Ingegärd Eliasson

Urban nocturnal temperatures, street geometry and land use

Abstract The climate in the city of Goteborg, Sweden, was investigated for a three year period. In this paper the nocturnal temperature distribution is analysed in relation to differences in street geometry and land use. The seasonal and monthly air temperature difference between a street canyon and a nearby open area, as well as case studies of the vertical and horizontal temperature distribution within and between different urban units and urban districts are discussed. In spite of a good relationship between the local surface temperature and the sky view-factor, both continuous measurements and case studies show small variations in air temperature within the city centre. Air temperature variations between urban districts of different land use are greater. The average horizontal temperature decrease of 4°C in the transition zone from the city centre to a large park southwest of the centre are, in fact, of the same order as the average urban-rural air temperature difference.

The influence of green areas on nocturnal temperatures in a high latitude city (Göteborg, Sweden)

The air temperature pattern in three urban parks and their surrounding built-up areas was studied over a one and a half year period in Goteborg, Sweden. The measurements were made at mobile and permanent stations on nights with clear skies and light winds. The maximum temperature difference found between a park and a built-up area was 5.9°C (summer), and the extension of the cool park climate into the built-up area was over 1100 m from the park border. Both the extension and the magnitude of the temperature difference depended on the size of the park and the distance from the park border. The green area cooled at a faster rate than the built-up area, although there were large variations within each area. At the rural station, located at an open site, the cooling was less than at the open part of the green area. Differences in sky obstruction between the sites explained some of the variations in cooling and temperature. However, the relationship between the urban-park temperature difference and sky view factor was not statistically significant.

Thermal Comfort and Outdoor Activity in Japanese Urban Public Places

Subjective thermal comfort and outdoor activity in a park and a square in a satellite city northeast of Tokyo were investigated through structured interviews, observations, and comprehensive micrometeorological measurements. Results showed that the park was on an average 1.1°C cooler than the square. The relatively warmer thermal conditions in the square in comparison to the park resulted in a heat load of greater intensity for humans in the square. In general, there was a low relation between the thermal environment and the use of the two places in terms of total attendance. However, the use of the park was influenced more by the thermal conditions than by the use of the square, which can mainly be attributed to the different functions of the two places. Finally, examples of the differences between the use of the sun, the attitudes toward it, and outdoor exposure in Japan and Sweden are highlighted and discussed.

Psychological mechanisms in outdoor place and weather assessment: towards a conceptual model.

The general aim has been to illuminate the psychological mechanisms involved in outdoor place and weather assessment. This reasoning was conceptualized in a model, tentatively proposing direct and indirect links of influence in an outdoor place–human relationship. The model was subsequently tested by an empirical study, performed in a Nordic city, on the impact of weather and personal factors on participants’ perceptual and emotional estimations of outdoor urban places. In line with our predictions, we report significant influences of weather parameters (air temperature, wind, and cloudlessness) and personal factors (environmental attitude and age) on participants’ perceptual and emotional estimations of outdoor urban places. All this is a modest, yet significant, step towards an understanding of the psychology of outdoor place and weather assessment.

Urban modification of the surface energy balance in the West African Sahel : Ouagadougou, Burkina Faso

Abstract Surface–atmosphere energy exchanges in Ouagadougou, Burkina Faso, located in the West African Sahel, were investigated during February 2003. Basic knowledge of the impact of land cover changes on local climate is needed to understand and forecast the impacts of rapid urbanization predicted for the region. Previously collected data showed a large dry season urban heat island (UHI), which dramatically decreased with the onset of the rainy season and corresponding changes to the natural land cover thermal and radiative properties. Observations of local-scale energy balance fluxes were made over a residential district, and building surface temperatures were measured in three separate locations. Net all-wave radiation showed an increase with urbanization owing to the higher albedo, lower heat capacity, and thermal conductivity of the bare dry soil compared to the urbanized surface. The combination of material and geometry resulted in a decrease in albedo toward the urban center. Despite the higher alb...

