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Towards a prescription for the greater use of climatic principles in settlement planning
Energy and Buildings, 7 (1984) 1 - 10
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Towards a Prescription for the Greater Use of Climatic Principles in Settlement Planning T. R. OKE The University of British Columbia, 1984 West Mall, Vancouver, B.C. (Canada)
SUMMARY
Despite some notable exceptions, relatively little o f the large body o f knowledge concerning urban climate has permeated through to working planners. This lack o f application has been bemoaned by climatologists for many years. The reasons for this state o f affairs are many, but amongst those most cited are the inherent complexity o f the subject, its interdisciplinary nature and lack o f meaningful dialogue between planners and the climatological research community. Whilst this is true, these potential barriers have n o t prevented application o f knowledge from parallel disciplines by planners, nor the adoption o f climatological principles by architects and engineers at the building scale. The paper reviews the possibility that part o f the problem is attributable to the nature o f research in urban climatology and the difficulty o f translating its results into robust tools for application. A critical assessment o f the preparedness o f urban climatology to be an applied science reveals progress and promise but also weaknesses. The latter include a lack o f quantitative techniques and relationships, lack o f standardization, generality and transferability, and the absence o f clear guidelines for those wishing to learn and use climatological principles in settlement planning. There is also a dearth o f first-hand information from the low latitudes which must be filled if climate is to play a significant role in planning the massive urbanization which is under way there.
INTRODUCTIO N
Present consensus When asked to review or analyze the state of the field relating climate to settlement 0378-7788/84/$3.00
planning, one has little difficulty in arriving at a clear two-part consensus. The first part is that climate has an important role to play in settlement design and must be included as an integral part of the education of every urban planner. Support for this view is readily available -- it was the gist of the resolution received by the first International Federation for Housing and Planning (IFHP) symposium on this topic [1] and has been the position of the World Meteorological Organization (WMO) (see [2] for a recent summary) and the International Council for Building Research, Studies and Documentation, for many years. On the other hand, the second part of the consensus is that, with a few notable exceptions, very little use is being made of the available urban climate knowledge and expertise in the planning process [1-8].
Reasons for rejection Not unreasonably this lack of application has led to frustration and introspection in the urban climatological community. They ask -why are the fruits of their research largely being ignored by the planning and construction professions? It is this reviewer's opinion that the essential answers were provided by Page about 15 years ago [8]. He pointed out that potential users of any scientific information may end up rejecting it because they consider it to be irrelevant, incomprehensible or inapplicable. Translated into the case at hand, the planner may consider the work of the urban climatologist to be irrelevant if the latter has n o t realized the nature of the planner's problem and has failed to relate his work to it. This implies the climatologist to be ignorant of the planning process. If the climate information is relevant but the planner considers it to be incomprehensible it implies the user is ignorant of the science. In © Elsevier Sequoia/Printed in The Netherlands
the last case, if the user and the supplier both see the relevance of the information to the problem but the user finds it to be inapplicable, this suggests a lack of communication between them. In their attempts to rationalize the reasons for the lack of application most urban climatologists note that the large body of information they have communally amassed, clearly indicates that much of the impact of urbanization on the atmosphere is negative. Such an assessment is based on many grounds, such as: -- the comfort, safety or health of the inhabitants, -- the economic productivity of the workers, - - t h e costs of the failure, deterioration and maintenance of buildings, -- the wastage of vital fuel and water resources, -- the disruption of transportation, etc. Further, they often assume that the same data are relevant to the design of cities in order to rectify existing deficiencies or to avoid the creation of new ones. Thus many climatologists d o u b t that the problem lies with them. The same group would see a more likely reason to lie with the planners and their inability to understand the complexities of the atmosphere. They would point to the fact that many planners do n o t have a natural science background and their training is usually deficient with respect to exposure to the value of climatology. This view may be couched in sympathetic terms such as the fact that it is not easy to grasp the workings of the atmospheric system because it is a fluid with confusing space and time variability, with non-linear interactions and feedbacks, and although some of its effects are easily grasped others are more abstract and subtle. Nevertheless, the underlying implication is that, understandable as it may be, the planner or the training of planners is at fault. Hence they adhere to the lack-of-comprehension argument. Of the answers provided by Page it is probably the 'lack of communication' reasoning that finds the most support. The problems of interdisciplinary dialogue are well known and involve overcoming different philosophies and goals as well as different backgrounds, literature, jargon, means of expression etc. Between planner and climatologist these are evident as mismatches be-
tween the users request for help and that provided by the scientist or agency. In detail this may relate to the volume or format of the data, the technical expertise necessary to understand or make use of it, the total unavailability of the appropriate information, etc. Suppliers are always advised to engage in direct dialogue with the user because it is routinely assumed that the latter requires to be educated about their needs. It should also not be overlooked that urban planning is more forcefully applied in some countries than others, and that the input of environmental concerns to planning is equally uneven. As an example, my Canadian students find the film of Stuttgart's use of climatology in urban development [9] to be interesting, not only because of the demonstration of applied climatology, but because of the legislative planning framework of the city. The idea that a North American city would insist that climatologists be consulted in all new developments seems unrealistic to them. A i m s o f this paper
Undoubtedly each of these reasons is involved in the explanation of why urban climate information is not finding greater utility in the planning of settlements. It is my intention here to concentrate upon the first -- the state of the science (pure and applied). It occurs to me that other disciplines also face the problems of complexity and communication when seeking application of their interests in urban design, yet they have been more successful. Indeed it has not stopped the adoption of similar climatological principles at the building scale. Whilst urging climatologists to continue working to lessen these obstacles, this paper also asks them to critically assess their readiness to supply the kind of scientific information needed by urban planners, and by those whose job it is to train students aspiring to that profession. As a contribution to this process of assessment, the paper provides a brief review of the field of urban climatology and the resource base it has generated for urban planning purposes. It restricts its view to scales greater than those of a single building up to those of a whole city. It concludes that urban climatology must become a more predictive science if it is to be of real value in planning, and some suggestions for improvement are given.
work on the urban atmosphere [21, 22]. This grew o u t of an increasing awareness of Man's role in environmental modification, especially urban air pollution. The field received considerable input from meteorologists interested in observation and simulation of the planetary boundary layer. There was new emphasis upon atmospheric processes, extension of work into layers well above the surface, and the construction of models. These developments were paratlelled by the growth of a new physical climatology in which the energy and water budget concepts occupied a central theme. Much of this work was associated with very large observation programmes. Foremost amongst these were the Metropolitan Meteorological Experiment (METROMEX) in St. Louis, founded on the objectives of verifying the nature of urban effects on cloud and precipitation b u t including many other aspects of urban meteorology [23], and the Complete Atmospheric Energetics Experiments (CAENEX) in the Russian cities of Zaporozh'ye and Rustavi [24, 25] designed to elucidate the surface and atmospheric radiation budget in polluted environments. From this activity there emerged a general appreciation about the impact of cities upon almost all of the climatic elements (including their space and time domains) and a start was made towards a similar understanding of the urban meteorological process. Despite these advances, and recent shifts in the emphasis of urban climate research, the bulk of the o u t p u t has been couched in a descriptive mould characterized by case studies of the urban climate features of specific cities. The work has also remained concentrated in the mid-latitudes with almost an absence of knowledge from tropical cities.
