CITY IMPACT ON AVERAGE TEMPERATURE AND SWING

For this thesis, the relevance of the heat island and the modifications of the thermal conditions in urban zones is the way in which the Comfort Triangles can show the changes of both the average temperature and the temperature swing produced by the built environment, compared with peripheral areas outside the zone of direct urban influence. In this section, the Comfort Triangles are applied to visualise the changes produce and demonstrate the value of this tool for analysing the environmental modifications.

10.7.1. Summary of results

Table 10.4 presents the results of measurements of the intensity of the heat island during the days of the experiments. Table 10.5 indicates the variation in the average temperature registered in the urban zone TMu, and the average temperature in the airport outside the area of direct influence on the urban area, with similar characteristics to the surrounding rural zone, TMr, together with the temperature swings ATu and ATr registered in both zones.

Table 10.4. Temperatures measured during the heat island experiment and maximum and minimum temperatures registered in the Met Station on the same day.

City Date Min temp Heat island Max temp

Table 10.5. Average temperatures and thermal swings in the four experiments.

City Date Average

10.7.2. Application of the Comfort Triangles at the urban scale.

Finally, the results of these studies are presented in the format of the Comfort Triangles, demonstrating the nature of the modifications registered both in temperature and environmental conditions, produced by the difference between the urban area and the surrounding natural habitat.

Although the increase in temperature and modification of the temperature swing are potentially favourable in the cold climate of Río Gallegos and the winter season in temperate Buenos Aires, it has been pointed out that this effect is the result of high energy use in the urban area, which also contributes to greenhouse gas emissions.

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Average temperature

Swing

Figure 10.14. Difference between the environmental conditions in the central area (orange square) and the peripheral zone of the Federal Capital, Buenos Aires (green circle), in summer, with the mild temperature increase and the similar thermal swing.

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Average temperature

Figure 10.15. Difference between the environmental conditions in the central area (orange square) and the peripheral zone of the Federal Capital of Buenos Aires (green circle) in winter, with an increase in both the average temperature and the temperature swing.

It should be noted that the measurements in Buenos Aires corresponds to the Federal Capital area only, while the measurements in other cities covers the central area and the surrounding rural zone.

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Tmed At

Centre Rural Triangle

Figure 10.16. Difference between the environmental conditions in the urban centre and the peripheral areas of Río Gallegos, Argentina, in winter, with the temperature difference to achieve comfort (18°C) or to reach typical indoor temperatures.

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Average temperature

Swing

Figure 10.17. Difference between the summer environmental conditions in the urban zone (orange square) and the peripheral zone of Tampico, Mexico (green circle), with an increase in the average temperature and the temperature swing.

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Average temperature

Figure 10.18. Comparison between the meteorological data from two stations in Buenos Aires, the Central Observatory (orange square) in the urban area and Castelar in the suburbs, further from the River Plate (green circle). Data for a 10 year period for the months of January, summer with orange line, and june, winter with blue line (Evans and de Schiller, 1991).

In the hot climate situation of Tampico and the hot summer season in temperate climates such as Buenos Aires, the increase of air temperature is highly unfavourable. In both cases the Comfort Triangle format indicates that the rural areas are more comfortable although at the upper limit of the comfort zone, while the conditions in the urban area are beyond the comfort zone. In contrast to the situation measured in cold climates, this variation is not due to heat losses from heated buildings, but rather from heat generated by vehicular circulation, heat from air-conditioning systems and the accumulation of heat in building materials exposed to the sun during the day

All of these heat island experiments indicate a clear impact of the built environment on the temperature distribution in the urban areas surveyed. Although the measurements presented here only represent a specific and not necessarily representative day, the values from urban and rural meteorological stations in the same area and for the same decade also show significant temperature differences. For example, Figure 10.18.

compares the conditions in the central observatory in the Federal Capital, Buenos Aires, 4 km form the River Plate, with those of Castellar outside the Federal Capital, in a suburban area 21 km from the river (Evans and de Schiller, 1991).

In this case, the increased distance from the River Plate Estuary produces a larger temperature swing due to the lower heat capacity of the land, compared with the large surface of water moderating the temperature variation. The difference in average temperature is 1,2°C, while the difference in thermal swing is 0,8°C, small but significant variations affecting the energy demand for heating in winter as well as the cooling demand to achieve summer comfort.

10.8. CONCLUSIONS

This chapter demonstrates the significant effect that the built environment has on the modification of average temperatures and temperature swings. Although these differences are relatively small compared with the potential modifications achieved in indoor spaces which are analysed in the following chapter, they can have an important influence in the heating and cooling demand of buildings.

Indeed, the heating effect of cities can be equivalent to a latitude shift of 4°C; that is the temperatures in the urban area are similar to those found in rural areas 4°C closer to the Equator, applying the ‘latitude shift’ concept originally proposed by Olgyay (Cooke, 2004). The variation between urban and rural met stations in Argentina was studied by Camillioni and Barros (1991), who also reported a similar 3°C difference in the average temperatures for the two situations.

However, as the Comfort Triangle Graph shows, urban areas not only modify the average temperatures but also the temperature swing. This demonstrates the utility of the approach and enables the visualisation of important differences between meteorológical data recorded in airports outside the urban area and the conditions found arround buildings within urban centres.

In the next chapter, the even larger variations in the conditions produced by architectural design decisions, such as orientation, building form and design of the building skin, demonstrated by a series of examples and case studies.

CHAPTER 11. CASE STUDIES AT THE ARCHITECTURAL