The urban heat island in Salamanca (Spain) and its relationship to meteorological parameters

Abstract

An urban heat island (UHI) was found to exist in Salamanca, Spain -a medium-sized European city that has a continental climate and relatively little industrial pollution. We demonstrate that urban heating can occur in cities with these characteristics, giving rise to a microclimate that may alter the biological rhythm of the zone e.g. under these conditions, spring arrives earlier in the urban zone then in the rural zone. The study was carried out using data from 2 recording stations (one in and the other outside the city) for the years 1996-1998. The existence of a nocturnal UHI was observed, with a highest annual mean value of 3.6°C and a lowest annual mean value (cool island) of -0.9°C. The most intense nocturnal UHI was seen in autumn, while the strongest sinks occurred in spring and summer. As in other types of city with different characteristics, the UHI was seen to vary according to the atmospheric situation. The meteorological variables that most affected the UHI were found to be: (1) wind, which at speeds of >~6 m s -1 prevented the development of UHIs; (2) cloudiness, which altered the flux of incident solar radiation (the intensity of the nocturnal UHI was greater with high clouds); and (3) atmospheric pressure, which characterised the days of atmospheric stability or instability, leading to variations in the intensity of the UHI. KEY WORDS: Urban heat island · Local climate · UHI sink · Wind · Cloudiness · Synoptic conditions · SalamancaResale or republication not permitted without written consent of the publisher Clim Res 34: 39-46, 2007 mainly derives from the absorption of the solar radiation that reaches the ground and buildings from dawn until the sun reaches its maximum height. When the sun's radiation reaches buildings, the ensuing successive reflections lead to more energy being confined within their sphere of influence than in rural settings (Wilby 2003). Materials forming the surface of cities usually have a greater heat absorption capacity than natural soils, so that in urban zones the energy is stored for longer than in rural zones (Hoyano et al. 1999). In turn, long-wave radiation coming from the ground is less able to cross the pollution layer, thus further contributing to heating the urban zone.Meteorological factors also alter the energy balance existing between the ground and the top of the atmosphere. For example, wind causes turbulence that homogenises the air temperature (Jáuregui 1988, Morris & Simmonds 2000, and clouds absorb or reflect solar radiation, thereby varying the amount of radiation that reaches the ground (Labajo et al. 1988).Following the results of a pioneer study by Sundborg (1950) into the temperature conditions of an urban zone; and those of Morris & Simmons (2000) and Morris et al. (2001) who related the intensity of an UHI to meteorological factors, we were prompted to determine the existence of a UHI in a mediumsized European city and to analyse the temporal evolution of its intensity and its relationship to meteorological variables. To conduct ...

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