1. Introduction

The fact that built-up areas have a pronounced effecton the climate is a well-known phenomenon. One ofthe earliest studies was undertaken by Luke Howard(1833), who studied the anomaly between temperatureswithin and beyond the city margin of London and doc-umented that the city was warmer than the surround-ing rural areas. Howard’s work on London was basedupon a series of simple instrumental measurementstaken between 1806 and 1830. In recent years, ourknowledge of how the urban area affects climate hasgrown markedly. This is a result of both the increasingnumber of studies which document and interpret cli-mate variations within cities, and the development ofnew measuring techniques. During the 1950s and 1960sthe importance of building density in shaping heatislands was highlighted by studies such as those byChandler (1962, 1964, 1965). He showed that thepotential maximum urban heat island for London is10°C. For Swedish conditions Sundborg (1950) investi-gated the role of local climatological factors and tem-perature conditions in the urban area of Uppsala.

Road climatology on the other hand has a much shorterhistory. Lindquist carried out the first studies in Swedenin the middle of the 1970s. His studies dealt with suchsubjects as the development of an ice warning system(Lindquist, 1975, 1976) and the different techniques thatcould be used to find the most suitable locations forfield stations which would be representative of the roadnetwork. Studies of road climatology adopted the tech-niques used in earlier urban climatological studies, suchas mobile temperature measurements and stationarynetworks of automatic weather stations.

The present study aims to describe how the road cli-mate interacts with the urban climate in a large city.With automated traffic surveillance systems beingintroduced into urban areas, there is a need for moredetailed information on the interaction between urban

and road climates. Traditionally road climate studiesare carried out in rural areas and focus on phenomenasuch as the interaction between local topography androad surface temperature (RST) variations, (e.g. Bogrenet al., 2000; Postgård, 2000) or the effect of differentweather scenarios on the development of RST variationand the local risk of road icing (e.g. Eriksson, 2001).

Since the early 1990s the knowledge and equipment usedin association with road climatological applications haschanged greatly. Today road surveillance systems areused in most northern European countries and they arealso beginning to make an impact in North America. Thishas led to systems being used in a more advanced waythan was the case ten years ago. An example is the devel-opment of models that can be used to change speed lim-its automatically on highways according to the prevailingweather conditions. Another type of application is thedevelopment of winter indices that are used in determin-ing the allocation of money for winter maintenanceaccording to the severity of a specific winter or throughcomparison of different areas (e.g. Bogren & Gustavsson,1994; Cornford & Thornes, 1996; Gustavsson, 1996).

2. Causes of variations in road surfacetemperature

Three main factors can be identified as being importantin the development of spatial RST variations: (i) radia-tion, (ii) advection, and (iii) ground heat flux(Gustavsson, 1999). The radiation factor controls boththe daytime heating, owing to the degree of short-waveradiation reaching the road surface, as well as the night-time cooling by long-wave radiation. This factor hasbeen quantified in a number of studies using the sky-view approach (e.g. Bradley, 2000).

During a clear, calm night the variation of air temperatureis largely controlled by local topography, and the lowesttemperature can be found in valleys and hollows owing

Meteorol. Appl. 8, 481–489 (2001)

Road Climate in Cities:A Study of the StockholmArea, South-East SwedenTorbjörn Gustavsson, Jörgen Bogren and Cecilia Green, Laboratory of Climatology,Department of Earth Sciences, University of Göteborg, Box 460, SE 405 30, GöteborgUniversity, Sweden

The difference between air and road surface temperature in urban and rural areas is an importantconsideration when modelling the road climate. In this study the effect of the urban heat island in theStockholm area on road climate is examined. Factors such as distance from the city centre, traffic andtopography are analysed in order to assess their impact on the spatial variation of road and air temperature.

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to the pooling of cold air and the stabilisation effect fromthe valley sides. Cold air advection can have a substantialeffect on the spatial variation in RST along a stretch ofroad. Studies by Bogren & Gustavsson (1991) have doc-umented variations in RST of the order of 5–6 °Cbetween large valley bottoms and nearby hill summits.

The ground heat flux is to a large extent controlled bythe properties of the ground itself; in road climatologythis relates to the layers of material used in the roadbed.Gustavsson & Bogren (1991), for example, demonstratethat variations in road construction materials can pro-duce large variations in road surface temperature.

In urban climate studies the radiation factor is alsoregarded as one of the main factors in the differentialcooling of urban and rural areas. The geometry of thestreet canyons, often described using the sky-view fac-tor, controls the radiation budget and thereby the degreeof cooling on clear nights (Oke & Maxwell, 1975).

Another factor studied in urban and road climatologyis the release of anthropogenic heat in urban areas. RSTcan be affected by both anthropogenic heat release andby the warming effects of traffic, but it is not clear howimportant these factors are in relation to other factorscontrolling RST.

