radiation absorption and urban texture

Radiation absorption and urban texture

Koen Steemers, Nick Baker, David Crowther, Jo Dubiel and Marialena Nikolopoulou

The Martin Centre for Architectural and Urban Studies, Department of Architecture, University ofCambridge, 6 Chaucer Road, Cambridge CB2 2EB, UK

E-mail: [email protected]

This paper explores the relationship between the geometry of urban neighbourhoods and the microclimaticeffects of solar radiation. It presents both the modelling techniques and key �ndings with the aim ofquantifying solar absorption and its relationship to the urban heat island effect. The impact on the urbanenvironment and building energy consumption is discussed, which raises potential implications fordesign. This work discusses only one main environmental factor, solar radiation, which is part of a largerongoing research agenda addressing wider aspects of the urban microclimatic.

Cette eÂ tude explore la relation entre la geÂ omeÂ trie des quartiers urbains et les effets microclimatiques desrayonnements solaires. Elle preÂ sente aÁ la fois les techniques de modeÂ lisation et les reÂ sultats cleÂ s destineÂ s aÁquanti�er l’absorption solaire et sa relation avec l’effet d’ õÃ lot de chaleur urbain. Sont eÂ tudieÂ es lesreÂ percussions sur l’environnement urbain et la consommation d’eÂ nergie par les baÃ timents dans la mesureouÁ elles pourraient eÃ tre eÂ ventuellement prises en compte lors de la conception. Cette eÂ tude traite un seulfacteur majeur de l’environnement: les rayonnements solaires, qui font partie d’un programme de recherchepermanent prenant en compte des aspects plus globaux du microclimat urbain.

Keywords: urban microclimate, solar absorption, urban form, heat island, modelling

Introduction

There is an increasing need to understand and beable to quantify the microclimatic characteristicsof our cities. On the one hand, the concentrationof activities provides a necessary and stimulatingsocial, cultural and economic milieu. On theother hand, the high density of buildings andtransport use consumes energy and alters themicroclimate. This can affect both the well-beingof people and, crucially, by increasing thedemand for sealed indoor environments andair-conditioning, the potential for low-energybuilding design strategies. Air-conditioned andarti�cially lit buildings dramatically increasebuilding energy consumption (and buildingsaccount for half of our total energy use andCO2 emissions). In addition, uncomfortable and

unhealthy urban conditions discourage peoplefrom walking, cycling or using public transport,thereby encouraging private vehicle use, whichin turn leads to more energy consumption andatmospheric pollution. Poor urban microclimatesthus have signi�cant implications for energy useas well as health.

However, it is clear that urban microclimatesvary considerably, both from city to city andwithin cities, some areas, for example, beingmore affected by the ’heat island’ phenomenonthan others. This variation can be seen to resultfrom a complex interaction between urban form,climate and human activity. Urban form interactswith climate, particularly in relation to solaraccess, with implications for temperatures, day-light availability and passive solar design solu-

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BUILD IN G RESEARCH & IN FO RMATIO N (1998) 26(2), 103–112 103

tions, as well as active solar systems (collectors,photovoltaic cells, etc.). In other words, if we areto extend renewable energy use in cities (whichis now where most people live and where mostbuilding stock is located), there is an urgent needfor developing understanding and tools to facil-itate bioclimatic urban design. This paper, whichis part of a broader study, focuses on one aspect:the interrelationship between radiation absorp-tion and urban built form, revealing the magni-tude of the effects and discussing the potentialimplications of these insights. It thus aims toshow only one link between urban form andmicroclimate, and outline various techniques andnew de�nitions that were developed. It hopes todemonstrate the need for continued and oftennew theoretical research in this �eld, in order tobegin the transfer of the knowledge to urbandesign and practice (i.e. to establish the linkbetween urban form and environmental charac-teristics).

