URBAN HEAT ISLAND IN WARSAW (POLAND) AND ITS BIOCLIMATIC CONSEQUENCES

Urban heat island (UHI) is microclimatic phenomenon that occurs in urbanised areas. It is manifested in significant increase of air temperature in urban area in relation to the surrounding rural neighbourhoods. UHI is effect of: - physical characteristics of the surfaces: materials composing urban surfaces, such as concrete and asphalt, absorb rather than reflecting solar radiation, - lack of natural evaporative surfaces (vegetation) that, in rural areas, contribute to maintain a stable energy balance, - augmentation of the vertical surface that both provide an increased surface absorbing and reflecting solar radiation as well as block winds that contribute to the lowering of the temperature (canyon effect), - human activities that mainly consists in heat produced by heating and cooling plants, industrial activities, vehicles, etc. Therefore during the day large quantities of heat accumulate in the city. At night heat is gradually released into the atmosphere, making it warmer than outside the city. A commonly used measure of UHI is a difference of the minimum air temperature between urban and rural areas. In Warsaw Agglomeration the UHI in particular days can be up to 10-12°C. Medium, long-term UHI values in dense urban or industrial areas can reach 2.5°C. However, warming effect in the city is weakly observed on open areas inside the city, in residential suburban estates with no-dense settlements. UHI is not observed inside the forest growing within the city border. In spite of the growing of the city UHI in Warsaw did not change significantly during the period 1981-2010. UHI phenomenon is also seen when considering thermal conditions felt by urban population. While classic UHI approach is best observed in nights and early morning hours then in bioclimatic considerations daytime surplus of heat in the city centre is more crucial for people living in the city. To present spatial distribution of bioclimatic UHI in Warsaw the GIS methods were used. The research was carried out with support of Central Europe Programme of EU in the frame of UHI project “Development and application of mitigation and adaptation strategies and measures for counteracting the global Urban Heat Islands phenomenon (UHI)”.