3. Methodology

3.1 Study area

This study was conducted in and around Stockholm inSweden. Stockholm has a population of approximately1 million people and is located between the sea to theeast and Lake Mälaren to the west ( Figure 1).

3.2 Data

The climatological data used in this study are from twomain sources: field stations in the Swedish RoadWeather Information System (RWIS) and synopticweather stations located within the study area.

The locations of most of the RWIS-stations are shownin Figure 1, together with their identifying number aslisted in Table 1. In total, information from 28 differentRWIS stations has been analysed. At these stationsmeasurements of air and road surface temperature andhumidity are taken every half hour. At some field sta-tions the wind speed and direction are also recorded.Table 2 shows the types of sensors used at the RWISstations. All data are stored by the Swedish NationalRoad Administration (SNRA) on a central computer,and the SNRA also performs quality checks on thatdata. SNRA carries out a manual service of all stationsduring the winter measuring period (the beginning ofSeptember to the end of April), which is the periodwhen road slipperiness might occur.

In order to analyse the influence of local climatologicalfactors on the variation of temperature and road icing,the RWIS stations have been classified according totheir individual location (urban, suburban or rural),their distance from the city centre, and the amount ofscreening (Table 1). Field surveys together with topo-graphical maps were used for the classification.

Data from three synoptic weather stations have alsobeen used in the study. These stations are located at theairports of Bromma and Arlanda, and in the city centreat Observatorielunden (see Figure 1). Observatorie-lunden and Bromma are located within the urban areawhile Arlanda is outside the city. Data from these sta-

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Figure 1. Map of the study area (A = Arlanda; B = Bromma; O = Observatorielunden)

tions have been included to give provide measurementsand observations describing the weather situation, i.e.cloud amount, wind speed and direction.

4. Results

Previous studies (e.g. Bärring et al., 1985; Eliasson,1994; Bogren & Gustavsson, 2000) have shown that thechange in road surface temperature between urban andrural areas can be large. The following results aim todescribe the main reasons for this.

4.1 Urban heat island (UHI)

By comparing the observations from Arlanda andObservatorielunden (Figure 1) a good estimation of theheat island effect for Stockholm can be determined. Forthe winter period (November–March) a mean intensityof 1.4 °C for the years 1961–90, has been found. A max-imum heat island intensity of approximately 8 °C hasbeen found. This is in line with the calculated intensityachieved by use of Oke’s formulae, which relate theintensity to the number of inhabitants or the city struc-ture (Oke, 1987).

Using RWIS-stations located in traverses from the citycentre towards the rural areas, an intensity value forboth the air and road surface temperature can be calcu-lated. In Figure 2 the temperatures for three stations areshown. All stations are located in neutral areas so thatthe urban influence can be assessed in isolation.

In Figure 2(a) the air temperature pattern is shown fora cloudless day and night, with a light breeze, inDecember 1996. The diurnal range in temperature forthe city station is much smaller compared with both thesuburban and the rural site. The urban effect is mostclearly shown after sunset when the rural site under-goes much more rapid cooling than the urban site.Owing to the effect of the variable wind, the tempera-ture of the rural site fluctuates during the night. Themaximum temperature difference occurs just beforesunrise and is approximately 7 °C.

The road surface temperature ( Figure 2(b)) has a simi-lar pattern to the air temperature, with the rural sitebeing much colder than the urban site at night.However, the urban–rural RST variation is muchsmaller than the air temperature variation. The maxi-mum difference (more than 4 °C ) occurs at sunrise; thiscan be explained by the fact that the urban road surfacetemperature starts to rise two hours before sunset whenthe temperature at the rural site is still decreasing.

The above results are in line with present knowledgeregarding how urban areas can affect temperaturedevelopment. However, the relatively large variation inRST is somewhat more pronounced than expected,especially the relatively large effect from traffic inurban areas giving rise to heating of the road surfaceprior to sunrise. This has been observed in areas withvery heavy traffic but not previously in any Swedishbuilt-up areas.

The cooling rate of the urban station and the rural oneis similar during the early evening but the big differenceis the temperature level from which the cooling ofstarts. The afternoon temperature in the city is morethan 2 °C warmer than both the rural and the suburbanstation. If the general temperature had been around+3 °C instead, the temperature development during theevening could have resulted in a large variation in the

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Table 1. Description of the RWIS stations used in thestudy.

