Costos y precios

The urban heat island (UHI) is a particularly important example of how the urban area can influence climate and the most obvious climate manifestation of urbanisation (e.g. Oke 1987; Arnfield 2003; Fan et al. 2005). It is caused by a variety of factors which contribute to higher temperatures in the urban centre, either of the surface or the atmosphere, compared to the surrounding rural areas. The UHI can be particularly significant in exacerbating the effects of summer heat-waves, with consequent problems such as increased mortality (Johnson et al. 2005) and marked air pollution events (Stedman 2004). A UHI can also affect the regional scale flow by means of a thermodynamically driven circulation pattern (e.g. Oke 1995; Bornstein et al. 2000; Collier 2006) caused by the UHI modifying the local pressure field and the stability. If the synoptic winds are weak, and the temperature gradients are strong, then there can be a closed circulation pattern associated with the UHI, which is characterised by a strong updraft motion

over the city centre, convergent flow near the surface and divergent flow aloft (e.g. Lemonsu et al. 2002; Collier 2006). In the case of a strong synoptic flow the existence of a warm urban plume aloft extending up to 100 km downwind of the city has been established (Shea et al. 1978; Pujadas et al. 2000; Britter et al. 2003).

The existence of an urban heat island was first measured for London by Luke Howard in 1818 (Howard 1833), when a significant difference between urban and rural temperatures was observed. Modern investigations have subsequently been performed for many other cities and although the heat island climatology is dependent on the particular city structure and surroundings, the existence of the UHI has been confirmed beyond all doubt (Oke 1982).

Many factors are responsible for the UHI, including the following (e.g. Tumanov et al. 1999): • Anthropogenic heat emission (industrial activities, residential heating etc…).

• Modification of heat fluxes due to shadowing effects. • Differences in the albedo of urban and rural surfaces.

• Differences in the heat storage capacity of urban building materials compared to rural ones resulting in greater energy uptake during the day and release at night

• Compared to a rural environment less surface area is exposed to evapo-transpiration (though this is not the case for some specific cities).

A single, distinct type of UHI does not exist, and it is possible to define many different types, each with its own spatial and temporal characteristics (Oke 1982). Examples are the urban boundary layer heat island (increased temperatures in the atmosphere above the city), the canopy layer heat island (increased temperatures of the atmosphere between the ground and the mean building height) and the surface heat island (this refers to the difference in surface temperature between the urban and rural surface).

The intensity of the UHI is defined as the maximum difference in temperature between an urban and rural location within a defined time period, for example a diurnal cycle. The intensity of the canopy layer UHI depends strongly on the weather conditions and synoptic wind (e.g. Shea et al. 1978; Wong et al. 1978; Tumanov et al. 1999). It is highest during anti-cyclonic conditions, with clear skies and light winds (Klysik et al. 1999; Pinho et al. 2000; Morris et al. 2001; Gedzelman et al. 2003). Atmospheric fronts (whether warm, cold or occluded) act to enhance air mixing, with the effect of equalling urban and rural temperatures and weakening the UHI intensity (Tumanov et al. 1999). Strong winds will also significantly weaken or cancel out the UHI (Morris et al. 2001).

A distinct diurnal and seasonal course has been documented for the UHI intensity (e.g. Klysik et al. 1999; Gedzelman et al. 2003; Shepherd 2005). During daytime the UHI is less intense and can even vanish, whereas during the night it reaches its greatest intensity. This is due to the release of heat absorbed during the day by building materials. For many mid latitude cities the UHI intensity is greatest in summer months. During winter months, although the anthropogenic heat flux tends to be greater, there is also much less daytime solar energy to be absorbed and subsequently released by the buildings. This tends to weaken the UHI intensity.

Cold islands can also occur in urban areas, particularly during daytime when the UHI is less intense and can become negative. This is due to increased energy being partitioned into storage, which can be especially high due to the urban building vertical surfaces and heat capacities of the construction materials.

For many cities in the world an increasing trend in the UHI intensity has been identified (e.g. Lee 1992; Velazquez-Lozada et al. 2006; He et al. 2007) and this is expected to be due to an increase in population and the city area. However, it is not easy (or indeed possible) to establish a definite, linear relationship between the population (as a surrogate indicator of city size) and the UHI intensity. Although population was originally linked to the UHI development and its intensity, Oke (1987) found that any relationship would differ in North American and European cities. Also, cities located at tropical latitudes do not appear to fit into either range, probably due to different urban-rural contrasts in soil moisture. The geographic location of a city (and the corresponding regional climate, characteristics of the rural surroundings and the influences of local topography) is clearly a controlling factor in determining the UHI intensity (e.g. Nitis et al. 2005). It can also be seen that characteristics such as city structure, population density, building compactness, sky view factor, the percentage of artificial surfaces and vegetation fraction in the city are important factors in controlling the UHI development (Oke 1987). Other factors such as the frequency of suitable synoptic conditions and regional climatic fluctuations might also affect long term trends (Chandler 1965). Nonetheless population growth could be an important factor in the UHI development, since it is accompanied by increases in urban surface area, housing, roads, public transport and other services, all of which affect the surface energy balance. In recent years some major cities (for example London) have seen a decrease in population in the centre, with a migratory flux towards the suburbs. However the decrease in population is not necessarily

accompanied by a reduction in city size, or a change in the surface energy balance. In this case relationships previously established in the case of city growth can no longer be expected to apply and population change alone is not capable of explaining observed trends in the London UHI intensity (Lee 1992).