REVIEW OF URBAN CLIMATE -- GENERAL The field of urban climatology has been characterized by the strong desire of its proponents to discover and describe the detailed environment in which Man lives. This includes both the impact of the local topography (hills, valleys, rivers, lakes and other subtleties of surface morphology and cover) in producing different climates for settlement, and the impact of the settlement itself upon the climate in its vicinity. Together they form the complex feedback enigma that permeates both pure and applied research in the field. It was just such natural curiosity that prompted Luke Howard's pioneering study of the heat island of London [10]. He was followed by many others in the different cities of Europe during the latter part of the nineteenth century [e.g. 11 - 14]. Much of this work was made possible through the expansion of the meteorological observing network, thus providing the essential input for climatographic studies. The reliance on standard networks was radically altered by the emergence of microclimatology, especially in Germany and Austria during the 1930s. Here the emphasis was upon small time and space scales, and Schmidt's introduction of the mobile traversing technique provided a superb base for quickly gathering such information in urban areas [15]. By World War II, North America and Japan were involved in similar work. Following the war there was renewed interest in urban climate and several fairly elaborate studies were conducted in the same parts of the world where earlier interest had been shown [e.g. 16 - 20]. This work concentrated on the horizontal distribution of several climatic elements, with air temperature being the dominant one. There was also an interest in relating near-surface temperature to the urban land-use or other morphological features, [19, 20], finding statistical relationships between urban-rural differences and prevailing weather conditions [16, 17, 20], exploring the effect of city size on such differences [17, 20], and probing the vertical extent of urban influence [ 17]. The work was undertaken by meteorologists and to an increasing extent by climatologist-geographers. In the late 1960s and the early 1970s there was a tremendous upsurge of interest and
REVIEW OF URBAN CLIMATE INFORMATION RELEVANT TO URBAN PLANNING -
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Given the fairly substantial b o d y of urban climate information n o w available it is a common assumption that there is much to apply in urban planning. However, having many case studies of urban effects falls far short of having something relevant to the planning process. What are needed are: an ability to demonstrate the importance of climate information in the design of settlements,
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-- the predictive power to foretell the climatic impact of alternative design strategies. The former requires a clear, easily communicated and arresting set of examples of the positive and negative impacts of climate on settlements. These examples should translate the climatic effects into real values such as economic costs (savings in capital outlay, running and maintenance), health, comfort and safety implications, potential for structural failure or damage, etc. Whilst there are scattered examples of climate and planning (see the next section of this paper) there is at present no comprehensive source of information for use by planners at the training or operational level. It is necessary to have such a d o c u m e n t if planners are to be made aware of the need for climatology and if they, in turn, are to impress their clients or governing bodies with the same message. To develop its predictive power the field must establish general relationships in the form of laws, formulae, rules, guidelines, models and other techniques capable of producing reliable forecasts or estimates of future climatic conditions in the vicinity of planned built environments. Here again the general resource base for applications purposes has not been brought together in a single publication. The following is a brief survey of some of the candidates for the resource base. They are arranged according to three groupings: empirical methods, physical (scale) models, and numerical models.
Empirically-based methods This class of methods includes statistical algorithms, parameterizations, engineering formulae and qualitative conceptualizations. They are linked by the c o m m o n feature of reliance upon an empirical data base. Dependence upon observed input is both a strength and a weakness. On the one hand it means they are founded on 'real world' conditions, on the other it often restricts the m e t h o d to the specific location where the data were gathered (unless sufficient geographical diversity is incorporated in the data set or the relationship only includes universal concepts). A good example of this type of approach (and its limitation) is the multiple regression equation developed by Sundborg [16] to express the intensity of the heat island in
Uppsala, Sweden, in terms of the prevailing weather (cloudiness and wind speed). This type of relation provides a means of retrodicting a heat island climatology, but does not guarantee its value as a forecast tool even for the same city. Its empirical coefficients are unlikely to be very useful for either purpose in a different city. Similar expressions are available to relate: the heat island in terms of rural stability and city size [26]; the heat island on clear nights to wind speed and city size [27]; and the m a x i m u m heat island to the radiation geometry of the city centre [28]. Other urban climate features have been similarly treated including relations between: urban humidity and other weather variables [29]; increase of thunder days and city size [30]; and between the urban boundary layer wind speed and climate effects such as temperature, humidity and mixed layer depth [31]. There are numerous engineering formulae for use in urban areas which involve empirical relationships. Examples include the power law form of the wind profile with an exponent related to the roughness of urban areas [32], the use of pressure and force coefficients in wind load calculations [33], the assessment of the roughness length of urban areas from geometry [34], the development of urban dispersion coefficients [35] and adjustments to stability criteria to account for greater urban turbulence [36] and the assessment of r u n o f f coefficients sensitive to urban hydrologic characteristics. A number of these examples form the 'relationship functions' that Page [37] called for so as to enable users in the building and planning fields to be able to forecast the state of the urban climate from standard observations at non-urban sites. A major resource, based on empirical observation, is the 'model' of the urban atmosphere retained by an urban climatologist because of his accumulated experience, based on case study. However, unless this is converted into a form relevant to a planner it is unlikely to be called upon.