Station Id Distance Environment* Sitefrom city characteristicscentre (km)

0202 33.7 R Screened0203 25 SU Bridge0207 17.4 SU Screened0209 12 SU Screened0210 22 R Neutral0211 27 R Open0212 23 R Neutral0213 18.5 SU Neutral0214 23.3 R Screened0215 8.1 U Open0216 9.5 SU Bridge0217 10.3 U Open0218 17.2 SU Bridge0219 18 R Screened0220 24.5 R Neutral0222 8.7 U Bridge0223 20.4 R Bridge0224 31.7 R Open0226 20.4 U Neutral0232 14.2 U Open0233 24.4 SU Bridge0234 31.2 R Open/neutral0235 4.6 U Built up0236 3 U Bridge (missing data)0237 0 U reference Neutral0240 9.2 R Open0241 21.5 R Screened0243 7.7 U Neutral

*U = urban; SU = suburban; R = rural

Table 2. Instrument specification for the RWIS-stations.

Parameter Range Accuracy Instrument

Air Lambrecht temperature –50 – +50°C ± 0.26°C (8091100)

Humidity 0 – 100% ± 2.5% LambrechtSurfacetemperature –60 – +70°C ± 0.26°C Pt-100 DIN 43 760

Winddirection 0 – 360° ± 22.5° Vaisala WAV151

Wind speed 0.3 – 75m/s ± 0.2m/s Vaisala WAA151Precipitation Aerotech Telub

Optic Eye

risk of black ice formation. This result will therefore beof great importance when dealing with the forecastingof road surface temperature as well as the prediction ofslipperiness on roads in and around urban areas.

4.2 Distance from city centre

Lindqvist (1970, p. 55) demonstrates that the change intemperature between open and built-up areas is verysharp, and Oke (1987) used mobile temperature mea-surements to show that such changes occur as distinctsteps or cliffs.

In order to examine both the air and road surface tem-perature differences between urban, suburban and ruralareas, data from all RWIS-stations have been analysed.RWIS-station No.0237 (city centre) has been used as areference for this comparison. In Figure 3(a) the varia-tion in minimum air temperature compared with thereference station is plotted against distance from thecity centre. The temperatures are for a clear night inDecember 1996 with a light breeze. The magnitude ofthe UHI is 5 °C (at 0700 hours) taking the datarecorded at the meteorological stations. A linear trendcan be fitted to the RWIS-data giving an R2 of 0.54. Thestations are plotted with symbols representing the loca-

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Figure 2. Air temperatures (a) and road surface temperatures (b) for three RWIS stations, (0213 suburban, 0237 urban and 0234rural) during a clear night with a light breeze, 16–17 December, 1996.

tion (urban, suburban or rural). If only the urban sta-tions are compared there is no significant correlationbetween distance and variation in minimum air temper-ature. This is also valid for the suburban stations sincethe coldest station is located relatively close to the citycentre (approximately 12 km). It is only by comparingthe urban and the rural stations that a clear trendbecomes evident. The rural stations become muchcolder with increasing distance from the city and themaximum variation amounts to 8 °C.

The same conclusion holds for the variation in minimumRST (Figure 3(b)). The difference between the warmestand coldest stations is much smaller for surface temper-

ature (4° C) than air temperature (8 °C). Otherwise thetrends are the same, i.e. the variations between the urbanstations are relatively small and not correlated with thedistance from the city centre. The rural sites, on theother hand, are colder with increasing distance from thecity, as was the case for minimum air temperature.

4.3 Topoclimate

The combined effect of different micro and local clima-tological factors – topography, land use, vegetation andother factors relating to surface characteristics – is oftenreferred to as the topoclimate of an area (Thornthwaite,

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Figure 3. Difference in minimum air temperature (a) and minimum road surface temperature (b) plotted against reference sta-tion no 0237 versus distance from city centre for the studied RWIS stations, 16–17 December, 1996. Met. stations areObservatorielunden (urban) and Arlanda (rural)

1953). The importance of local climatological factors,especially local topography, has been dealt with in anumber of studies related to road climatology (e.g.Bogren & Gustavsson, 1991; Bogren et al., 2000).

As can be seen in Figures 3(a) and 3(b), the magnitudeof the urban effect differs significantly and is by nomeans dependent only on the relative distance from thecity centre. Because the field stations in the RWIS arelocated in several different topoclimatological areas thedata from these stations were used to determine theeffect of different parameters in relation to the decreas-ing influence of the urban heat island with increasingdistance from the city centre.

In Figure 4(a) and (b) each station site is labelled with ashort description covering the most significant topo-graphical factor for that site. The variation in minimumair temperature versus the reference station is plotted inFigure 4(a). It can be seen that for stations in the urbanarea the influence of local topography is not very large(the air temperature is up to 2°C lower than the referencestation). However, for the stations located in suburbanand rural areas, the influence of local topography ismuch more pronounced. Here the temperature differ-ence varies between –8°C and –1°C colder than the ref-erence station indicating that the local environmenttogether with the distance from the city centre determinethe variation in minimum air temperature.

