URBAN CLIMATOLOGY AND DESIGN

URBAN CLIMATOLOGY AND DESIGN Simos Yannas Lecture AA EE 16 October 2002 These notes edited 28 October 2002 Topics Factors shaping urban microclimates;...
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URBAN CLIMATOLOGY AND DESIGN Simos Yannas Lecture AA EE 16 October 2002 These notes edited 28 October 2002 Topics Factors shaping urban microclimates; improving the microclimates of open spaces in cities.

INTRODUCTION Cities are places of concentrated energy consumption and the main producers of waste, pollution and climate change. Urbanisation and human activity in cities have led to microclimatic changes that have important implications for inhabitants health and comfort as well as for energy use and environmental quality. These topics are discussed from the viewpoint of an environmentally sustainable approach to urban and architectural design focusing on design of urban open spaces. The environmental design objectives for open spaces are to influence their microclimates so as to improve amenity and use of such spaces, as well as contributing to a cleaner and healthier urban environment to establish the contribution from such climatic improvements to the wider urban microclimate of cities. THE CLIMATOLOGY OF CONTEMPORARY CITIES According to Oke (Oke 1987), urban surfaces are rougher, drier and warmer than those in the surrounding country side. Important contributors to these conditions are: ?? ?? ?? ??

the reduced airflow and humidity in cities large amounts of anthropogenic heat aerosols and greenhouse gases the urban form.

Airflow Patterns and Humidity Airflow The resistance to airflow presented by the solid volumes of buildings slows down the wind; as a result, wind velocities in cities are generally lower at ground level, than those in the open country. The result is a reduced rate of heat dissipation by convective cooling. This might be felt in the form of higher urban temperatures near ground level. It produces effects of wind protection which may be felt as a positive contribution to outdoor thermal comfort in cool climates. On the other hand, tall buildings lead to complex air flow patterns and produce turbulence which affects the activity and comfort of passers by outdoors. Humidity Cities have much reduced areas of green and water compared to the country side and thus reduced levels of humidity and lesser contribution from evaporative cooling processes (see below). This modification also results in higher air temperatures in the urban environment. Anthropogenic Heat Production All human activities both inside and outside buildings produce heat as a by product. Space heating and cooling in buildings, artificial lighting, and the use of domestic and office appliances are the main heat sources indoors, that are eventually transferred to the urban environment by transmission through building elements and by air exchange. In Northern Europe during winter, the amount of heat dissipated within the urban canopy layer by buildings and traffic can exceed that contributed by solar

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radiation, about 25 W/m2 in Europe (Barry ?a?Chorley 1997). For the urban microclimate, buildings are permanent heating appliances discharging heat around the clock all year, Fig. 1. This effect is more pronounced the less insulated a city’s buildings are thermally, and the higher the built density. These features increase the rates of heat production and/or outward dissipation. Built form and construction thus become critical considerations for the outdoor environment, as well as for indoors.

Fig 1 Buildings operate like heat radiators day and night all year

Air Pollution The two main families of pollutants have different and rather opposite effects on the urban climate. Greenhouse gases such as carbon dioxide (CO2) and methane (CH4) are fairly transparent to incoming solar radiation (shortwave radiation) but absorb part of the outgoing thermal (longwave) radiation emitted by terrestrial surfaces, Fig 2 (left). This reduces the net cooling of the urban environment resulting in a warming effect at ground level. Aerosols, on the other hand, absorb solar radiation reducing the amount reaching the ground, Fig.2 (right). This results in a change in a city’s radiation climate with proportionate decrease in direct radiation and increase in diffuse, as well as lower temperatures at ground level. The implications for design of buildings as well as outdoor spaces are considerable.

Fig 2 Greenhouse gases (left) lead to a warming-up of the urban environment by reducing the rate of heat dissipation, whilst aerosols (right) lead to lower urban temperatures by reducing the amount of solar radiation reaching the ground.

Urban Morphology Individually or in combination, the various properties of the urban tissue have multiple effects on the urban microclimate; these are summarised below. They influence the magnitude of incident solar radiation, the air movement inside and outside buildings, and the mean and peak temperatures of urban surfaces and outdoor air.