Urban Heat Island Circulation in Göteborg, Sweden

SummaryThe paper presents a study of the Urban Heat Island Circulation (UHIC) in Göteborg, Sweden. Observations and recordings have been carried out from 1981 to 1986 during winter nights with anticyclonic weather conditions. The UHIC develops in general at a large negative net radiation balance, when the heat island intensity is at least 2.5°C, the wind speed less than 3 m/s and the sky is clear. If the weather conditions are favourable the UHIC starts 4–6 hours after midnight and stops a few hours after sunrise. An increase in cloud cover during the late night will not prevent the development of the UHIC. The UHIC layer extends to 40–70 metres in the vertical direction and to 10–13 km in the horizontal direction. The UHIC layer is capped by an inversion and the flow is almost independent of the direction of the regional flow. The UHIC is of great importance for the concentrations of air pollutions in Göteborg, as it transports both polluted and clean air. The study is a part of a clean air programme in the Göteborg area.

Urban–rural vapour pressure differences and their role in the development of urban heat islands

Data for a 4-year period from Goteborg, Sweden was used to investigate the urban–rural humidity differences during clear and calm nights. The purpose was to analyse the impact of the Urban Moisture Excess (UME) on the Urban Heat Island (UHI) with focus on the longwave radiation balance and the latent heat flux. The statistical analysis demonstrated that a hot summer with reduced precipitation increased the mean UME to 3 hPa (‘normal’ summer conditions demonstrated an UME of about 1 hPa) and during some nights the UME was as high as 7 hPa. The analysis also demonstrated a positive correlation between UME and UHI, with the maximum UHI preceding the maximum UME by between 2 and 5 h. An analysis of the nocturnal development of UME demonstrated that the UME could develop from three combinations of urban and rural vapour pressure changes. The initial level of the rural vapour pressure, and thus the conditions for dewfall proved to be important. The influence of an UME of 3 hPa on the downward longwave radiation was calculated to about 3 W m−2, which is 12% of the urban–rural radiation difference giving a weak enhancing effect of the UME on the UHI. Advection of dry air from the UHI circulation was considered to influence the urban vapour pressure change. After correction of the advective effect on the urban vapour pressure change, the urban–rural difference in the latent heat flux ranged from −5 to −25 W m−2, which demonstrates that UME also has a considerable negative feedback on the UHI development. Copyright

Infrared thermography and urban temperature patterns

Abstract A thermal scanner in the 2-μm waveband is used for airborne studies of urban temperature patterns. The urban geometry is modelled by the Sky View-Factor (SVF). Comparisons of surface temperature and SVF show a good correlation. The largest difference in surface temperature (4°C) was recorded between dense canyons and open areas. Ground measurements correspond well with the temperature calculated from the infrared images. Different examples of inlra-urban thermal variations and the utility of the instrument and software for the determination of differences in temperature arc discussed.

COOLING RATES, SKY VIEW FACTORS AND THE DEVELOPMENT OF INTRA‐URBAN AIR TEMPERATURE DIFFERENCES

Abstract Intra‐urban cooling rates were monitored for a year in the centre of Göteborg on the Swedish west coast. Five sites with different building geometry ‐ from dense canyon to open space ‐ within a radius of 300 m were analysed. Results showed two modes of cooling during the night. In the first mode, the cooling was site‐dependent ‐the open space cooled the most and the dense canyon the least. In the second mode, which began about 3–4 hours after sunset, all sites cooled at the same rate. Our hypothesis is that in the early part of the evening both radiative divergence and sensible heat dominate the cooling. However, as the evening progresses, a spatially homogeneous inversion is established which controls the radiative cooling, making the cooling independent of both surface geometry and surface type. From April to November the sites cooled rapidly around sunset and the cooling slowly decreased during the rest of the night. However, between December and March, the cooling was less intense, with an almost constant rate during the entire night. It has been suggested that this might be the result of the draining of stored solar heat during the summer and a continuous flux of anthropogenic heating during the winter.

Urban Geometry, surface temperature and air temperature

Abstract This paper presents a local climatological investigation of urban climate and urban structure carried out in Goteborg, Sweden. The surface and air temperature pattern are studied on a city scale and on a canyon scale. Data from ordinary weather stations and three specially designed weather stations are used. Recordings of temperature, humidity and wind are also made by an infrared thermography system (Thermovision 870), measurement cars and dataloggers. The geometry of the city is described by the sky view factor (SVF). In general the results show a lesser temperature variation than expected. The surface temperature varies a few degrees between different urban geometries, but the air temperature variation is very small. Further studies are planned to determine the temperature variation within the city and between the city and the suburbs.