One of the ways in which urban texture in�u-ences indoor and outdoor climate is as a resultof the fact that different city layouts absorbdifferent proportions of solar energy. This canoccur simply because of differences in interre-�ection in cavities such as street canyons andcourtyards. This phenomenon is well summed-up by Givoni (1989): ’The radiation falling onvertical walls is partly re�ected, mostly towardsother walls of nearby buildings [. . .] In an urbanarea, a great part of these bounced-off rays hitwalls of adjacent buildings. In this way, theprocess begins of radiation bouncing back andforth a number of times between the walls ofdifferent buildings. At the end of this process, ina densely built urban area, only a small part ofthe solar radiation impinging on walls isre�ected upwards to the sky, while most of itis absorbed.’

The effect may be expressed as a decrease in theoverall re�ectance of the urban texture whencompared with a �at plane of the same materialand same ’paint’ re�ectance. The comparisonminimizes the effect of the complex combinationsof micro-structural surface characteristics, focus-ing on the effects of built form. The decrease ofre�ectance is an effect of the extra geometricalcomplexity as well as the ’paint’ re�ectance forthe radiation band concerned. Thus the effect canbe applied at any wavelength. However ourinvestigations are chie�y concerned with visible

wavelengths, containing around 40% of theenergy of the solar radiation reaching the Earth’ssurface.

Reducing pollution and building energydemand for cooling

The decrease in re�ectance of an urban texturerepresents the radiant energy that is absorbed andconverted to heat, rather than being re�ected backout to the sky. There may be occasions when thisextra heat may be bene�cial in saving energy, forexample in colder climates and seasons (typicallyin the UK the reduction in heating requirements isin the order of 10% (Lacy, 1977)). However, overlarge parts of the world it will be a hindrancebecause of the need for additional cooling andbecause of the contribution to the urban heatisland effect.

Sailor (1995) has demonstrated with meteorologi-cal simulations that an increase in re�ectance of0.14 over downtown Los Angeles and of 0.08over the entire basin would decrease peaksummer temperatures by 1.58 C, potentially lower-ing energy demand and atmospheric pollution by5%. This is a useful potential contribution tomeeting, for example, CO2 emission targets andsets a useful benchmark. However, it raises thequestion ’What reduction in re�ection is depen-dent on urban texture, and how do differentcities from different climates affect re�ectance?’

The decrease in re�ectance due to urban texturemay be expected to vary with solar altitude, withlayout in relation to sun position, and with thesurface re�ectances of the city components. Itwill also depend on characteristics of the urbangeometry, which we have termed the ’urbantexture’. For example, a city with a complexstructure of cavities would be expected to trapmore radiation than an open city with largeblocks of deep-pan buildings. To study theseeffects and their magnitudes, a series of investi-gations has been carried out, both on the genericforms and on case study areas, which exemplifythree very distinct urban textures. The case studyareas, in London, Toulouse and Berlin, werechosen to represent a range of typical urbantextures. The London site is orthogonal in planbut random, particularly in terms of buildingheights. The Toulouse site has a typical ’random’medieval street pattern with a major boulevardrunning through part of it, and all buildings are

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of very similar height. Berlin has a grid plan andmoderately uniform building heights, but thegrid in this area is disrupted diagonally. Theinvestigations include laboratory measurementsof photometric re�ectance, in parallel with com-puter simulations which allow the estimation oftotal extra energy trapped. The techniques andresults are described below.

Physical urban models

Three-dimensional physical models were made inexpanded polystyrene of a representative centralarea of the three cities: London, Toulouse andBerlin.

The main aim of the experiment was thedetermination of the re�ectance of each modelas a function of the ’paint’ re�ectance. However,determination of the hemispherical re�ectancerequires the measurement and subsequent inte-gration of light re�ected from the model in alldirections (over the whole hemisphere). This wasapproximated by light measurements taken inone plane only, as explained below (this resultantre�ectance is referred to as the ’planar re�ec-tance’).

Each model was identical in plan area and scale,1:500, representing a real-life ground area of

400 3 400 m. For the experiments, each modelwas illuminated by a light beam, representingthe sun at different angles of illuminance. Thelight re�ected back from the model was thenmeasured at a series of angles in the same planeas the light beam, using a specially constructedluminance meter. Three angles of illuminationwere used, corresponding to three realistic sunpositions during the day and seven angles ofobservation (Fig. 1).