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? ? the area of exposed external surfaces is an indicator of potential incidence of solar energy, as well as a measure of thermal coupling between indoors and outdoors. Compared to flat, open ground, a built site has a larger external surface area on which to receive a larger total amount of solar radiation. ? ? view of the sun is often obstructed due to overshadowing by neighbouring buildings, an effect which increases with latitude and reaches its peak on high density sites in midwinter. A surface’s view of the sun at any given time is largely determined by the built form and by street widths and orientations. ? ? heat dissipation by longwave radiation depends closely on sky view which is restricted by narrow streets and dense built form; it is also inhibited by the accumulation of greenhouse gases. The morphological characteristics and built density of the city influence: 1. 2. 3. 4. 5.

solar access and shading of building surfaces and open spaces thus affecting air and surface temperatures view of sky and thus daylighting and cooling of buildings and open spaces the air permeability of the urban tissue and thus the ventilation and cooling of the city the solar reflectance and thermal capacity of the urban tissue and thus the temperature peaks and fluctuations in air and surface temperatures the volume of green areas and water bodies and thus among other things air temperature and its moisture content.

Urban Heat Island On the whole the parameters and processes described above result in the manifestation of urban microclimates that are distinctly different from conditions in rural areas. A characteristic of these is the phenomenon known as the urban heat island, a concentration of heat leading to higher temperatures than in countryside. Maximum intensities of the heat island have varied in the range 5-15 K and are generally manifested at night, Fig. 3.

Fig 3 Temperature isotherms in London by Chandler on an evening of May 1959 showing magnitude of heat island effect. This is because in urban areas, restricted sky view from street canyons leads to lesser cooling by longwave radiation compared to open country where, given a clear sky and good sky view, outward longwave radiation is at its highest. Thus rural areas cool faster and the urban-rural temperature difference becomes more acute at such times. In the daytime the difference reduces to 1-2 K. An

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inversion phenomenon has been observed in tropical seaside cities leading to the creation of a cool island during afternoon in city centre as the warm air rises pulling cooler air from the sea that cools the centre. In desert areas Pearlmutter (1998) observed this phenomenon as a “cool island” owing to the shading and heat storage provided by the surrounding elements. It led to warming-up of the urban canyon after sunset as heat stored in the canyon walls started affecting the air temperature. Implications of the urban heat island effect: -

in winter the higher air temperatures in streets and open spaces of cities may be sensed as more comfortable and may also lead to reductions in space heating energy use owing to lower heat losses from buildings

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in summer the higher air temperatures in streets and open spaces may be sensed as exacerbating summer conditions, as well as increasing cooling loads for buildings leading to increase in installations of air conditioning and higher consumption of electric energy

-

increase in peak electric demand from larger number of air conditioning appliances requiring the commissioning of new power stations

-

air pollution increasing in line with energy consumption

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possible health issues from hyperthermia especially for the elderly.

The importance of these tends to vary depending on other characteristics of a place. In cool climates the higher city temperatures will be welcome in winter both for comfort and for reduced energy use for space heating. On the other hand reductions in available solar radiation due to the presence of aerosols may counteract some of the positive effect. In summertime the higher air temperatures lead to higher demand for cooling. In climates with mild winters, for example, those of Southern Europe (eg cities such as Rome, Lisbon, Athens) it may be possible to eliminate conventional heating altogether thus economising on capital cost, as well as on fuel cost. However, cooling loads are more than double those in suburban areas in some Mediterranean city centres (Santamouris 2000). MICROCLIMATIC CONSIDERATIONS IN URBAN DESIGN Key objectives of environmental design for open urban spaces are: ?? ?? ?? ??

solar access and solar control thermal inertia natural ventilation and wind protection natural cooling

Solar Access and Solar Control In most climates, solar access and solar control are the most important strategies of environmental design of outdoor spaces. By solar access we denote the selective exposure of spaces to solar radiation. Solar control is the selective avoidance of components of the incident solar radiation. Whether and when solar access or solar control should be preferred for a given space depend on use of that space, as well as on the climatic conditions of its location. In outdoor spaces a person’s exposure to direct solar radiation is equivalent to the thermal sensation of a higher air temperature. For example, someone standing outdoors in a Mediterranean location on a sunny winter day around noon, exposure to typical solar radiation values of 300-400 W/m2 on a horizontal plane is equivalent to o a rise in air temperature by 4-6 C. With the outdoor air temperature in the range 10-15 degrees centigrade, which is typical for such location in mid-winter, this translates to a temperature sensation o of 14-20 C. This means that whilst without sunshine one would probably not sit outside for any length of time, on a sunny day and wearing appropriate clothing, conditions of thermal comfort can be achieved.

