublimated by the incident solar radiation on exposed road sections. In shaded sections where solar Figure 3.6 Variation of mean minimum air temperature with altitude for twelve sensor sites in Hereford and Worcester, 1988/89. The three weather types are discussed in section 3.1.1(d).

4 9 radiation is unable to penetrate, road-surface temperatures may remain low and early morning traffic can then compact the hoar frost into ice.

Hence areas with a low sky-view factor can be more hazardous than exposed sections if the road-surface temperature falls below zero.

(c) Road Construction

Road construction is important because heat is stored in the road structure and released differentially according to its thermal properties.

Depth of construction is important too: usually the greater the depth of construction the warmer the road. As a result, motorways are normally warmer than other roads, and concrete roads are warmer than blacktop roads. Also, the seasonal variation in incident solar radiation must be considered. In late Autumn and Spring when frost formation is still a hazard, sufficient radiation may be stored in the road from the daytime input to offset the night time cooling.

Where a road crosses a bridge it is likely to be colder due to its shallower construction and, as a result, smaller ‘thermal memory’. This term is used to describe the length of time which a road structure-or any structure-retains the stored heat which it gains from daytime solar radiation. The thermal memory depends upon the depth of construction, the construction materials used and the amount of incident solar radiation received. Certain bridges, particularly those over water, may appear warmer as a result of radiation to the underside of the bridge from a relatively warm water surface. In urban areas, elevated viaduct sections, although limited in depth of construction, can remain warm due to the effect of the urban heat island (see below) and traffic. Steel bridges normally cool quicker than the adjacent road thanks to their high thermal conductivity and poor heat retention. They present particular problems as they can produce a short icy section on an otherwise safe road.

The construction of major roads such as motorways and trunk roads tends to reduce the effects of minor topographical features. For example, embankments reduce the effect of cold-air pooling by raising the road above the base of frost hollows and valleys. Cuttings reduce altitudinal variations and also reduce the sky-view factor.

(d) Urban Heat Island

The urban heat island effect is the phenomenon observed in towns and cities where the built-up area can be several degrees warmer than the suburbs or surrounding rural area. The actual magnitude of the heat island at any single location in the city will depend upon the season and land use at that location. Urban heat island intensity is a function of city size, population density and urban morphology. The temperature

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difference is due to the industrial and domestic heat sources, the low sky-view factor and heat retention by the fabric of the city.

The urban heat island effect is strongest in the autumn and spring, and is at its weakest in the midst of winter. The effect is most noticeable when wind speeds are low, reducing mixing of the air over the city.

Within the city the heat island effect means that the effects of topography, weather and traffic are usually less influential on road surface temperatures than on non-urban roads.

(e) Traffic

Traffic tends to keep a road warm at night by acting as a shadow factor restricting the loss of radiation. Also traffic stirs the air above the road surface, mixing in warmer air from above on cold nights. In addition radiation from the engine and exhaust plus the frictional heating from tyres mean that minimum temperatures can be up to two degrees warmer than for an untrafficked road. On multi-laned roads vehicles tend to concentrate on the ‘slow’ lane which means that the ‘fast’ lane and slip roads may well be cooler at night.

3.2.4 Observed thermal fingerprints in differing weather conditions