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1. Introduction

Micro- and local climatological processes vary in rela-tion to small-scale geographical features. Consequentlymeasurements that aim to cover these variations mustbe properly and carefully designed. For local climato-logical studies it is common to use either a stationarynetwork of field stations or vehicles equipped withtemperature sensors. Mobile measurements have a longtradition; for example Schmidt (1927; 1930) andPeppler (1929) published some early work usingmobile platforms to describe climate variations. Sincethen this approach has been used in a large number ofstudies concerned with topo-climatology as well asurban climatology (e.g. Huovila, 1964; Hocevar &Martsolf, 1971; Oke & Maxwell, 1975; Söderström &Magnusson, 1995). Also Lomas et al. (1969) discussedmobile surveys in agro- and topo-climatological studiesand especially the possibility of using mobile measure-ments to describe spatial variations in temperature inareas close to roads. An advantage of using a mobileplatform is that a large area can be covered in a rela-tively short period and that the same instruments areused. This results in a temperature pattern that can beanalysed in relation to, for example, topographical fac-tors whilst avoiding differences related to instrumentalerrors or calibration.

Mobile temperature measurements, or thermal map-ping, have been used in applied road climatologicalstudies since the middle of the 1970s. Lindqvist (1976)published an early report where methods for detectingroad sections with a high frequency of ice formationwere discussed. Other early work within this area isthat by Sugrue et al. (1983) and Thornes (1985).Thermal mapping has since been developed and istoday a method which is usually used prior to an

installation of a Road Weather Information System(RWIS). Thornes (1991) describes the thermal mappingtechnique in great detail and also gives examples of howthe temperature data varies according to weather aswell as other important factors. The thermal mappingtechnique has also been used in a number of studieswhere the influence of different topographical objectswas studied (e.g. Bogren & Gustavsson, 1989, 1991).

An important development of the traditional mobilemeasurements was the inclusion of surface temperaturedetectors which could be used for road climatologicalstudies. By use of infrared detectors the importantfactors influencing the local risk of road icing could bemeasured. Traditionally thermal mapping is used as amethod to detect locations which differ in temperaturecompared to neutral areas; for example these could beareas where no gathering of cold air takes place. Theselocations are further compared with field observationsand analyses of topographical maps to identify themost suitable locations for field stations in an RWIS.

The present study emanates from a long tradition inusing the thermal mapping technique for climatologicalstudies. The need for the evaluation of the technique,both with respect to instrumentation and ways ofanalysing the recordings, has grown during the last fewyears. It is very important that thermal mapping is per-formed in a correct way since the technique is very sen-sitive to errors. This study focuses on the measuringtechnique and how to analyse the temperature record-ings in order to determine factors that control the vari-ation in surface temperature. The results from thisstudy will provide recommendations about how thethermal mapping technique could be developed inorder to give high-quality data for analyses of road cli-mate variations.

Meteorol. Appl. 6, 385–394 (1999)

Thermal mapping – a technique for roadclimatological studiesTorbjörn Gustavsson, Laboratory of Climatology, Department of Earth Sciences, Universityof Göteborg, Box 460, SE 405 30 Göteborg, Sweden

The present study deals with the thermal mapping technique in a road climatological perspective. Thetemperature mapping technique is analysed in relation to instrument set-ups, data collecting andpossible sources of errors in converting measured radiation to surface temperature values. Furthermore,analyses of thermal mapping recordings are discussed. Factors that give rise to a varying temperaturepattern are presented (i.e. road-bed material, radiation and advection), and also how the thermalmapping technique can be used to determine the relative importance of these factors. The conclusion ofthe study is that the technique is very useful but it is very important to be aware of the limits anddifferent source of errors involved.

2. Data and analysis

In Sweden thermal mapping has been used for morethan 20 years as a tool for road climatological studies.All major roads have been thermally mapped duringdifferent weather conditions to analyse the influence oflocal topography and other factors that determine theroad surface temperature (RST). Many roads have alsobeen mapped on a number of occasions, with severalyears in between. These recordings have been used toanalyse the influence of changes in road coatings andthe clearance of forests close to the road etc. All themeasurements have been saved on data disks, whichconstitute a very large data bank available for analysesof variations in road climate conditions.

The examples of thermal mappings used in this studyare carried out along road no. 47 between Falköpingand the road crossing no. 47/no. 48, in the county ofSkaraborg, in the south-western part of Sweden. Thisparticular road has been chosen as a test road as itpasses through a large number of different local climateenvironments. The road is furthermore covered with auniform asphalt layer, which is essential for the inter-pretation of the RST. For the study of road-bed mate-rial a test road outside Luleå, in the northern part ofSweden, was used. A homogeneous asphalt layer alsocovers this road, but the construction material differsalong the road. Each stretch composed of a differentmaterial from the standard type (gravel) is marked onthe roadside, so making it possible to perform detailedanalyses of the temperature variation.