Readings were taken for each of the three citymodels, and a �at reference plane of the samematerial and paint re�ectance, with which thecity results were compared. Each of the modelsand the �at plane were painted at differentre�ectance values, 90%, 60%, 30% and 15%. Anexample of the distribution of re�ected light indifferent directions, for all the three sun anglesand for different paint re�ectances, is shown inFig. 2.

In order to obtain an approximate value torepresent the overall light energy being re�ectedfrom the model, it had been assumed that there�ected light energy in the third dimension,either side of the xz illumination plane, variesconsistently and in proportion to the light energymeasured on the xz plane. This simplifyingassumption was checked by using angles ofobservation on a different plane from the sun,

Model in vertical position

x

1 67.5a1 45a

1 22.5a

0a

2 22.5a

2 45a

2 67.5a

z

Observation point

Illumination point

x

y

Fig. 1. Model setup for re�ectance experimental measurements.

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RADIATION ABSORPTION AND URBAN TEXTURE

at 22.58 and a set at 45 8 from the illuminationplane. The results compared well.

Another simplifying assumption was that thetexture of each form was assumed to be roughlysimilar however orientated so that it made nosigni�cant difference which way the models weremounted. To test this, each form was rotated 90 8and a set of additional readings obtained.

The results represent the total energy re�ectedfor each sun angle for a particular urban modelwith a particular paint re�ectance. Cosine correc-tions have been applied to these �gures both forthe observation point (to produce energy weight-ed results rather than luminance weighted), andthe illuminating source (to refer the results to thesame unit of energy in order to make themcomparable). The average planar re�ectance fordifferent paint re�ectances for each form are thencompared with the corresponding values for the�at plane, and the percentage reduction betweenthe two are plotted (Fig. 3).

The overall �ndings are summarized below:

· In broad terms the study has shown andquanti�ed that, for all surface re�ectances,urban forms absorb upto 40% more sunlightenergy than �at planes. This is important inunderstanding such issues as the urban heatisland effect, at least in part (reduced windspeeds, reduced evaporation, urban heatsources, etc., are not discussed in this paper).

· Furthermore, it is apparent that the complex-ity, or occlusivity, of the urban textureaffects the amount of light that is absorbed.Thus the reduction of light re�ected from themodelled surfaces (compared with planesurfaces) is much greater for Toulouse (upto 40%) than for London and Berlin (15%)for realistic re�ectances of around 20%(Fig. 4). A link between a description ofurban form (e.g. occlusivity) and re�ectancecan thus be established to predict theradiation absorption of any urban texture.

Fig. 2. Measured distribution of re� ected light for the London model for three sun-angles and four different paintre� ectances.

sun at 67.5sun at 45.0sun at 22.5

reflectance 2 87%

67.50

45.00

22.500.00

22.50

45.00

67.50

2.00

2.50

1.50

1.00

0.50


reflectance 2 62%

67.50

45.00

22.500.00

22.50

45.00

67.50

2.00

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1.000.50


reflectance 2 36%

67.50

45.00

22.500.00

22.50

45.00

67.50

2.00

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0.50


reflectance 2 17%

67.50

45.00

22.500.00

22.50

45.00

67.50

2.00

2.50

1.50

1.000.50

40%

20%

0%

2 20%

0 20 40 60 80 100

Reflectancereduction (%)

67.545.022.5Mean

Paint reflectance (%)

Fig. 3. Planar re� ectance reduction for London com-pared to � at surface (%) – laboratory.

LondonToulouseBerlin

Reflectanceredction (%)

60%

40%

20%

0%0 20 40 60 80 100


Fig. 4. Mean planar re�ectance reduction for the threecities compared to a �at plane (%) – laboratory.

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STEEMERS ET AL.