Physical models In urban climatology the classic controlled experiment is the simulation of airflow around buildings and across urban areas with the use of an environmental wind tunnel, or a water flume. As long as scale and dynamic
similitude are maintained, this form of modelling has been strikingly successful in simulating the extremely complicated flow patterns in the built environment. It allows recreation of past conditions, and prediction of those yet to be experienced in the vicinity of planned structures. The circulation within street canyons can be simulated [38] including the dispersion of pollutants, and the comfort and safety of pedestrians [32]. The flow around large building complexes and large parts of cities can also be studied [39, 40]. In some tunnels the thermal conditions can be controlled so that heat island perturbations to the flow field can be added [41]. Thus the ability to 'fine tune' wind climates is at hand, although one should add that the cost of such studies is relatively high. Scale models have also been used to simulate shadow patterns and heat loading on building arrays using an appropriately oriented light source [42]. The effects of urban geometry on canyon nocturnal cooling also have been investigated [28]. Quasi-controlled experiments have been conducted outdoors using scale models. These include simulating the urban heat island [43], surface roughness change [34] and the effects of urban geometry on albedo [44]. Numerical models This group of models includes any linked sets of mathematical equations and analytical techniques designed to reproduce aspects of the behaviour of the urban climate. These models have gained considerable attention in the past decade. Ultimately they promise to simulate the climate for any contemplated set of urban characteristics. A few models have been developed with the aim of modelling climatic conditions in the urban 'canopy' layer below roof-level, including models of radiative exchange in canyons [45, 46], street vortex circulations [47], the surface energy budget [48, 49] and the interaction between these budgets and human c o m f o r t [ 50]. The majority of such models deal with conditions above roof-level as part of the larger field of boundary layer modelling. The range of models is extremely broad: there are one-, two- and three-dimensional models, static and time-dependent ones, those whose central concern is the thermal climate and others
only designed to study circulation effects. Here it is neither possible nor appropriate to describe the working of such models. For detailed descriptions the following example studies should be consulted: radiative transfer effects in polluted urban air [51]; surface energy budget and temperature [52]; combined radiation, energy budget and mixed layer dynamics [ 53] ; simple circulation [ 54] ; comprehensive three-dimensional temperature and wind fields [55]. Such models hold great potential for predicting urban effects. As with controlled laboratory models, the numerical model gives opportunities to exert control over any desired input condition(s) and to follow the effect(s) through the climate system. Hence McElroy [52], after validating his model against field data, was able to investigate the potential climatic impacts of future land-use changes in the city of Columbus, Ohio. It would also be valuable to be able to simulate the effects of changes in urban climate due to changes of urban characteristics such as anthropogenic heat output, pollutant emissions, surface water availability, urban expansion, growth of the number of tall buildings etc. It is also possible to merge urban climate and urban air pollution dispersion models so as to provide the capability of simulating future pollutant concentration distributions. Clearly urban climate models could become powerful planning, forecasting and research tools but we should note some problems hindering their widespread use. There is great need for the present models to be validated, but appropriate data sets are scarce or nonexistent. In some cases the models require relatively exotic inputs which have no obviously measurable counterpart in the field, or are so demanding that standard climatic data is not sufficient. Similar problems occur with model output. Finally, there are restrictions on most models which constrain their use to idealized conditions, e.g. no cloud, no advection, etc. Inspection of the preceding relationships reveals that very few have direct utility in urban planning. This highlights the need to translate the climatological resource into terms easily appreciated by the intended user, and to conduct new research specifically
targeted to answer the questions asked by planners. A list of such questions might include: --is there an optimum proportion of the city to devote to vegetation? --is there an optimum arrangement of such green space? - - i s there a preferred orientation for roads and buildings to maximize solar radiation influences? - - a r e there optimal height:width ratios for street canyons to maximize energy conservation or pollutant dispersal? Only a genuine working dialogue between planners and climatologists can provide the fullest and best list of such questions and their answers. REVIEW OF URBAN CLIMATE --APPLICATIONS Examples of climate applications at the building scale abound. The role of buildings in providing shelter from the physical stresses imposed by the environment is basic. The strategies adopted by Man to c o m b a t negative climatic stresses are well represented by the traditional architectural building forms. However, in the recent past there has been the increasing tendency to overcome these stresses by employing 'brute force' engineering solutions, thereby creating a dependency on scarce energy, water and materials resources. Realization of this insensitivity prompted a resurgence of interest in applying climatic knowledge to building design, especially with respect to conserving fuel use and taking advantage of solar and wind energy. Similar examples of climate input to the planning and design of large groups of buildings, up to the scale of whole settlements (the scales of this discussion) are n o t so readily available. There are some examples of the use of solar and wind principles in the layout of cities from ancient civilizations, but the record from the modern era is chequered and largely unimpressive. The suggestions of Schmauss [56] to lay out cities according to prevailing winds, which were adopted in planning new Russian cities [57], are now seen to be t o o simplistic. A better scheme would be to base such plans on the direction and frequency of occurrence of winds during conditions conducive to high
ground-level concentrations of pollution, if that is the primary concern. Holford [58] notes the well-intentioned plans for three new Capital cities: Brasilia, Brazil; Canberra, Australia; and Chandrigarh, Punjab. In each case climatic constraints were considered in the early planning stages, but following construction none is free of climatic problems. Somewhat better outcomes can be reported for a number of settlements located in extreme climatic environments. The towns of Kiruna and Svapparava in Sweden, and Leaf Rapids, Inuvik and Fermont in Canada, and Noril'sk in Siberia all experience very cold winters. Their plans were drawn up with due allowance for the special problems experienced by those wishing to live and conduct economic activities where wind chill, ground heaving, snow drifting etc. are severe [59 61]. The town of Kitimat in Canada is an example of planning to accommodate very wet, snowy and windy conditions [60]. At the other end of the spectrum there are examples of towns designed to c o m b a t problems of excess heat load, water loss and sand drifting in tropical desert and other arid lands [1]. In these extreme cases the role of climate is so dominant that it is readily given high priority in the planning process. The literature also includes carefully considered suggestions for 'ideal' city climate plans such as the 'Metutopia' of Landsberg [62] and the 'material-energy conservation city' of Page [63]. These are not to be classed with the interesting but overly futuristic views of planned city systems which include totally enclosed (domed) cities, subterranean and sub-oceanic cities and extraterrestrial space communities. There are also examples where climate forms a fundamental planning criterion in the design of housing developments, industrial parks, commercial core renewal and growth, major new institutions, sports facilities, transportation corridors, airports, etc. When considered in more detail, it often emerges that the reason climate was included in planning such developments is one or more of the four following concerns: (1) wind -- desire to avoid problems associated with wind loads on structures leading to structural failure or high maintenance costs; concerns with energy losses from