During thermal mapping the air temperature is mea-sured at two levels: 0.3 m and 2.0 m above the road sur-face. The surface temperature is measured with aHeimann KT15 radiation thermometer, which has aspectral sensitivity in the interval 8–14 µm and aresponse time of 1 second (Heimann InstructionManual, Heimann GMBH, Wiesbaden, Germany).Each parameter is logged every 10 m during the mea-surement and the driver can feed manual markings tothe computer about factors that can help in the analysisof the recordings, such as change in surface conditions,screening by bridges etc. Before and after each mea-surement the prevailing weather conditions are notedtogether with information about geographical location,time and date of the measurement. The weather condi-tions for the examples used in this study are discussedin each section. However, situations have carefullybeen selected in order to find the most suitable condi-tions for each of the aspects that are examined.

3. Measuring technique

Surface temperature is a very difficult parameter tomeasure. The surface is what one could characterise asthe transition zone between two different media. Largetemperature differences often occur in this zone owing

to the change from turbulent to laminar flows close tothe surface, as well as other factors. One way of per-forming measurements of the surface temperature is toregister the energy flux and relate that to the tempera-ture by use of the Stefan–Boltzmann law:

I = ε σ T4

where I is the energy flux density, σ is theStefan–Boltzmann constant, ε is the emissivity and T isthe temperature of the surface given in degrees kelvin.

Infrared (IR) scanners are instruments which have sen-sors that are sensitive to a specific wavelength-band.The interval 8–12 µm is most commonly used since itcorresponds to the atmospheric window (i.e. where theatmosphere is nearly transparent for energy flow). Thisis also the range which is of most interest in studies ofapplied climatology.

Using the IR technique is not trouble-free. For exam-ple the emissivity factor can be especially difficult tohandle, since a small variation can cause a large varia-tion in temperature calculations. In Figure 1 the tem-perature variation is shown for a calculation using anemissivity of 0.95. A change from 0.95 to 0.85 gives asurface temperature difference of approximately 8 °C,given that I = 300 W m–2. The emissivity of a surfacevaries with type of material, texture etc. Tabulated val-ues (Marshall, 1981) give an emissivity for concrete of0.92–0.94 and asphalt 0.967. Another parameter of spe-cial importance in road climatology is whether the sur-face is wet or dry, which will significantly affect theemissivity of the surface. Measurements using an IRsensor have shown that the RST can differ by approxi-mately 0.8 °C between a dry and a moist asphalt sur-face. Therefore it is of great importance to carry outmeasurements during situations with constant surfacestatus, either dry or moist. It is very difficult to inter-pret temperature recordings produced during situa-tions with alternating surface status; therefore these sit-uations should be avoided.

The radiation received by the IR sensor comes mainlyfrom three different sources, and the total radiance(Itot ) can be calculated using the following equation:

Itot = It-object τ ε + It-amb τ (1 – ε) + It-atmos τ (1 – ε)

where It-object is the radiation from the road surface,It-amb is the reflected radiation from surroundingobjects, It-atmos is the radiation from the atmosphere(controlled by the temperature of each object), τ is thetransmission and ε is the emissivity.

A problem to be overcome during thermal mappingmeasurements is to minimise the influence of the lasttwo factors in the equation for Itot. One way of doing thatis to have the scanner mounted in a nadir position andto install the sensor close to the surface. If the scanner

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is mounted with a view angle differing from nadir, anumber of problems occur. First of all the size of themeasuring area increases, which in a way can be anadvantage, but if the oblique angle view is used, theimportance of radiation from the surroundings and theatmosphere increases. Furthermore the emissivitychanges with view angle and thereby the risk of misin-terpretation increases. The intensity of the radiationalso differs in relation to the view angle, and if anoblique set-up is used, a recalculation must be carriedout in order to compare measurements using differentset-ups.

The marked points in Figure 1 refer to variations inemissivity owing to a varying view angle (I = 20°, II =40°, III = 60°). Data from Lagourade et al. (1995) havebeen used to give the relation between the apparentemissivity and the view angle. However, detailed stud-ies performed over an asphalt surface have shown amuch larger variation (Bergendahl, 1998). A view angleof 45° gives an apparent emissivity of 0.87 (IV in Figure1), which has a pronounced effect on the calculated sur-face temperature, i.e. the RST will be 6 °C lower com-pared with a nadir mounting. Scott (1986) has calcu-lated that an increased view angle will result in adecrease in the apparent emissivity of 4% (V in Figure1) using a view angle of 40°. This will give a tempera-ture difference of 3 °C. Further studies are needed inthis area to fully understand the effect from a varyingview angle on the RST.