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Fig. 4 Psychrometric charts showing comfort conditions at different outdoor temperatures On the other hand, when the air temperature is high, exposure to direct sun is likely to be undesirable. For example, a person sitting outdoors over a period of time with the air temperature above 25oC will almost certainly require protection from the sun for thermal comfort (with the exception of sitting out for a sun tan). With good solar protection (i.e. sitting in complete shade), and where needed the parallel contribution from air movement, we can sustain air temperatures above 30C with little discomfort. It follows from these considerations that in outdoor spaces where solar access and solar control can be provided when required, use of these spaces can be extended in time so that with the parallel application of other microclimatic measures it may cover most of the year. For Southern Europe, and similar climates in other regions (for example, parts of the Americas), aiming at solar access at times and solar control at others are seasonal objectives. For the cooler and cloudier climates of Northern Europe (and similar climates in other regions) sunny days are almost always welcome, irrespective of outdoor air temperature, because they are comparatively rare and people go out and take over all available outdoor spaces whether it is winter or summer. Finally, in tropical climates, where the outdoor air temperature is high all year round, though not excessively high, direct exposure to sun is almost always undesirable and providing solar protection can make the difference between comfort and discomfort. For a given geographic latitude and climate, the ?rientation of streets and their sectional profiles are key factors influencing the view of the sun from the ground or from building surfaces. For example, a street running in the direction North-South has a symmetrical view of the sun, with one side of the street in (potential) view of the sun in the morning and the other in the afternoon, Fig. 5 (right). On the other hand, with a street that runs East-West only the southern side of the street is potentially “seen” by the sun, Fig. 5 (left). In both cases the view of the sun from ground level is a function of the ratio between the height (h) and the width (w) of the street (h/w ratio).

Fig. 5

Differences in solar access as a function of street orientation

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As an example, consider a location at geographic latitude of 40oN, with an urban tissue of row buildings and a street running east-west. If we required the whole of a building elevation that faced to the south to be in view of the sun down to ground level at midday on a sunny day in mid-December, the h/w ratio would need to be equal to, or less than, 0.5. Given a height for the buildings on the other side of the street of 15 metres, the width of the street should be at least 30 metres to satisfy this relationship (15/30= 0.5), Fig. 6. With this configuration up to 60% of the ground between the buildings could also be in view of the sun by mid-March, when the solar altitude angle reaches a peak of 50 degrees around noon. By contrast, if the street was only 15 metres wide (i.e. h/w = 1.0) the sun would not see the ground area of the southern elevation before March and in December only parts of the elevation 7.5 metres above ground would be seen by the sun.

Fig. 6 Explanatory Diagram If we draw a line joining the top of an obstructing element and a point on a surface of interest the angle ? formed by this line and a horizontal from the point of interest is known as the obstruction angle, Fig. 7.

tg ? = h / w where, h

? w

?, obstruction angle h, average building height w, street width

Fig.7 The obstruction angle

Thermal Inertia In urban centers throughout Europe and many other parts of the world the external walls of buildings are of brickwork, stone or concrete all of which are materials with high heat strorage capacity, Table1. The same applies to roads and pavements. The heat storage capacity of all these surfaces functions as a daily heat sink reducing temperature fluctuations in outdoor spaces. Reduction of fluctuation means a reduction in peak temperatures both winter and summer. This outcome is desirable during daytime in summer, but potentially undesirable on winter days (as it will keep temperatures low). Heat flow from buildings to the urban environment is a function of the thermal resistance of building elements. The lower the thermal resistance, the faster the rate of heat flow from buildings. Uninsulated

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buildings require more energy for space heating and cooling. As a result they dissipate more heat to the outside contributing to the rise in urban temperatures. Table 1. Volumetric Thermal Capacity of Common Materials, in Wh/m3 K Water Stone High Density Concrete Plaster Brickwork

1158 650 483 440 374

Ventilation and Wind Protection The lower wind velocities experienced in cities contribute to the urban heat island effect by reducing the rate of heat dissipation. These also affect dissipation of pollutants, and thus air quality and ventilation of outdoor spaces. The way air moves in towns is a function of the geometry of the urban tissue. Where streets are parallel to the direction of the wind air movement is freer. The wider streets are, the less the resistance to wind. Open spaces in the center of urban blocks are normally outside the path of the air flow. In cool climates, the reduced wind velocity can be welcome for thermal comfort outdoors and wind protection for buildings. However, tall buildings and movement of air through narrow street canyons lead to turbulent flows leading to problems for surrounding spaces, buildings and passers by. For row buildings, the alignment of streets with wind direction provides the most effective ventilation conditions. Best results are achieved when wind direction is at an angle of 45 degrees to the street. The scope of varying wind speed or aligning wind direction with a city’s main avenues depends on other climatic factors as well. A general criterion is that wind velocity should not exceed 5m/s as above this threshold wind becomes disruptive, and above 10m/s unpleasant. In cool climates wind protection is one of the main environmental objectives especially where building is low density. Natural Cooling Lower outdoor air temperatures at night permit heat dissipation from building surfaces and outdoor spaces. Heat is dissipated from the urban tissue by longwave radiation to the sky and by convection to adjacent air layers. Heat dissipation by longwave radiation to the sky is the main mechanism by which the terrestrial thermal balance is maintained and the heat absorbed from sunshine during the day is released. The temperature outside the Eatrth’s atmosphere is at absolute zero (-273?C). High in the atmosphere air temperature is around -40?C. Thus the sky provides a powerful heat sink. The intensity of the outgoing thermal radiation is higher in the direction of the zenith, in the spectral range of 8-14µm called the atmospheric window. In cities reduced view of the sky can inhibit this mechanism of heat dissipation. Vertical building surfaces and street decks in narrow urban canyons have restricted sky view and tend to retain heat; howver, by the same token they also have reduced view of the sun and thus absorb less solar radiation.