buildings; danger or discomfort for pedestrians; difficulty with access due to snow or sand drifting, etc. [e.g. 32, 39, 40, 58] : (2) solar radiation -- need to avoid mutual shading of buildings; attempts to maximize solar energy potential (active and passive) through building orientation; need for shading to reduce solar load, etc. [e.g. 64] : (3) precipitation/runoff -- concern a b o u t storm precipitation frequency and amount in order to design adequate flood control, stormwater routing and drainage systems; need to withstand snow loads [e.g. 64, 65] : (4) air pollution -- need to prevent or minimize negative impacts of pollutant emissions in the immediate vicinity and downwind of potentially harmful or unsightly sources [e.g. 6 6 - 68]. Notice that these four areas of concern share two very important characteristics: they are subject to legislated planning requirements, and they are fields with proven applied science methodologies. The planning requirements include state or municipal laws such as building and planning codes, health and safety regulations, solar and wind rights, local city ordinances, etc. Hence the planner has been forced to respond to climatic impacts and has successfully managed to do so. Such regulation has arisen in response to clearly demonstrable hazards or benefits. The methodologies are those associated with the emerging environmental engineering sub-disciplines of wind, solar and air pollution engineering and urban hydrology. Each has borrowed methods from meteorology/climatology/hydrology and have embellished upon them or developed new ones. CONCLUSIONS FROM THE REVIEW
The preceding review has five general conclusions. {1) Urban climate knowledge -- this relatively y o u n g field has managed to amass a b o d y of knowledge concerning the general impact of cities on their atmospheric environments. All of the climatological elements have been shown to be affected. This largely descriptive fund of knowledge is, however, deficient in its geographical coverage, especially in respect of tropical cities. (2) Methodological d e v e l o p m e n t s - whilst aspects of city climatography are well
developed, other elements of the science have progressed slowly, if at all. The field has not developed clear methodologies (e.g. to estimate urban effects on climate [69]) or techniques (e.g. to transfer results from one city to another, or one climate region to another). (3) Urban meteorological process knowledge -- only in the last 15 years has there been any emphasis upon study of the physical mechanisms underlying urban climate effects, and cause-and-effect linkages. This has hampered attempts to build urban climate models and in providing data against which to validate them. It also has delayed the development of physically-, rather than empiricallybased relationships. The former are likely to be of greater general value because they are often not b o u n d to site- or climate-specific conditions. (4) Resource base for applied purposes -many of the relationships, algorithms, 'rulesof-thumb' and models of various kinds express results that are only of interest to other climatologists. Even those which may have relevance to planners are often presented in climatic terms so that potential users are not clearly alerted to their value. (5) Application to urban planning -evidence of climate information being used in the planning of urban areas on scales greater than a few buildings is relatively scarce, and where attempts have been made the o u t c o m e has not always been successful. The most positive examples display a c o m m o n set of ingredients. These include early recognition and demonstration of a potential benefit or disbenefit to a project due to climatic characteristics. Often only a single element is the focus of concern, with wind, solar energy, precipitation or pollution being the most common. These elements have clearly demonstrable impacts and well-developed techniques for illustrating and implementing solutions.
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The preceding conclusions clearly indicate the need for certain improvements to be made to the scientific base of urban climate and to the applied science which relates it to urban planning.
Development of the science of urban climatology In the general field of urban climate, there is need to encourage all initiatives leading towards the acquisition of predictive power. This can be forwarded by insistence on methodological rigour, including attention toward proper definition of the urban surface and its physical properties, recognition of different urban scales and care to ensure scaleconsistent design of experiments, observation, interpretation and modelling. Methods to estimate urban effects and to establish the transferability of results must also be developed. At present there is no recognized description or set of requirements to characterize an urban observation station, and no formal guidelines to aid the exposure of instruments in urban surroundings, nor to help design an urban observation network. Without some form of standardization there is little basis for intercomparison of data. Predictive power also requires that urban climate work aims to seek general relationships. Thus, where possible, research should be directed towards general hypothesis testing rather than case study description. Ideally relationships should be of a quantitative nature but general rules, guidelines, and conceptual frameworks may be equally valuable. Work on urban meteorological process must continue to seek the causative basis of urban climate. Such understanding is essential to the intelligent application of science. It is also necessary in the construction of realistic numerical models of the urban atmosphere. These promise to become powerful planning tools but are presently in need of validation before becoming operational.
Development of applied urban climatology Such basic work ought to be accompanied by more genuinely applied research. Here again we encounter the need for the development of standard methods. Once the climate c o m m u n i t y has managed to convince a planner of the need to consider assessing the role of climate, there must be a standard set of procedures available to do so. These should probably include a checklist of potential climate features needing consideration, and a clearly defined set of methods in order to conduct an initial screening of their importance. This process should include filters at the
macro-, meso-, local and microclimatic scales and relate climatic characteristics to the real concerns of the planner. In order for this first-order assessment to take place there is need to have climatic data relevant to the planned project. This is commonly not directly available, hence the results from nearby stations have to be extrapolated to the site location. Although some procedures are available there is great need for the climatological c o m m u n i t y to promulgate research on this topic and to arrive at a recommended set of procedures. Ultimately we seek a co-ordinated set of checklists, procedural manuals, standard design solutions for different climatic regions, and advice as to when and where to call upon specialized climatological services. We must also recognize that planning requires decisions, not all of which can be based on full knowledge, therefore we must be prepared to work from basic principles and offer advice in the absence of the best solution. If we do not, those decisions will still be made by those less sensitive to atmospheric impacts, or they will be ignored. We face this same dilemma in what may well be the challenge to the urban climate/ urban planning field in our time. Namely, whether or not there is to be an infusion of intelligent climate reasoning and practice into the design of cities in the developing (largely tropical) world. The rate of urbanization in these areas is staggering and the potential for environmental deterioration is great. Given that the c o n t e n t of this review is at all representative of the status of pure and applied science in our field, we are not well placed to meet the challenge. Knowledge of the climates of existing cities in these areas is poor. We cannot await its growth, which promises to be slow. Therefore, if we are not to sit idly by, we must muster available expertise, resources and means of international communication, and do our best to tackle the problem. In doing so we will be in danger of 'over-selling' a fledgling field, and this must be borne in mind by those entrusted with the task.
ACKNOWLEDGEMENTS Research in urban climatology at the University of British Columbia is supported
by the Natural Sciences and Engineering Research Council of Canada. Thanks are extended to M. L. Oke who processed the manuscript.