In Figure 2, repeated temperature measurements alongthe test road (no. 47) are shown. Measurements werecarried out four times during the same clear and calmnight in order to follow the temperature developmentof the stretch of road and also to enable analyses of thequality of the instruments used. If the instrument is ofhigh quality and the measurements are carried out in aproper way, the correlation between the individualmeasurements should be high, which is the case for theexample shown in the figure (there is a correlation coef-ficient of 0.77 between runs 2 and 3). The magnitude ofthe temperature differences changes through the night,which will affect the correlation in a negative way.However, the temperature pattern stays the samethrough the night, and this is clearly seen in Figure 2.Other factors that can influence the quality of the mea-surements are whether the scanner is kept at constanttemperature or not, whether the lens of the scanner iscleaned, and whether the instruments are calibrated. Allthese factors have a large influence on the accuracy ofthe measurements.

4. Analysis of the thermal mapping recordings

Three main factors can be distinguished which deter-mine the road surface temperature: (a) road-bed mate-rials, (b) radiation, and (c) advection and stagnation ofcold air. In the following sections the importance ofthese factors is discussed and examples are presentedwhich show how the parameters can be studied by the

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Figure 1. Plot of the effect of variation in the emissivity for determining surface temperature. The surface temperature isexpressed as a difference from a reference value (ε = 0.95). Points I–V are values from different studies (see text for more infor-mation).

use of thermal mapping. The analysis of the influenceon the RST of the three factors is carried out in such away that the studied parameter should be the main fac-tor determining the variation in temperature. The influ-ence of road-bed material, for example, is studied alonga stretch of road with very small topographical varia-tions and open roadsides, so that no obstruction of theradiation balance occurs as a result of nearby obstacles.

4.1. Road-bed materials

The heat flow and storage in the road bed is determinedby the thermal properties of the material used in theroad’s construction. This factor influences the temper-ature reaction during cooling as well as during warm-ing. Information about the type of material used is notalways available, and therefore it is very important toperform the thermal mapping in such a way that thisfactor is covered. An example of the large influence ofthe construction material is presented in Figure 3. Themeasurement was carried out during a cloudy, windynight (4/5 December 1997) along the test road outsideLuleå, previously described. The road passes throughopen arable land, and there are no large variations inaltitude (<15 m). The temperature recording shown inFigure 3 is from one of the test sections, locatedbetween 800 and 940 m from the start point. This sec-tion has construction material consisting of iron-richsand, a waste material from mining. In the sectionsadjacent to the test section the material is gravel. Ahomogenous asphalt layer covers the entire stretch.During the measurement the road surface was dry, butthere was snow along the hard shoulders. It might be

expected that drifting snow on the road surface couldhave an effect. However, the influence of this factor isconsidered marginal because the temperature differ-ences are consistent and well correlated with the varia-tion in road-bed material. The examples discussedbelow are from a test stretch where the influence of dif-ferent built-up materials is examined. It is not commonto have such large variations along an ordinary road, butthis can occasionally occur, for example, between graveland solid rock. These two materials can alternate alongroads which pass through rock cuttings where the insitu rock is used near the cuttings and gravel in between.

In Figure 3 the air and surface temperatures from mea-surements carried out at 22.30 (local time) and the sur-face temperature from 07.00 are shown. The air tem-perature along this test section is constant (–13 °C), butthe surface temperature shows a very large variationassociated with the change in construction material,being approximately 2.5 °C lower than that of the sur-rounding stretch at 22.30 and 2.0 °C lower at 07.00. Asthe change is very distinctive and is repeated in a simi-lar way between the two measurements, the materialused in the road bed must cause the pattern. The airtemperature showed no variation along the stretch onboth occasions. This is a fact that also indicates that thematerial is the main factor causing this variation in sur-face temperature.

4.2. Radiation

Incoming solar radiation and outgoing long-wave radi-ation from the surface are major influences on the RST.

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Figure 2. Recordings of surface temperature along road no. 47 on four different occasions during the same night (I = 20.00, II= 23.00, III = 01.00, IV = 03.00) for 27 March 1994.

I

II

III

IV

Objects that screen off the sun influence the radiationbalance both during the day and night (by obstructingthe long-wave radiative cooling). That screening off thesun has a pronounced effect on the RST has beenshown by, for example, Bogren (1991).