· In general, the results show, as one mightexpect, that the extra amount of lightabsorbed by an urban form tends to increaseas surface re�ectance decreases (Fig. 3). Thisis con�rmed by the vernacular white hilltowns in Greece.

· A clear pattern is for more light energy to beabsorbed at low angles of elevation thanwhen the angle of illumination is closer tothe normal. Thus more low angle winter sunis absorbed, becoming more pronouncedwith lower re�ectances (e.g. the timbervernacular of Scandinavia).

To investigate whether there are correlationsbetween directionality and energy absorbed bythe urban form, the models were rotated 908 for adifference re�ectance each. For Berlin and Londonthe orientation does not seem to have a signi�canteffect on the results (a 7% and 8% difference,respectively). For Toulouse, there seems to be amore striking effect, the results varying by 17%,but this is due to the wide boulevard which has astrong directional characteristic.

Luminance variations in the city

The next step is to identify the richness of thetexture in terms of its cavities, that is to say thecourtyards and narrow streets, which provideopportunities for interre�ection, as opposed toroofs of buildings, which tend to re�ect more ofthe incident light. In order to get an idea of thevariation of luminance across the urban texture, aluminance meter ( with an angle of acceptance of1 8 ) was used to take multiple point measurements,rather than a single integrated reading as above.

The experimental set-up was similar to theprevious experiment. However, in order tomeasure the effects solely due to interre�ectionand not due to existence of shadows, theobservation point was directly behind the illumi-nating source. The experiments were carried outfor the three cities and for each of the three sunangles. Readings were taken along a straight linefrom one side of the model to the other at 20 mmintervals.

The main �nding is to con�rm that there areindeed variations in observed luminance levelswithin an urban texture as seen by the sun.These variations were least for Berlin, which theprevious study found to absorb least energy, and

they were highest for Toulouse and London (Figs5 7). The results for Toulouse were distorted bythe presence of trees with 30% paint re�ectance,the line of measurement passing through a tree-�lled courtyard and across the tree-�lled boule-vard (Fig. 6). For London, a noticeable troughcan be observed for the top curve (22.58 ),coinciding with the position of Tottenham CourtRoad (Fig. 5), although this becomes less obviousfor the other two angles.

A general point as with the previous study isthat the actual luminance readings are consis-tently higher for the higher angles of illumina-tion that is the re�ectance is higher for high’sun’ angles.

100

75

50

25

0

Luminance(cd/sqm)

0 20 40 60 80Distance (cm)

22.5 deg45 deg67.5 deg

Fig. 7. Variation of luminance across the model forthe Berlin case study – 14% re�ectance.


806040200Distance (cm)

0

25

50

75

100Luminance(cd/sqm)

Fig. 6. Variation of luminance across the model forthe Toulouse case study – 14% re�ectance.

Luminance(cd/sqm)


100

75

50

25

00 20 40 60 80

Distance (cm)

Fig. 5. Variation of luminance across the model forthe London case study – 15% re�ectance.

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Computer simulations

RADIANCE is one of the most sophisticatedlighting simulation packages available and iscapable of accurately calculating direct, diffuseand interre�ected light using its innovative algo-rithm based on backward ray-tracing (Ward,1994). Although in this project we are primarilyinterested in the accurate numerical output ofRADIANCE, it is also well known for its photo-realistic images. The examples in Fig. 8 are of thethree modelled areas at 20% ’paint’ re�ectance,with sunlight at an altitude of 458 in the south-west.

Simulations were carried out for each of the threecase study areas. For this, the geometry of thebuildings in each of the 400 3 400 m areas had tobe entered in to the computer. Some simpli�ca-tion in geometry is obviously necessary, and forinterre�ection, the level of complexity of themodelling could be important. Here we areconcerned mostly with the effect of streetcanyons and courtyards, so an appropriate levelof detail was used. All surfaces were de�ned asLambertian diffuse re�ectors, and each modelwas simulated as having a uniform grey colourwith speci�ed uniform ’paint’ re�ectance. Thisallowed the effects on re�ectance of the geometryand urban texture to be isolated.