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1
Towards a Prescription for the Greater Use of Climatic Principles in Settlement Planning T. R. OKE The University of British Columbia, 1984 West Mall, Vancouver, B.C. (Canada)
SUMMARY
Despite some notable exceptions, relatively little o f the large body o f knowledge concerning urban climate has permeated through to working planners. This lack o f application has been bemoaned by climatologists for many years. The reasons for this state o f affairs are many, but amongst those most cited are the inherent complexity o f the subject, its interdisciplinary nature and lack o f meaningful dialogue between planners and the climatological research community. Whilst this is true, these potential barriers have n o t prevented application o f knowledge from parallel disciplines by planners, nor the adoption o f climatological principles by architects and engineers at the building scale. The paper reviews the possibility that part o f the problem is attributable to the nature o f research in urban climatology and the difficulty o f translating its results into robust tools for application. A critical assessment o f the preparedness o f urban climatology to be an applied science reveals progress and promise but also weaknesses. The latter include a lack o f quantitative techniques and relationships, lack o f standardization, generality and transferability, and the absence o f clear guidelines for those wishing to learn and use climatological principles in settlement planning. There is also a dearth o f first-hand information from the low latitudes which must be filled if climate is to play a significant role in planning the massive urbanization which is under way there.
INTRODUCTIO N
Present consensus When asked to review or analyze the state of the field relating climate to settlement 0378-7788/84/$3.00
planning, one has little difficulty in arriving at a clear two-part consensus. The first part is that climate has an important role to play in settlement design and must be included as an integral part of the education of every urban planner. Support for this view is readily available -- it was the gist of the resolution received by the first International Federation for Housing and Planning (IFHP) symposium on this topic [1] and has been the position of the World Meteorological Organization (WMO) (see [2] for a recent summary) and the International Council for Building Research, Studies and Documentation, for many years. On the other hand, the second part of the consensus is that, with a few notable exceptions, very little use is being made of the available urban climate knowledge and expertise in the planning process [1-8].
Reasons for rejection Not unreasonably this lack of application has led to frustration and introspection in the urban climatological community. They ask -why are the fruits of their research largely being ignored by the planning and construction professions? It is this reviewer's opinion that the essential answers were provided by Page about 15 years ago [8]. He pointed out that potential users of any scientific information may end up rejecting it because they consider it to be irrelevant, incomprehensible or inapplicable. Translated into the case at hand, the planner may consider the work of the urban climatologist to be irrelevant if the latter has n o t realized the nature of the planner's problem and has failed to relate his work to it. This implies the climatologist to be ignorant of the planning process. If the climate information is relevant but the planner considers it to be incomprehensible it implies the user is ignorant of the science. In © Elsevier Sequoia/Printed in The Netherlands
the last case, if the user and the supplier both see the relevance of the information to the problem but the user finds it to be inapplicable, this suggests a lack of communication between them. In their attempts to rationalize the reasons for the lack of application most urban climatologists note that the large body of information they have communally amassed, clearly indicates that much of the impact of urbanization on the atmosphere is negative. Such an assessment is based on many grounds, such as: -- the comfort, safety or health of the inhabitants, -- the economic productivity of the workers, - - t h e costs of the failure, deterioration and maintenance of buildings, -- the wastage of vital fuel and water resources, -- the disruption of transportation, etc. Further, they often assume that the same data are relevant to the design of cities in order to rectify existing deficiencies or to avoid the creation of new ones. Thus many climatologists d o u b t that the problem lies with them. The same group would see a more likely reason to lie with the planners and their inability to understand the complexities of the atmosphere. They would point to the fact that many planners do n o t have a natural science background and their training is usually deficient with respect to exposure to the value of climatology. This view may be couched in sympathetic terms such as the fact that it is not easy to grasp the workings of the atmospheric system because it is a fluid with confusing space and time variability, with non-linear interactions and feedbacks, and although some of its effects are easily grasped others are more abstract and subtle. Nevertheless, the underlying implication is that, understandable as it may be, the planner or the training of planners is at fault. Hence they adhere to the lack-of-comprehension argument. Of the answers provided by Page it is probably the 'lack of communication' reasoning that finds the most support. The problems of interdisciplinary dialogue are well known and involve overcoming different philosophies and goals as well as different backgrounds, literature, jargon, means of expression etc. Between planner and climatologist these are evident as mismatches be-
tween the users request for help and that provided by the scientist or agency. In detail this may relate to the volume or format of the data, the technical expertise necessary to understand or make use of it, the total unavailability of the appropriate information, etc. Suppliers are always advised to engage in direct dialogue with the user because it is routinely assumed that the latter requires to be educated about their needs. It should also not be overlooked that urban planning is more forcefully applied in some countries than others, and that the input of environmental concerns to planning is equally uneven. As an example, my Canadian students find the film of Stuttgart's use of climatology in urban development [9] to be interesting, not only because of the demonstration of applied climatology, but because of the legislative planning framework of the city. The idea that a North American city would insist that climatologists be consulted in all new developments seems unrealistic to them. A i m s o f this paper
Undoubtedly each of these reasons is involved in the explanation of why urban climate information is not finding greater utility in the planning of settlements. It is my intention here to concentrate upon the first -- the state of the science (pure and applied). It occurs to me that other disciplines also face the problems of complexity and communication when seeking application of their interests in urban design, yet they have been more successful. Indeed it has not stopped the adoption of similar climatological principles at the building scale. Whilst urging climatologists to continue working to lessen these obstacles, this paper also asks them to critically assess their readiness to supply the kind of scientific information needed by urban planners, and by those whose job it is to train students aspiring to that profession. As a contribution to this process of assessment, the paper provides a brief review of the field of urban climatology and the resource base it has generated for urban planning purposes. It restricts its view to scales greater than those of a single building up to those of a whole city. It concludes that urban climatology must become a more predictive science if it is to be of real value in planning, and some suggestions for improvement are given.
work on the urban atmosphere [21, 22]. This grew o u t of an increasing awareness of Man's role in environmental modification, especially urban air pollution. The field received considerable input from meteorologists interested in observation and simulation of the planetary boundary layer. There was new emphasis upon atmospheric processes, extension of work into layers well above the surface, and the construction of models. These developments were paratlelled by the growth of a new physical climatology in which the energy and water budget concepts occupied a central theme. Much of this work was associated with very large observation programmes. Foremost amongst these were the Metropolitan Meteorological Experiment (METROMEX) in St. Louis, founded on the objectives of verifying the nature of urban effects on cloud and precipitation b u t including many other aspects of urban meteorology [23], and the Complete Atmospheric Energetics Experiments (CAENEX) in the Russian cities of Zaporozh'ye and Rustavi [24, 25] designed to elucidate the surface and atmospheric radiation budget in polluted environments. From this activity there emerged a general appreciation about the impact of cities upon almost all of the climatic elements (including their space and time domains) and a start was made towards a similar understanding of the urban meteorological process. Despite these advances, and recent shifts in the emphasis of urban climate research, the bulk of the o u t p u t has been couched in a descriptive mould characterized by case studies of the urban climate features of specific cities. The work has also remained concentrated in the mid-latitudes with almost an absence of knowledge from tropical cities.
