The influence of weather types on the urban heat islands magnitude and patterns at Paranhos, Oporto - a case study from November 2003 to January 2005

Abstract

Oporto is a medium size city on the NW coast of Portugal with approximately 300 000 inhabitants that has experienced an intense urbanization process especially after 1970. Results from daily field work from November 2003 to January 2005 show several different thermal nocturnal pattern in Paranhos, Oporto, some of which we conclude can easily be related to different weather types. Our main purpose is to demonstrate the diversity of Oportos climatic subsystem resolution processes under generically similar weather types, as found during our period of itinerant measurements. Our research underlines the idea that the latitude, the strength of the winds from west, the Douros river presence and the altimetric diversity are not sufficient to erase the influence of urban metabolism on the energy city balance. From the analysis of the various examples selected, it is possible to state that there is a significant relationship between the weather type and the magnitude and pattern of the urban heat islands. The urban heat-island was specially evident on days of strong stability, weak barometric gradient, weak wind and frequent periods of calm, conditions normally associated with the presence of anticyclonic situations, but which we have found also under the influence of cyclonic weather types. We conclude however that it is not absolutely true that the urban heat-island is weaker under cyclonic than anticyclonic weather types. On the majority of the days with cyclonic weather and good mixing conditions we frequently verified a disturbance in the explicatory capacity of the two geographic factors considered - distance from the sea and altitude. Nevertheless, these two geographic factors proved decisive in explaining Oportos thermal nocturnal pattern on days under the effect of dry and very hot or very cold air masses. It is also significant that there was no particular intensification in the heat-island during the coldest time of the year. In our opinion, this did not occur because, on the one hand, the annual thermal amplitude is quite weak and, on the other, because the state of Portugals economic development is not compatible with the generalised use of equipment (eg. central heating of buildings) which can create a more comfortable ambience in the interior of buildings. We conclude that the energetic excesses which sustain Oportos climatic subsystem and the positive thermal anomalies particularly significant in some points of the city are above all associated with intense traffic, great compactness of the constructed area and irregular topography in these areas. Although our analyses of some climatic elements clearly show several examples of what we call the new balances of Oporto's Climatic Subsystem, they also reveal the difficulties concerning the distinction between the intrinsic climatological variability and effects induced by anthropogenic processes.