Modelling was done under conditions of directlight, simulating the sun at different positionsbut without a diffuse sky component. Theillumination of the model depended only onsolar position, and not on the varying effect ofthe atmosphere for different solar altitudes. If there�ectance under conditions of diffuse illumina-tion is of interest, this may be estimated from theresults under direct illumination by using thefact that any diffuse sky distribution may bedivided into areas corresponding to the locationsof the direct sources modelled, and taking aweighted mean of the direct illumination results.

One of the most important parameters to controlin RADIANCE modelling is the number ofinterre�ections calculated. This was set at therelatively high value of 7, to yield accurate results.

Spot measurements

Figure 9 shows a plan view of the Toulouse site, at20% ’paint’ re�ectance with diffuse skylight. Seveninterre�ections were calculated. In analogy to thespot luminance measurements carried out in thelaboratory, RADIANCE can be used to measurethe luminance at any visible spot. This showedthat typically, the luminance of the ground withincourtyards was only between 10% and 30% of theluminance of exposed streets and roofs.

Con�rming physical model results

As a parallel to the laboratory measurements, asimilar series of measurements was made usingcomputer simulations. The simulations were of theLondon site, and used the same illumination andmeasurement positions as the laboratory measure-

Fig. 8. RADIANCE images, from southerly directions,of London (top), Toulouse (middle) and Berlin (bottom)models in sunny conditions.

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ments. From the simulation results, the averageluminance of the model as appearing from themeasurement position could be calculated. Usingthe same procedure as in the laboratory experi-ments, calculations were made of the reduction inluminous energy re�ected by the model comparedwith a uniform �at plane of the same ’paint’re�ectance. The entire procedure was carried outfor ’paint’ re�ectances which match those used inthe laboratory for London. The results are plottedin Fig. 10. The mean results for the three solarpositions are also plotted.

It can be seen that, as with the laboratorymeasurements , the luminous energy re�ectedinto the measurement plane decreases withdecreasing ’paint’ re�ectance and with decreasing

’sun’ angles. These initial results con�rmed the�nding of the physical tests.

Measuring overall re�ectances

Although measurement positions in one plane cangive information about the relative abilities toabsorb radiation of varying urban textures withvarying illumination and ’paint’ re�ectance, toquantify the total extra luminous energy absorbedin each case we need to measure the totalhemispherical re�ectance under the chosen con-dition of illumination. This would be equivalentto using measurement positions over a wholehemisphere, then integrating the results. Analternative experimental procedure could use anintegrating hemisphere. However, these wouldboth be dif�cult and costly physical experimentalprocedures; instead we can use the RADIANCEsoftware to produce equivalent results.

Using the same method of comparing urbantextures to �at planes of the same ’paint’re�ectance and material, total hemisphericalre�ectance for direct illumination was calculatedfor each of the three urban sites. This was donefor re�ectances of 20%, 40%, 60% and 80% foreach site, and with illumination incidence anglesof 22.5, 45 and 67.5 degrees. The results arepresented in Fig. 11 for London and arerepresentative of results for the other locations.The comparison of the three sites is shown inFigure 12, showing mean results for the threesolar incidence angles.

It can be seen that, again, the extra capturedenergy increases with increasing incidence angleof illumination, and with decreasing ’paint’re�ectance. The effect of the urban geometry

Fig. 9. RADIANCE view from above of Toulouseunder diffuse sky conditions.

67.54522.5mean

100806040200Paint reflectance (%)


60%

40%

20%

0%

Fig. 10. Reduction in planar re� ectance for Londoncompared with � at plane – computer simulation.

67.545.022.5mean



60

40

20

0

Fig. 11. Reduction in hemispherical re�ectance forLondon compared with � at plane – computer simula-tion.

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can clearly be seen in Fig. 12. The complexmedieval texture of Toulouse captured mostextra luminous energy, with a decreased re�ec-tance of up to 24% compared with the �at planevalue, at the realistic building surface re�ectanceof 20%. Next in magnitude is London, with�gures of up to 20% decrease. Berlin was themost re�ective, with up to 17% decrease.