REVIEW OF URBAN CLIMATE -- GENERAL The field of urban climatology has been characterized by the strong desire of its proponents to discover and describe the detailed environment in which Man lives. This includes both the impact of the local topography (hills, valleys, rivers, lakes and other subtleties of surface morphology and cover) in producing different climates for settlement, and the impact of the settlement itself upon the climate in its vicinity. Together they form the complex feedback enigma that permeates both pure and applied research in the field. It was just such natural curiosity that prompted Luke Howard's pioneering study of the heat island of London [10]. He was followed by many others in the different cities of Europe during the latter part of the nineteenth century [e.g. 11 - 14]. Much of this work was made possible through the expansion of the meteorological observing network, thus providing the essential input for climatographic studies. The reliance on standard networks was radically altered by the emergence of microclimatology, especially in Germany and Austria during the 1930s. Here the emphasis was upon small time and space scales, and Schmidt's introduction of the mobile traversing technique provided a superb base for quickly gathering such information in urban areas [15]. By World War II, North America and Japan were involved in similar work. Following the war there was renewed interest in urban climate and several fairly elaborate studies were conducted in the same parts of the world where earlier interest had been shown [e.g. 16 - 20]. This work concentrated on the horizontal distribution of several climatic elements, with air temperature being the dominant one. There was also an interest in relating near-surface temperature to the urban land-use or other morphological features, [19, 20], finding statistical relationships between urban-rural differences and prevailing weather conditions [16, 17, 20], exploring the effect of city size on such differences [17, 20], and probing the vertical extent of urban influence [ 17]. The work was undertaken by meteorologists and to an increasing extent by climatologist-geographers. In the late 1960s and the early 1970s there was a tremendous upsurge of interest and
REVIEW OF URBAN CLIMATE INFORMATION RELEVANT TO URBAN PLANNING -
-
Given the fairly substantial b o d y of urban climate information n o w available it is a common assumption that there is much to apply in urban planning. However, having many case studies of urban effects falls far short of having something relevant to the planning process. What are needed are: an ability to demonstrate the importance of climate information in the design of settlements,
-
-
-- the predictive power to foretell the climatic impact of alternative design strategies. The former requires a clear, easily communicated and arresting set of examples of the positive and negative impacts of climate on settlements. These examples should translate the climatic effects into real values such as economic costs (savings in capital outlay, running and maintenance), health, comfort and safety implications, potential for structural failure or damage, etc. Whilst there are scattered examples of climate and planning (see the next section of this paper) there is at present no comprehensive source of information for use by planners at the training or operational level. It is necessary to have such a d o c u m e n t if planners are to be made aware of the need for climatology and if they, in turn, are to impress their clients or governing bodies with the same message. To develop its predictive power the field must establish general relationships in the form of laws, formulae, rules, guidelines, models and other techniques capable of producing reliable forecasts or estimates of future climatic conditions in the vicinity of planned built environments. Here again the general resource base for applications purposes has not been brought together in a single publication. The following is a brief survey of some of the candidates for the resource base. They are arranged according to three groupings: empirical methods, physical (scale) models, and numerical models.
Empirically-based methods This class of methods includes statistical algorithms, parameterizations, engineering formulae and qualitative conceptualizations. They are linked by the c o m m o n feature of reliance upon an empirical data base. Dependence upon observed input is both a strength and a weakness. On the one hand it means they are founded on 'real world' conditions, on the other it often restricts the m e t h o d to the specific location where the data were gathered (unless sufficient geographical diversity is incorporated in the data set or the relationship only includes universal concepts). A good example of this type of approach (and its limitation) is the multiple regression equation developed by Sundborg [16] to express the intensity of the heat island in
Uppsala, Sweden, in terms of the prevailing weather (cloudiness and wind speed). This type of relation provides a means of retrodicting a heat island climatology, but does not guarantee its value as a forecast tool even for the same city. Its empirical coefficients are unlikely to be very useful for either purpose in a different city. Similar expressions are available to relate: the heat island in terms of rural stability and city size [26]; the heat island on clear nights to wind speed and city size [27]; and the m a x i m u m heat island to the radiation geometry of the city centre [28]. Other urban climate features have been similarly treated including relations between: urban humidity and other weather variables [29]; increase of thunder days and city size [30]; and between the urban boundary layer wind speed and climate effects such as temperature, humidity and mixed layer depth [31]. There are numerous engineering formulae for use in urban areas which involve empirical relationships. Examples include the power law form of the wind profile with an exponent related to the roughness of urban areas [32], the use of pressure and force coefficients in wind load calculations [33], the assessment of the roughness length of urban areas from geometry [34], the development of urban dispersion coefficients [35] and adjustments to stability criteria to account for greater urban turbulence [36] and the assessment of r u n o f f coefficients sensitive to urban hydrologic characteristics. A number of these examples form the 'relationship functions' that Page [37] called for so as to enable users in the building and planning fields to be able to forecast the state of the urban climate from standard observations at non-urban sites. A major resource, based on empirical observation, is the 'model' of the urban atmosphere retained by an urban climatologist because of his accumulated experience, based on case study. However, unless this is converted into a form relevant to a planner it is unlikely to be called upon.