Previous studies (e.g. Bärring et. al., 1985; Gustavsson,1995) have documented that the sky view factor (SVF)has a strong correlation with the temperature, since thisfactor is a measure of the degree of sky reduction. TheSVF can, for example, be determined from fish-eyephotographs. However, this is a very time-consumingmethod, and information is only obtained from therestricted place where the photograph was taken. Asthe radiation condition is a very important parameterto have information about, there is a need for a methodthat can be used for entire stretches of road.

A surface temperature recording from road no. 47 dur-ing late afternoon (18.00) just before sunset is shown inFigure 4(a). The weather situation during the day andthe following night was a clear sky (≤ 1 oktas) with aweak wind (< 2 m s–1). From 16 km onwards the roadpasses through a forest, alternating between open andtree-covered sections. The orientation of the road isnorth–north-east to south-south–east, giving a pro-nounced screening effect during the morning and after-noon. Open and screened sections of the road can bedistinguished by means of the temperatures: open areashave a surface temperature of approximately +10 °Cand the screened ones of approximately +4 °C. Thistype of temperature recording gives information aboutthe road’s surroundings or more precisely about the

obstruction of the radiation that occurs on that specificsection of road.

Analysis of daytime recordings was carried out in orderto determine if this type of measurement gives enoughinformation about the radiation conditions or SVFalong a road stretch. The mean surface temperature wascalculated for sections of the road showing a consistenttemperature level. These mean temperatures were fur-ther compared with SVF photographs from selectedplaces and scaled by use of the maximum and minimumtemperatures and SVF values, respectively (see Figure4(b)). The open area had SVF = 1.0 and the lowest SVF(0.60) was obtained at 21 km. By comparing these val-ues with the air temperature pattern from a recordingcarried out during a clear night, it possible to verify ifthis technique is useful or not. In Figure 5 the sky viewvalues are plotted against the difference in air tempera-ture for a clear, calm night. The differences in air tem-perature are calculated using open, neutral areas as areference level. The correlation between the two vari-ables is high, showing that the sky obstruction is animportant factor influencing the variation in tempera-ture in forested areas.

The first 16 km of the measured route in Figure 2 passthrough open, undulating terrain. Analysis of the tem-perature pattern from these sections of road shows thatthe radiation plays an important role in the open terrainas well as in the forested part, as previously discussed.The surface temperature shows a very diversified pat-tern that cannot be explained by variation in the airtemperature; rather the pattern is associated with the

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Figure 3. Variation in surface and air temperatures along a test road outside Luleå in the northern part of Sweden on 4/5December 1997.

detailed orientation of the road. In valleys the bottomand the valley slope facing the east will receive lessshort-wave radiation during the afternoon and evening.And as shown in Figure 2, this causes large variation insurface temperature (3 °C at sunset).

By comparing the surface temperature pattern fromsunset onwards, it becomes possible to determine theperiod for which this variation in received radiation hasan effect (see Figure 2). The repeated measurementalong road no. 47 shows that temperature differencesdecrease in intensity. However, after 5 hours, the after-

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Figure 4. (a) A daytime recording at 18.00 on 27 March 1994 of the surface temperature in a forested part of the study area. (b)Calculation of the sky-view factor from the surface temperature recording presented in (a).

Figure 5. Plot of calculated sky view values versus variation in air temperature at 20.00 on 27 March 1994.

(a)

(b)

noon radiation is still an important factor influencingthe surface temperature. This is very important to takeinto consideration if the thermal mapping is used tofind the most suitable location for field stations in anRWIS. Mapping must be carried out during the winterperiod when the intensity of the incoming radiation isweak to be able to fully evaluate the influence of thelocal topography etc. Another aspect is that this factormust be taken into consideration since it controls thesurface during autumn and spring. The selection ofstation sites and detailed documentation of the areas andof the reasons for choosing that specific site are veryimportant aspects of the establishment of an RWIS.

4.3. Advection and stagnation of cold air

Advection, especially cold air flows, and stagnation ofcold air in valleys and low points can significantlyinfluence the road surface temperature. If the air tem-perature is measured during thermal mapping, theinfluence of low temperatures in cold air pools and cold

hollows can be analysed. Studies by Gustavsson (1995)and Gustavsson et al. (1998) have shown that a verydiversified air temperature pattern develops in undulat-ing terrain shortly after sunset. It has also been shownthat a significantly lower than average surface tempera-ture can be found in valleys during clear nights (e.g.Bogren & Gustavsson, 1991).