Physical modelling vs. computersimulations

Three techniques for estimating re�ectance havebeen described: laboratory measurements con-�ned to one plane, RADIANCE simulations mea-suring in the same plane, and RADIANCEsimulations to estimate hemispherical re�ectance.All techniques agree in the ranking of the threeurban sites, and in showing that luminous energyabsorption increases with increasing incidenceangle (decreasing angle of elevation) and withdecreasing ’paint’ re�ectance. Thus we may becon�dent that any of the techniques may be usedfor this type of comparative study.

In quantifying the total luminous energy absorbedby an urban texture, RADIANCE simulationswould be expected to produce the most accurateresults, as they are able to consider the wholehemisphere. However, it is also helpful to assessthe usefulness of the laboratory procedure, whichcan be carried out with relatively simple equip-ment. The principal inaccuracy of measuringre�ected light in only one plane can be assessedby considering Fig. 13. This compares the resultsof the three procedures for an illuminationincidence angle of 45 8 for the London model.The results for the other incidence angles weresimilar. Both the sets of results measured in the

E W plane were integrated, using the procedureexplained earlier. By considering the two sets ofRADIANCE results only, it can be seen thatmeasuring only in the single plane considerablyunderestimates the extra light absorbed. This canbe explained by the preferential re�ection of lightinto the plane of measurement, which may beexpected for such a geometry with many planesorthogonal to the measurement plane, and aLambertian diffuse surface. The laboratory proce-dure produced results with a magnitude betweenthat of the simulation results for measurements inthe plane and the simulation results for hemi-spherical re�ectance, except at high re�ectances.This can be understood when it is rememberedthat the surfaces used were real materials with anuneven microtexture, which would not re�ect intothe measurement plane quite so much as theperfect Lambertian surfaces used in the computersimulation.

The �nal conclusion is that the use of a singleplane for measurement underestimates the lightabsorbed, as does the use of perfect Lambertiandiffuse surfaces rather than realistic buildingsurfaces. These points must both be borne inmind when making measurements of urbantexture re�ectance effects. These results lead usto believe that for real surfaces, at realisticre�ectance values of 20%, we might expect theincrease in luminous energy absorbed by the realurban texture compared with a �at plane to beeven greater than 25%.

Discussion

It has been shown here that all urban textureshave an increased ability to absorb solar radiationthrough interre�ection due to the effect of the

LondonToulouseBerlin



60

40

20

0

Fig. 12. Reduction in hemispherical re�ectance com-pared with � at plane for London, Toulouse and Berlin– computer simulation.



60%

40%

20%

0%

2 20%

0% 20% 40% 80%60% 100%

Radiance: hemispherical reflectance

Radiance: planar reflectance

Laboratory: planar reflectance

Fig. 13. Comparison of laboratory results with compu-ter simulation for London – illumination incidenceangle 458 .

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urban geometry, and that it can be measured andpredicted. All measurement methods con�rm thatthe extra radiation absorbed increases with lowersun altitude, and with decreasing re�ectance. Allmeasurement methods show this energy-trappingability of the urban texture is greatest for theToulouse site, which has re�ectance decreasescompared with a �at plane of 25% for realisticbuilding re�ectances of 20%, taking the mean ofthe three representative solar positions. The nextin magnitude is London, with the correspondingre�ectance decreases being up to 20%, then Berlinwith a �gure of 17%. These �gures are all forhemispherical re�ectance for the Lambertiandiffuse surfaces modelled in RADIANCE. Thecomparison between laboratory and computersimulations has shown that these are likely to beunderestimates : �gures may be higher for morerealistic building surfaces which will not havesuch a regular re�ectance distribution function.Thus the maximum value measured in thelaboratory, of a 40% re�ectance decrease forToulouse, can be realistic.

To consider if this discovery is signi�cant tourban microclimate in general, a question arises:is the increase in absorptivity signi�cant in termsof the heating effect of absorbed radiation, onecause of the urban heat island effect? Thisquestion is discussed below.