Physical models In urban climatology the classic controlled experiment is the simulation of airflow around buildings and across urban areas with the use of an environmental wind tunnel, or a water flume. As long as scale and dynamic
similitude are maintained, this form of modelling has been strikingly successful in simulating the extremely complicated flow patterns in the built environment. It allows recreation of past conditions, and prediction of those yet to be experienced in the vicinity of planned structures. The circulation within street canyons can be simulated [38] including the dispersion of pollutants, and the comfort and safety of pedestrians [32]. The flow around large building complexes and large parts of cities can also be studied [39, 40]. In some tunnels the thermal conditions can be controlled so that heat island perturbations to the flow field can be added [41]. Thus the ability to 'fine tune' wind climates is at hand, although one should add that the cost of such studies is relatively high. Scale models have also been used to simulate shadow patterns and heat loading on building arrays using an appropriately oriented light source [42]. The effects of urban geometry on canyon nocturnal cooling also have been investigated [28]. Quasi-controlled experiments have been conducted outdoors using scale models. These include simulating the urban heat island [43], surface roughness change [34] and the effects of urban geometry on albedo [44]. Numerical models This group of models includes any linked sets of mathematical equations and analytical techniques designed to reproduce aspects of the behaviour of the urban climate. These models have gained considerable attention in the past decade. Ultimately they promise to simulate the climate for any contemplated set of urban characteristics. A few models have been developed with the aim of modelling climatic conditions in the urban 'canopy' layer below roof-level, including models of radiative exchange in canyons [45, 46], street vortex circulations [47], the surface energy budget [48, 49] and the interaction between these budgets and human c o m f o r t [ 50]. The majority of such models deal with conditions above roof-level as part of the larger field of boundary layer modelling. The range of models is extremely broad: there are one-, two- and three-dimensional models, static and time-dependent ones, those whose central concern is the thermal climate and others
only designed to study circulation effects. Here it is neither possible nor appropriate to describe the working of such models. For detailed descriptions the following example studies should be consulted: radiative transfer effects in polluted urban air [51]; surface energy budget and temperature [52]; combined radiation, energy budget and mixed layer dynamics [ 53] ; simple circulation [ 54] ; comprehensive three-dimensional temperature and wind fields [55]. Such models hold great potential for predicting urban effects. As with controlled laboratory models, the numerical model gives opportunities to exert control over any desired input condition(s) and to follow the effect(s) through the climate system. Hence McElroy [52], after validating his model against field data, was able to investigate the potential climatic impacts of future land-use changes in the city of Columbus, Ohio. It would also be valuable to be able to simulate the effects of changes in urban climate due to changes of urban characteristics such as anthropogenic heat output, pollutant emissions, surface water availability, urban expansion, growth of the number of tall buildings etc. It is also possible to merge urban climate and urban air pollution dispersion models so as to provide the capability of simulating future pollutant concentration distributions. Clearly urban climate models could become powerful planning, forecasting and research tools but we should note some problems hindering their widespread use. There is great need for the present models to be validated, but appropriate data sets are scarce or nonexistent. In some cases the models require relatively exotic inputs which have no obviously measurable counterpart in the field, or are so demanding that standard climatic data is not sufficient. Similar problems occur with model output. Finally, there are restrictions on most models which constrain their use to idealized conditions, e.g. no cloud, no advection, etc. Inspection of the preceding relationships reveals that very few have direct utility in urban planning. This highlights the need to translate the climatological resource into terms easily appreciated by the intended user, and to conduct new research specifically
targeted to answer the questions asked by planners. A list of such questions might include: --is there an optimum proportion of the city to devote to vegetation? --is there an optimum arrangement of such green space? - - i s there a preferred orientation for roads and buildings to maximize solar radiation influences? - - a r e there optimal height:width ratios for street canyons to maximize energy conservation or pollutant dispersal? Only a genuine working dialogue between planners and climatologists can provide the fullest and best list of such questions and their answers. REVIEW OF URBAN CLIMATE --APPLICATIONS Examples of climate applications at the building scale abound. The role of buildings in providing shelter from the physical stresses imposed by the environment is basic. The strategies adopted by Man to c o m b a t negative climatic stresses are well represented by the traditional architectural building forms. However, in the recent past there has been the increasing tendency to overcome these stresses by employing 'brute force' engineering solutions, thereby creating a dependency on scarce energy, water and materials resources. Realization of this insensitivity prompted a resurgence of interest in applying climatic knowledge to building design, especially with respect to conserving fuel use and taking advantage of solar and wind energy. Similar examples of climate input to the planning and design of large groups of buildings, up to the scale of whole settlements (the scales of this discussion) are n o t so readily available. There are some examples of the use of solar and wind principles in the layout of cities from ancient civilizations, but the record from the modern era is chequered and largely unimpressive. The suggestions of Schmauss [56] to lay out cities according to prevailing winds, which were adopted in planning new Russian cities [57], are now seen to be t o o simplistic. A better scheme would be to base such plans on the direction and frequency of occurrence of winds during conditions conducive to high
ground-level concentrations of pollution, if that is the primary concern. Holford [58] notes the well-intentioned plans for three new Capital cities: Brasilia, Brazil; Canberra, Australia; and Chandrigarh, Punjab. In each case climatic constraints were considered in the early planning stages, but following construction none is free of climatic problems. Somewhat better outcomes can be reported for a number of settlements located in extreme climatic environments. The towns of Kiruna and Svapparava in Sweden, and Leaf Rapids, Inuvik and Fermont in Canada, and Noril'sk in Siberia all experience very cold winters. Their plans were drawn up with due allowance for the special problems experienced by those wishing to live and conduct economic activities where wind chill, ground heaving, snow drifting etc. are severe [59 61]. The town of Kitimat in Canada is an example of planning to accommodate very wet, snowy and windy conditions [60]. At the other end of the spectrum there are examples of towns designed to c o m b a t problems of excess heat load, water loss and sand drifting in tropical desert and other arid lands [1]. In these extreme cases the role of climate is so dominant that it is readily given high priority in the planning process. The literature also includes carefully considered suggestions for 'ideal' city climate plans such as the 'Metutopia' of Landsberg [62] and the 'material-energy conservation city' of Page [63]. These are not to be classed with the interesting but overly futuristic views of planned city systems which include totally enclosed (domed) cities, subterranean and sub-oceanic cities and extraterrestrial space communities. There are also examples where climate forms a fundamental planning criterion in the design of housing developments, industrial parks, commercial core renewal and growth, major new institutions, sports facilities, transportation corridors, airports, etc. When considered in more detail, it often emerges that the reason climate was included in planning such developments is one or more of the four following concerns: (1) wind -- desire to avoid problems associated with wind loads on structures leading to structural failure or high maintenance costs; concerns with energy losses from
