Diurnal air temperatures in built-up areas in relation to urban planning

Temperature changes generated by the urban landscape influence people's health and comfort as well as energy consumption and air quality. It is therefore important for urban planning purposes to learn about air temperature variations between different land use categories for both extreme situations and during average conditions. Air temperature data were collected at 18 sites within the city of Gothenburg, Sweden, over an 18-month period. These data, together with land use information from the Master Plan of Gothenburg, have been used in a statistical analysis of air temperature variations and land use. A characterisation of three built-up land use categories (urban dense, multi-family and single houses) was performed as well as an analysis of observed temperature differences between these categories during both day and night and different weather conditions. At night the built-up areas were always warmer with the highest temperature deviation (8 degreesC) during clear conditions in the urban dense category. The 'multi-family' and 'single houses' categories, however, showed a similar temperature difference, around 4 degreesC, compared to the outskirts. On clear days built-up categories were colder than areas outside the city, a phenomenon referred to as a cool island. On cloudy days these areas were usually warmer, around 2.5 degreesC. Air temperature thus varies within the built-up area on a diurnal basis and during different weather situations but what is the relevance of these results for urban planning? How can this information change the way we plan? The consequences of air temperature variations on human comfort and energy consumption are discussed. Human comfort is measured as physiological equivalent temperature (PET), an index that expresses thermal stress on people. It is shown that PET values are generally higher within the densely built-up category but the energy demand is lower. In fact due to local climate variations a household within the densely built-up category uses 11-20% less energy on a yearly basis compared to a suburban one. This paper presents information that is of concern for the urban plan and the authors hope that it will increase awareness of climate variations in urban areas. (C) 2002 Elsevier Science B.V. All rights reserved.