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Figure 4. Variation in minimum air temperature differences (a) and road surface temperature (b) due to topoclimatological fac-tors, compared with reference station 0237.

The variation in minimum road surface temperatureversus the reference station is shown in Figure 4(b). Aswith the air temperature, the importance of local topo-graphical factors at each station site becomes moreimportant with increasing distance from the city centre.The screening effect, for example, results in a very smalldifference in night-time surface temperature comparedwith the reference station. This is because the screeningreduces the amount of long-wave radiational cooling.

The temperature development for the reference stationand a screened station (approximately 23 km from thecity centre) is shown in Figure 5. The surface tempera-ture is lower at the screened site during the day com-pared with the open reference station owing to thereduced direct solar heating of the road. During thenight the effect discussed above becomes obvious: thecooling of the screened road surface is much slowerthat that of the reference station. The variation in RST,just before sunset, amounts to approximately 1°C.However, during the morning hours the difference intemperature is reversed and is also much larger becausethe screened site is not heated by direct short-waveradiation.

A weak trend is apparent whereby low air temperatureresults in lower surface temperature (R2 = 0.3). In ruralareas this relationship is normally much more pro-nounced (Gustavsson, 1990). Around the city ofStockholm several other factors are more important indetermining the minimum road surface temperature.For example, bridge stations tend to record low surface

temperatures because heat storage is much reducedcompared with on an ordinary road. The screened sta-tions are more influenced by the radiation conditionsthan by the local air temperature. The results presentedin this study lack data from stations sited in well-defined valleys. This type of location should showlower air and road surface temperatures in the urbanand suburban areas as well as the rural areas. That thisis the case has been documented by use of thermal map-ping in urban areas. These mobile temperature surveyswill be used in a future study to investigate more fullythe influence of local topography on temperature vari-ations in and around cities.

4.4 Influence from traffic

According to Thornes (1985) the length of time that theroad surface is screened from the sky by cars is animportant factor in the influence of traffic on RST.Other studies (e.g. Gustavsson et al., 1987; Gustavsson& Bogren, 1990) have concluded that other factors maybe of greater importance, such as the heat release fromengines or the increased turbulence generated by thecars. Farmer & Tonkinson (1989) examined the influ-ence of traffic on the diurnal temperature cycle andfound that a higher traffic density resulted in warmerroad surface temperatures.

In the city of Stockholm the early morning traffic startsto build up around 0530h. This start is identified by amarked change in RST – the nocturnal cooling trend is

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Figure 5. Temperature development for a screened RWIS station and the exposed reference station during a clear night inDecember.

interrupted and warming begins. Figure 2(b), whichplots RST for the urban, suburban and rural areas, indi-cates that this temperature change move successivelythrough the different areas: the warming effect isnoticed first in the urban area (0530 h), followed by thesuburban areas (0600 h), and then in the rural areas(0830 h), which is the time when sunrise occurs. Thetraffic in the rural area is very sparse and not sufficientto affect the RST. The rise in temperature in the ruralarea is caused entirely by the incoming radiation fromthe rising sun.

It is also important to look at the air temperature forcomparison (Figure 2(a)). The general level of the airtemperature is higher than the RST in the urban (~2 °C)and suburban (~1.5 °C) areas but not in the rural area.When the early morning traffic starts, the air tempera-ture levels out but there is no pronounced warming asfor the RST in the urban and suburban areas. The sig-nificant warming of the air does not start until sunrise.

The effect of the traffic results in a warming of the RSTthat is of the magnitude of 2 °C between 0530hr and0830hr (when the sun rises) in the urban area, and0.8 °C between 0600hr and 0830hr in the suburbanarea. In the rural area the RST continues to fall at a rateof 0.2 °C/h until sunrise. The cause of the rise in RST isprobably a combination of the heat release from thevehicles and turbulent mixing of the warmerurban/suburban air. This shows that it is very impor-tant to take into consideration the effect of traffic whencalculating the development of RST.

5. Conclusions

The results of this study show that it is important toconsider the climatological effect that is generated bylarge city areas when dealing with road climatology.The urban heat island has a marked impact on the airand road surface temperatures. This effect can be calcu-lated essentially as a function of the distance from thecity centre. This is especially true for air temperaturebut less so for road surface temperature. However, thisstudy shows that in order to calculate the urban effecton air and road surface temperature, factors such as thelocal topography and traffic density must be consid-ered. Radiation conditions and roadbed characteristicsin combination with the local topography have beenfound to be the major factors causing temperature vari-ations within the three environments (urban, suburbanand rural).

The results show that further studies and analysis areneeded to take the effects of topography and trafficfully into account. In assessing the influence of topo-graphical factors a more comprehensive set of mobiletemperature recordings are needed. It is also importantto establish how the volume of traffic can be used as aparameter in the equation to forecast RST develop-ment.

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