buildings; danger or discomfort for pedestrians; difficulty with access due to snow or sand drifting, etc. [e.g. 32, 39, 40, 58] : (2) solar radiation -- need to avoid mutual shading of buildings; attempts to maximize solar energy potential (active and passive) through building orientation; need for shading to reduce solar load, etc. [e.g. 64] : (3) precipitation/runoff -- concern a b o u t storm precipitation frequency and amount in order to design adequate flood control, stormwater routing and drainage systems; need to withstand snow loads [e.g. 64, 65] : (4) air pollution -- need to prevent or minimize negative impacts of pollutant emissions in the immediate vicinity and downwind of potentially harmful or unsightly sources [e.g. 6 6 - 68]. Notice that these four areas of concern share two very important characteristics: they are subject to legislated planning requirements, and they are fields with proven applied science methodologies. The planning requirements include state or municipal laws such as building and planning codes, health and safety regulations, solar and wind rights, local city ordinances, etc. Hence the planner has been forced to respond to climatic impacts and has successfully managed to do so. Such regulation has arisen in response to clearly demonstrable hazards or benefits. The methodologies are those associated with the emerging environmental engineering sub-disciplines of wind, solar and air pollution engineering and urban hydrology. Each has borrowed methods from meteorology/climatology/hydrology and have embellished upon them or developed new ones. CONCLUSIONS FROM THE REVIEW
The preceding review has five general conclusions. {1) Urban climate knowledge -- this relatively y o u n g field has managed to amass a b o d y of knowledge concerning the general impact of cities on their atmospheric environments. All of the climatological elements have been shown to be affected. This largely descriptive fund of knowledge is, however, deficient in its geographical coverage, especially in respect of tropical cities. (2) Methodological d e v e l o p m e n t s - whilst aspects of city climatography are well
developed, other elements of the science have progressed slowly, if at all. The field has not developed clear methodologies (e.g. to estimate urban effects on climate [69]) or techniques (e.g. to transfer results from one city to another, or one climate region to another). (3) Urban meteorological process knowledge -- only in the last 15 years has there been any emphasis upon study of the physical mechanisms underlying urban climate effects, and cause-and-effect linkages. This has hampered attempts to build urban climate models and in providing data against which to validate them. It also has delayed the development of physically-, rather than empiricallybased relationships. The former are likely to be of greater general value because they are often not b o u n d to site- or climate-specific conditions. (4) Resource base for applied purposes -many of the relationships, algorithms, 'rulesof-thumb' and models of various kinds express results that are only of interest to other climatologists. Even those which may have relevance to planners are often presented in climatic terms so that potential users are not clearly alerted to their value. (5) Application to urban planning -evidence of climate information being used in the planning of urban areas on scales greater than a few buildings is relatively scarce, and where attempts have been made the o u t c o m e has not always been successful. The most positive examples display a c o m m o n set of ingredients. These include early recognition and demonstration of a potential benefit or disbenefit to a project due to climatic characteristics. Often only a single element is the focus of concern, with wind, solar energy, precipitation or pollution being the most common. These elements have clearly demonstrable impacts and well-developed techniques for illustrating and implementing solutions.
ELEMENTS OF PROVEMENT
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The preceding conclusions clearly indicate the need for certain improvements to be made to the scientific base of urban climate and to the applied science which relates it to urban planning.
Development of the science of urban climatology In the general field of urban climate, there is need to encourage all initiatives leading towards the acquisition of predictive power. This can be forwarded by insistence on methodological rigour, including attention toward proper definition of the urban surface and its physical properties, recognition of different urban scales and care to ensure scaleconsistent design of experiments, observation, interpretation and modelling. Methods to estimate urban effects and to establish the transferability of results must also be developed. At present there is no recognized description or set of requirements to characterize an urban observation station, and no formal guidelines to aid the exposure of instruments in urban surroundings, nor to help design an urban observation network. Without some form of standardization there is little basis for intercomparison of data. Predictive power also requires that urban climate work aims to seek general relationships. Thus, where possible, research should be directed towards general hypothesis testing rather than case study description. Ideally relationships should be of a quantitative nature but general rules, guidelines, and conceptual frameworks may be equally valuable. Work on urban meteorological process must continue to seek the causative basis of urban climate. Such understanding is essential to the intelligent application of science. It is also necessary in the construction of realistic numerical models of the urban atmosphere. These promise to become powerful planning tools but are presently in need of validation before becoming operational.
Development of applied urban climatology Such basic work ought to be accompanied by more genuinely applied research. Here again we encounter the need for the development of standard methods. Once the climate c o m m u n i t y has managed to convince a planner of the need to consider assessing the role of climate, there must be a standard set of procedures available to do so. These should probably include a checklist of potential climate features needing consideration, and a clearly defined set of methods in order to conduct an initial screening of their importance. This process should include filters at the
macro-, meso-, local and microclimatic scales and relate climatic characteristics to the real concerns of the planner. In order for this first-order assessment to take place there is need to have climatic data relevant to the planned project. This is commonly not directly available, hence the results from nearby stations have to be extrapolated to the site location. Although some procedures are available there is great need for the climatological c o m m u n i t y to promulgate research on this topic and to arrive at a recommended set of procedures. Ultimately we seek a co-ordinated set of checklists, procedural manuals, standard design solutions for different climatic regions, and advice as to when and where to call upon specialized climatological services. We must also recognize that planning requires decisions, not all of which can be based on full knowledge, therefore we must be prepared to work from basic principles and offer advice in the absence of the best solution. If we do not, those decisions will still be made by those less sensitive to atmospheric impacts, or they will be ignored. We face this same dilemma in what may well be the challenge to the urban climate/ urban planning field in our time. Namely, whether or not there is to be an infusion of intelligent climate reasoning and practice into the design of cities in the developing (largely tropical) world. The rate of urbanization in these areas is staggering and the potential for environmental deterioration is great. Given that the c o n t e n t of this review is at all representative of the status of pure and applied science in our field, we are not well placed to meet the challenge. Knowledge of the climates of existing cities in these areas is poor. We cannot await its growth, which promises to be slow. Therefore, if we are not to sit idly by, we must muster available expertise, resources and means of international communication, and do our best to tackle the problem. In doing so we will be in danger of 'over-selling' a fledgling field, and this must be borne in mind by those entrusted with the task.
ACKNOWLEDGEMENTS Research in urban climatology at the University of British Columbia is supported
by the Natural Sciences and Engineering Research Council of Canada. Thanks are extended to M. L. Oke who processed the manuscript.
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