Trabajo publicado en :
17a Conferencia Internacional Passive and Low Energy Architecture (PLEA): Arquitectura, Ciudad y Ambiente. Cambridge, Gran Bretaña. Editores T. Steemers y S. Yannas James&James publishers, pp. 533-535. Año 2000
MARIA JOSE LEVERATTO, SILVIA DE SCHILLER & J. MARTIN EVANS
Research Center Habitat and Energy,
Faculty of Architecture, Design and Urbanism, University of Buenos Aires,
Pabellón 3, Piso 4, Ciudad Universitaria, (1428) Cap. Fed. Argentina.
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This study presents measurements and analysis of the Buenos Aires urban heat island. The objective is to identify one of the environmental impacts of urban development, relating climatic factors and urban design qualities. Data were obtained in a specially designed survey using miniature data-loggers in a series of simultaneous vehicular circuits combined with information about the location and its urban character. Results show a clear relationship between urban density and heat island intensity.
This study provides a quantitative assessment of the heat island of the city of Buenos Aires, located in a temperate sub-tropical climate. This phenomenon directly affects the quality of life of the population and indirectly contributes to climate change at the world scale. The identification of local impacts provides evidence to promote policy changes and improve the environmental quality of urban spaces.
The urban heat island has as series of important effects on the environmental conditions in the city. While small increases in outdoor temperature reduce heating demand in winter, energy demand for cooling becomes greater in summer. This leads to a vicious circle: air conditioning use increases with higher heat output from cooling towers, while greater heat output intensifies the heat island and demand for cooling.
Another effect, noted on calm days, is the generation of convection currents from the outer edges of the city towards the hotter central area, as warmer air rises. This produces a concentration of dust and pollution in the central area and reinforces the heat island intensity. However, in Buenos Aires, with a favourable wind regime, this effect is not as strong as in cities with lower wind speeds.
The urban heat island has been measured in many cities, using different techniques: a series of fixed stations distributed throughout the urban area (Santamouris, Mazzeo and others) or mobile stations (Chandler, Yamashita, Lansberg and others), using satellite-derived values (Nichol, Brazel and others) or combinations of these techniques. Mobile stations, chosen for this Survey, are simpler and provide more information on temperature distribution at a fixed point in time and the relation between temperatures and the characteristics of different areas. Fixed stations are valuable to obtain data on the variations of the heat island intensity over time.
In the specific case of Buenos Aires, a series of factors can be expected to contribute to reduce the heat island effect:
• South, south-east and east prevailing winds bring the moderating influence of the River Plate.
• The terrain is flat, unlike Santiago de Chile or Los Angeles, where mountains concentrate pollutants.
• The street grid encourages effective urban ventilation, with east-west central area streets open to river breezes.
The climatic data from 7 met stations in the metropolitan region of Buenos Aires do not provide sufficient data to identify the urban heat island as these stations are located to avoid city influences. They show the moderating influence of the River Plate and increased thermal range in inland areas (Evans, 1991). Other studies used limited fixed stations.
Seven simultaneous vehicular circuits were made, leaving from the city centre by different routes and returning to the original point after a journey of about an hour. The size of Metropolitan Buenos Aires does not allow circuits that reach the edges of the suburban area in this time limit, so they were planned within a 12-km radius from the centre, to the motorway which separates the Federal Capital district from the Buenos Aires conurbation.
The average temperature difference between the start and finish was 0.5°C in June (Winter measurements) and 0.8°C in October (Summer measurements). Measurements were made between 20 and 22:30 hours in the late evening. One passenger identified the location and evaluated the proportions of the street canyon while the other noted the vehicles location at two-minute intervals, together with data on vegetation and traffic conditions.
Temperatures were measured automatically every 10 seconds using a miniature data logger fixed outside each vehicle at a 1.40 metre height, adapted to obtain direct air flow over the thermocouple. The difference between data loggers was less than 0.4°K in all cases, the typical temperature step that the logger measures. The use of data loggers avoids errors that may arise in the recording of measurements in a moving car at night. It also avoids bias in the recording or rounding up or down, as well as simplifying the preparation of tables and graphics, as results can be introduced directly in a electronic spreadsheet.
Climatic data from the domestic airport, the city observatory, the international airport at the outer limits of the urban area and a fixed point near the Rio de la Plata were also obtained and analysed.