A temperature recording during a clear and calm situa-tion along road no. 47 is shown in Figure 6. The record-ing was carried out during 15–16 February 1994 at18.00 (i.e. approximately one hour after sunset). In thefigure the temperatures are presented as deviationsfrom the neutral temperature. The neutral temperaturerepresents the temperature of areas where no gatheringof cold air takes place. By use of this type of presenta-tion the cold air pools can clearly be identified. Alongthis specific route, four different valleys could be iden-tified where the accumulation of cold air occurs. Theseare located at 5–6.5 km, 10–11.3 km, 11.9–13 km and14–14.8 km from the starting point. As shown in Figure6 there is a close correspondence between the air tem-

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Figure 6. Recordings of (a) air temperature and (b) surface temperature along the test road for 18.00 on 15/16 February 1994.The temperatures are expressed as deviations from a reference temperature.

(a)

(b)

peratures and the surface temperature: the lowest tem-perature are found in valleys where stagnation as wellas pooling of cold air leads to low air temperatures.

A second measurement was carried out along the sameroute two hours later (20.00). By comparing these twomeasurements it is possible to draw conclusions aboutthe temperature development and especially the effectthat an accumulation of cold air will have on the surfacetemperature. In Table 1 the deviation from the neutraltemperature for both the air and the surface tempera-ture is shown. The air temperature deviation shows thatintense cold air pools are established shortly after sun-set. A measurement carried out just before sunsetshowed very small temperature variations along thestretch. In a study by Gustavsson et al. (1998) it wasconcluded that this could be explained by stabilisationof the surface-cooled air in sheltered locations such asvalley bottoms and in forests which are not very dense.The difference between the two measurements (I, II)indicates that the intensity does not increase to anygreat extent; the effect of a longer cooling period israther the extension of the cold air pools. The surfacetemperature shows quite a different development. Thesurface is more inert, and therefore a longer time periodis needed in order to adjust to the low air temperaturein valleys. This can be seen in Table 1 from the fact thatthe difference between the neutral temperature and thevalley-surface temperature increases between the twomeasurements even though the air temperature doesnot show the same clear trend.

By plotting the air and surface temperature deviationfrom the reference temperature for both measurementsit possible to determine the relative importance of theadvection-stabilisation of cold air. Thereby it is alsopossible to compare this factor with the other ones thatcan influence the surface temperature pattern. In Figure7 the result of such a comparison is shown. A linear fitis calculated for both situations, and the square of thecorrelation coefficient (R2) increases from 0.45 to 0.67;this indicates that the surface temperature is adjustingto the variations in air temperature along the route.

5. Conclusions

Thermal mapping is a very useful technique in appliedclimatological studies. However, attention must be

paid to a number of factors during the measurement.This holds especially for measurements using IR sen-sors. Particularly important is the mounting of the IRsensor and awareness of the factors controlling theradiation received by the sensor as well as the quality ofthe type of sensor used. Analysis of the recordings inorder to select the most suitable locations for fieldstations in an RWIS is also an important task. It is cru-cial that all parameters that control the road surfacetemperature are measured in order to interpret therecordings.

In the present study it was concluded that three majorfactors could be distinguished which together controlthe variation in RST.

(a) Road-bed material. Measurements along a testroute showed that the variation in road construc-tion material had a significant effect on the RST.The influence of this factor can be determined byuse of detailed information about the materialsused or from analyses of the temperature reactionduring cooling or warming episodes.

(b) Radiation. The obstruction of the radiation thatoccurs in forested or built-up areas can be deter-mined from daytime measurements of the surfacetemperature and furthermore can be related to thenight-time temperature differences.

(c) Accumulation of cold air. Cold air accumulation invalleys and stabilisation in wind-sheltered loca-tions will significantly influence the RST. Theinfluence of this factor will increase during thenight owing to the inertia of the road surface.

Thermal mapping measurements should be plannedand performed in such a way that the influence of theabove-discussed factors is determined. It may also benecessary to study road construction plans regardingthe variation in road-bed materials. The obstruction ofthe radiation can be determined either by direct mea-surements or in a general way by the method describedin this paper. For the actual locations of field stations itis important to select different areas where these factorsvary in importance and also to describe the station sitesvery carefully.

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Table 1. Cold air pool intensity and lowering of surface temperature in the valleys compared with the generaltemperature. Measurement I was carried out at 18.00 and II at 20.00 on 15 February 1994.

Valley number Measurement I Measurement II

Air Surface Air Surface

1 10.1 3.7 10.1 4.22 6.7 2.5 8.1 3.13 7.0 2.7 7.6 3.04 8.5 2.3 7.3 2.8

Acknowledgement

This study was supported by the Swedish NationalRoad Administration. Comments on the draft of thispaper by Dr Jörgen Bogren and Maria Karlsson, MScare also greatly appreciated.

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