3 INSTRUMENTS AND MEANS OF SUSTAINABLE ENVIRONMENTAL DESIGN The following are the main instyruments with which we can influence urban microclimates, improve comfort and reduce energy consumption in cities: ? ? surface properties ? ? water bodies ? ? vegetation

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? ? additive elements and transitional spaces ? ? urban morphology. The relationships of these with the strategies and objectives discussed above and their comparative importance in environmental design is shown in Table 2.

+/++

THERMAL INERTIA

NATURAL COOLING

GREEN

VENTILATION WIND PROTECTION

WATER

+/+

SURFACE PROPERTIES

SOLAR ACCESS SOLAR CONTROL

ADDITIVE ELEMENTS

URBAN MORPHOLOGY

Table 2. Correlation of Means and Objectives for Microclimatic Design of Outdoor Spaces

++

+ +++

+

+/+++

+/++

+

+/-

+

+

++

++

+/-

++

++

+++

+/+

+++

Key: + positive influence ++ very positive +++ extremely positive +/- positive or negative influence

Surface Properties and Choice of Materials The physical properties that influence temperatures of external surfaces and built elements of a city are: ? ? the solar reflectance (or reflectivity or albedo) and absorptance (or absorptivity), and for transparent surfaces, the solar transmittance (or transmissivity) ? ? thermal emissivity, the ability to emit thermal radiation ? ? thermal capacity, the ability to store heat ? ? thermal resistance, the ability to reduce the rate of heat flow. Solar reflectance, absorptance and transmittance: the control of the solar reflectances of surfaces according to climatic conditions and functional requirements is one of the most important means of bioclimatic design in the urban environment. Most external surfaces in cities are of low reflectance with values in the region 0.10-0.30 and average of 0.15 (Oke 1987), Fig. 8. This means that on average 15% of incident solar radiation is reflected from these surfaces on first incidence whilst the remaining 85% is absorbed. Absorption of solar radiation by building materials influences leads to heat storage as well as emittance of thermal radiation contributing to higher urban temperatures. Especially low is the reflectance of street decks which varies from 0.04 for freshly laid asphalt to 0.12 after some use. Light colours are preferable especially in areas with high values of h/w ratio where they also contribute to natural lighting.

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Fig. 8 Reflectance and absorptance of solar radiation in urban canyon; typically almost all of the incoming solar radiation is absorbed within canyon after two reflecxtions.

Roofs are the most exposed surfaces in a city. Givoni (Givoni 1997) reports differences in the order of 30-40 degrees Celsius between peak surface temperatures of white and black roofs (reflectances of 0.7-0.8 and 0.1-0.2 respectively). Keeping roofs at lower temperatures benefits urban microclimates whilst also reducing building cooling loads. Planting of roofs contributes to cooling and has a cleansing effect on adjacent air layers contributing to improvement of the urban microclimate. Thermal emissivity: In cities streets and most open spaces are surrounded by surfaces that have a high thermal emissivity. The positive aspect of this is that building materials have potential for dissipating heat absorbed during day by emitting thermal radiation at night. Thermal Resistance and thermal capacity: thermal resistance and thermal capacity have a regulatory role on thermal exchanges between buildings and open spaces. High thermal resistance (a measure of thermal insulation) reduces the rate of such exchange, essentially insulating the building from the outside. The thermal capacity of a building structure reduces temperature fluctuations. Thermal capacity of external walls influences street and outdoor space temperatures. Water Water possesses much higher thermal capacity than any of the other materials in the urban tissue, two to three times as high of building materials such as stone, brick and concrete, see Table 1 above. A large body of water (sea, river, lake, fountain) has a moderating influence on the air temperature in its vicinity. Water evaporation has a cooling effect on surrounding air. Under clear sky and solar altitudes above 30 degrees, the solar reflectance of water is in the range .03 to .10 (Oke 1987) which means that water absorbs almost all of the incident solar radiation and operates as a solar control device for its immediate surroundings. Thermal radiation is also absorbed by water. At nightime dissipation of stored heat balances heat loss by radiation but also supports evaporation which may continue overnight. Vegetation The microclimatic effects of vegetation are a function of the following of its characteristics: ?? ?? ?? ?? ?? ?? ??