It is worth noting that the measured reduction inre�ectance is in the visible region of the electro-magnetic spectrum. Due to the predominance ofnon-metallic surfaces in the urban surface, wewould expect that the absorptivity in the infraredregion would already be close to 100% irrespec-tive of the urban texture.

In our laboratory measurements, the maximumdecrease in re�ectance was found to be 40%, forToulouse. Assuming an initial re�ectance of 20%,thus would reduce the re�ectance to 12% (that is0:6 3 20%), an 8% absolute change. Sailor (1995)has already been quoted showing that an in-crease of optical re�ectance of 8% over the wholeof Los Angeles would lead to a reduction of1.58 C in peak summer temperatures. We couldinfer from this, that in our case the urban textureis accounting for an increment of similar magni-tude. This con�rms the link of this urbancharacteristic to the urban heat island effect.Furthermore, in comparison with measured heatisland effects, a 1.5 8 C increment from solar

absorption goes a long way to accounting forthe overall temperature increase compared to therural environs (typically upto 38 C) (Landsberg,1981).

For the other sites, the increase in absorption isless, only about 10% in the case of Berlin, andthus we would expect the temperature incrementto be proportionately less.

However, the re�ectance measurement is aver-aged over the whole urban surface. As can beseen from the luminance scans of the three cities(Figs 5 7), the luminance is not uniform, indi-cating that the ’extra’ absorption of energy istaking place in the cavities, that is, on the groundand facades of the streets and squares, and noton the roofs.

This non-uniform re�ectance decrease impliesthat the thermal impact in these spaces will begreater than the average effect. As we haveshown the decrease in urban re�ectance is moremarked in Toulouse with its narrow streets andbuildings, than in Berlin with the wider openspaces. This may at �rst seem counter to thetraditional view that the narrow streets of thesouthern European cities demonstrate an appro-priate climatic response. But we must rememberthat except when the street is running in adirection coinciding with the current solar azi-muth, a greater height to width ratio of the streetwill result in very little direct radiation reachingground level. Most will fall on building surfaceshigh up in the street canyon and the heatingeffect will be reduced by some combination ofconvection and wind. Some direct sunlighttypically increases the effective temperature of aperson by about 6 8 C, shading from direct sun ismore important than the increase in absorbedradiation.

We conclude then that the reduced re�ectancedue to urban texture is more likely to affect theurban temperature at the mesoscale than at themicroscale. Prediction of this effect is useful toset the mesoscale boundary conditions, but it willstill need a more spatially sensitive modellingprocedure to predict conditions at street level.Such work is ongoing at the Martin Centre andelsewhere, building on the �rst steps outlined inthis paper which were necessary in order tomake the next more detailed predictions. Inparallel with solar thermal analysis, the research

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addresses the link between urban form descrip-tors and air movement, pollution migration, airquality and building energy use. Image proces-sing techniques are exploited and developed todescribe form and predict the environmentalcharacteristics of any city texture.

Acknowledgements

The research �ndings described here are part ofthe results from a project funded by the EuropeanCommission’s APAS Programme (DGXII). Theproject was entitled ’Project ZED: Towards ZeroEmission Urban Development The interrelation-ship between energy, buildings, people, andmicroclimate ’ (Contract APAS-RENA CT94 0016),

co-ordinated by Dr Steemers at the Martin CentreUniversity of Cambridge Department of Architec-ture.

References

Givoni, B. (1989) Urban design in different climates.WMO Technical Document, No. 346.

Lacy, R.E. (1977) Climate and Building in Britain, B.R.E.,Watford.

Landsberg, H.E. (1981) The Urban Climate, AcademicPress, London.

Sailor, D.J. (1995) Simulated urban climate response tomodi�cations in surface albedo and vegetativecover. Journal of Applied Meteorology, 34, 1694 704.

Ward, G. (1994) The RADIANCE lighting simulationand rendering system, in Computer Graphics Pro-ceedings, Annual Conference Series.

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radiation absorption and urban texture

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