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 ...
Using data sets on the daily data of maximum atmospheric pressure at ground level collected at 14 weather stations located in the central zone of the Iberian Peninsula [Spanish central plateau (SCP)], the series of daily maximum pressure anomalies at each station, together with the difference between the daily value and mean daily value for each day of the year for the period between 1961 and 2003, have been established. The regional series of such anomalies were constructed for the whole study zone and for two differentiated parts of the same. As thresholds for the extreme values of the anomaly series, the values corresponding to the P 05 and P 95 percentiles were used. The series of annual frequencies of days with anomaly values below and above the threshold values were constructed for each of the weather stations, together with the average series for the whole study zone and each of its two parts. The corresponding regional average series of seasonal frequencies were also constructed.From an analysis of the trend of the series of the annual frequency of extreme anomaly values in daily maximum pressure it may be deduced that the lowest values show a decreasing annual trend, while the highest ones show an increasing frequency. This indicates that between 1961 and 2003 the number of days per year on the SCP with the highest extreme atmospheric pressure values at ground level increased along the study period. In contrast the number of days per year with lower extreme values decreased. Additionally, analysis of the seasonal frequency series indicated that it was the winter that dictated such behaviour.
RESUMENSe establece el comportamiento temporal de las frecuencias anuales de olas de calor y frío observadas entre 1961 y 2010 en la Meseta Central española y en las dos zonas que se pueden diferenciar en ella. A partir de los datos diarios de anomalías de temperatura se determinan las series de anomalías diarias de temperaturas máxima y mínima para las áreas de trabajo. Se obtienen los valores umbrales de dichas series de anomalías, determinados por los percentiles P 10 y P 90 . Se establece la existencia de ola de calor cuando se observan dos o más días consecutivos en los que las anomalías de temperatura máxima y de temperatura mínima superan, simultáneamente, los valores de los umbrales establecidos por el P 90 . Se identifican las olas de calor que han afectado a la Meseta Central española, y a las dos subzonas durante el periodo de estudio y se establecen sus frecuencias mensuales y anuales. Asimismo, considerando que existe una ola de frío cuando hay dos o más días consecutivos en que los valores de temperaturas máximas y mínimas diarias son inferiores, simultáneamente, a los umbrales establecidos por el P 10 , se identifican las olas de frío y se establecen sus frecuencias mensuales y anuales en el periodo de estudio. Los resultados indican que los meses con mayor número de olas de calor entre 1961 y 2010 son mayo (25 olas) y junio (23 olas). El análisis de tendencia de las series de frecuencias anuales obtenidas indica que existe una tendencia creciente de olas de calor a un nivel de confianza mayor del 99%. El modelo lineal establece que se ha producido un aumento en la frecuencia de olas de calor en la Meseta Central española del orden de 0.6 olas cada 10 años. En cuanto a las olas de frío, se detectan olas de frío todos los meses del año en número que oscila entre ocho y 16 olas. Los meses con menor número son abril (nueve), julio (ocho) y agosto (nueve), y los meses con mayor número son marzo, mayo, junio y octubre con 16 olas. Los años con mayor número de olas de frío son 1969, 1971 y 1977 con siete casos; en el resto de los años la frecuencia anual está comprendida entre uno y seis. El análisis de tendencia de la serie de frecuencias anuales indica que existe una tendencia decreciente de las olas de frío a un nivel de confianza del 99%. Si se considera un modelo lineal, en la Meseta Central española se ha producido, entre 1961 y 2010, una disminución de olas de frío del orden de 0.54 olas cada 10 años. ABSTRACTThe temporal behavior of the annual frequency of heat and cold waves observed between 1961 and 2010 is established for the Spanish Central Plateau and for the two sub-areas in it. The series of daily maximum and minimum temperature anomalies for the working areas were calculated from the daily data concerning temperature anomalies. The thresholds of these series of anomalies, determined by the P 10 and P 90 percentiles values, were obtained. Heat waves occur when there are two or more consecutive days on which the maximum and minimum temperature anomalies are simultaneously greater than the...
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