its high solar absorptance its lower thermal capacity and thermal conductivity compared to common building materials its ability to reduce air temperature by evapotranspiration its reduced emittance of thermal radiation its reduction of air velocity at ground level retention of dust and pollutants from the surrounding air its potential as protection from noise.

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The cooling effect of vegetation has been reported by many researchers comparing temperatures in urban parks with those recorded in street canyons. Rosenfeld et al (1997) reported on US findings; Santamouris et al (1998) observed that temperatures recorded at the central Athens’ National Gardens were much closer to those of the reference suburban sites. Macho et al (1994) have reported on how evaporative cooling and the shading and cooling effects of vegetation were successfully exploited on the site of the 1992 World Expo in Seville. The influence of vegetation is felt in its immediate vicinity; it may require a considerable surface area if it were to affect larger parts of the urban environment perhaps as large as a square if it were to function as an oasis. Additional elements and transitional spaces Pilotis, arcades, balconies, awnings, pergolas, and other coverings represent a typology of additive elements that define transitional or semi-outdoor spaces between buildings and open spaces of the city. ?heir key bioclimatic characteristic is protection from sun, wind and rain. Permeability of the building envelope at ground floor level through pilotis can contribute to ventilation and cooling of open spaces especially in narrow streets and spaces trapped in the center of urban blocks. Traditionally, verandas and balconies served useful functions as outdoor private spaces. Today noise and pollution limit these traditional uses in urban centers and perhaps the role of these elements should be rethought. Urban Morphology Streets, squares, patios and courtyards behave thermally partly as roofless rooms. Important parameters are the built density and building typology, the geometry and h/w ratio of the canyons. These determine basic conditions for ventilation, solar access, solar control and view of the sky. Stepped elevations of buildings with one or more set backs permit differentiation of the h/w ratio for a given mean height of buildings. SUMMARY AND CONCLUSION Key Issues 1 2 3 4

Urbanisation entails microclimatic changes which affect energy use, comfort, wellbeing and enjoyment of cities The climatic effects due to anthropogenic sources may be reversed with appropriate measures over a period of time If urban microclimates vary and change considerably we may need to rethink the way we design (environmental) buildings We need more data on urban microclimates and their effects in order to take them into account in design.

Designers need to consider and aim to influence the following: ?? ?? ?? ?? ?? ?? ??

Built form in order to affect airflow, view of sun and sky, and exposed surface area Street canyon geometry in order to influence warming-up and cooling processes, the resulting thermal and visual comfort conditions at street level, and pollution dispersal Building design to influence building heat gains and losses, and the thermal capacities and reflectances of external surfaces Urban materials and surface finishes to influence absorptance, heat storage, and emittance Water and vegetation to influence cooling processes Traffic reduction to alleviate air and noise pollution, and reduce heat discharges Use of renewable energy sources to alleviate pollution .

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More quantitative information is needed to assess their relative importance in different contexts, and to help inform design. Clearly, if the urban climate can change drastically in relatively short periods of time we need to make buildings climatically more adaptable.

READING AND REFERENCES Yannas, S. (2001). Toward More Sustainable Cities. Solar Energy Journal. Vol. 70 No. 3 pp 281294, Elsevier Science Publishers, Oxford. Oke, T.R. (1987). Boundary Layer Climates. (see Chapters 7-9). Methuen & Co., London. Akbari, H.M. et al (2001). Cool Surfaces and shade trees to reduce energy use and improve air quality in urban areas. Solar Energy Journal. Vol. 70 No. 3, pp295-310, Elsevier Science Publishers, Oxford. Givoni, B. (1997). Urban and Building Design Considerations in Different Climates. Van Nostrand Reinhold. Santamouris, M. (ed. 2000). Energy and Climate in the Urban Environment. James & James Publishers, London. Steemers, K. and S. Yannas (Eds 2000). Architecture, City, Environment. James & James Publishers, London. Girardet, H. (1992). The Gaia Atlas of Cities: new directions for sustainable urban living. Gaia Books.