During winter measurements temperatures registered automatically in the vehicles gave values between 11° and 13.8° C under generally cloudy conditions. Wind was blowing from the NW with a speed between 4 to 7 m/s, and gusts reaching 20 m/s. The temperature at the suburban airport was 10°C. Even though these cloudy and windy conditions usually tend to reduce the intensity of the heat island a maximum difference of 3.8°C between the dense, highly built, congested central area and the suburban airport was detected. Further temperature differences could be expected between the inner suburbs and the surrounding rural area.
Measurements under hot weather conditions were done an evening with clear sky after three sunny days. When data were collected with the vehicles, temperature at the suburban airport was 21.6°C with breezes of around 3 m/s from the Northeast. Figure 2 shows the isotherms obtained with data from these circuits, registering urban temperatures between 23.5°C and 26.9°C. Cooler areas were detected around parks and other open vegetated spaces with a mean reduction of around 1°C.
More detailed measurements should be done to further analyse the influence of this variable. The moderating effect of the river is clearly detected during these measurements, especially under winter conditions.
Finally, the survey showed that almost fifty percent of the avenues and streets analysed do not have trees. There is a potential for cooling if the amount of vegetation could be increased in those areas.
The studies and surveys undertaken highlight the direct environmental impact derived from urban morphology, and related to population density and building form.
Overlapping maps, as it can be seen in Figure 1, there is a strong relationship between population density and urban heat island intensity. This tendency is also noticeable in an infra-red satellite image (Murillo, 1996) which shows the increase of temperature along the main urban transport arteries, where these coincide with dense development.
In Buenos Aires, high density areas have more than 950 inhabitants per hectare with deep urban canyons. Our survey registered mean H/W values in the City center of around 2. Despite street’s orientation these dense built up areas have very little capacity to dissipate heat even under windy winter conditions. In summer the barrier of compact constructions block the cooling benefits of river breezes from the Northeast. It is significant to notice that a temperature difference of 3.5°C was registered between to urban sites only 2.5 km apart, one close to the river and open to the wind, and the other within the dense urban fabric.
However, these data do not allow the causal factors to be identified as effects are combined in these denser areas: heat losses from buildings, deeper street canyons and more paved areas, and larger and more congested traffic flows. Although the measurements made so far do not allow the detection of the relative importance of each of the contributory factors, it is clear that certain forms of urban development and architectural design tend to increase this effect and further links between microclimatic conditions and street design, should be made, considering that urban geometry is one of the basic physical factors influencing heat island conditions (Oke 1988).
These study also shows that architects must consider urban climate modification when planning energy efficient buildings. Buenos Aires urban heat island is likely to increase with time, and energy efficient buildings must be planned for more extreme design conditions than those presently measured in meteorological stations. According to our measurements, temperature differences are about four degrees higher than those measured at the city observatory, increasing cooling energy demand and discomfort. As these maximum differences are likely to occur in the evenings, they could have important implications in the demand for air conditioning in the residential sector.
This study is related to research undertaken at the Joint Centre for Urban Design, Oxford Brookes University, European Alfa-ibis Programme, and Project AR-026 ‘Sustainable Architecture’ supported by the University of Buenos Aires. Postgraduate students of the Bioclimatic Design Course, 1999 collaborated in the survey.
1. Oke, T. (1988): Street Design and Urban Canopy Layer Climate, Energy and Building 11, pp 103-113.
2. Evans, J.M. & de Schiller, S.(1991): Climate and Urban Planning: The example of the Planning Code for Vte.Lopez, Buenos Aires, Energy and Buildings, 15-16, pp 35-41.
3. Leveratto, M. J. (1995): El impacto de edificios en torre de gran altura y confort en espacios urbanos, Anais III Encontro Nacional y i Encontro Latino-Americano de Conforto no Ambiente Construido, Gramado, ANTAC, Porto Alegre, 1995
4. Murillo, F, & de Schiller, S, (1996) Efecto isla de calor y planificación urbana, Impacto de las variaciones climáticas en el desarrollo regional, un análisis interdisciplinaria, VII Congreso Argentino de Meteorología, Buenos Aires, 1996.
5. De Schiller, Silvia (1999), Impacto de la Forma Urbana en el Confort de Espacios Urbanos, Anais del 5 Encontro Nacional de Conforto no Ambiente Construido, Fortaleza, Brasil, (in press).