Abstract
The thermal environment is an important aspect of the urban environment because it affects the quality of life of urban residents and the energy use in buildings. Urban Heat Island (UHI) and Urban Cold Island (UCI) are complementary effects that are the consequence of cities’ structures interference with the local climate. This article presents results from five years of urban climate monitoring (2012–2016) in a small Portuguese city (Bragança) using a dense meteorological network of 23 locations covering a wide array of Local Climate Zones (LCZ), from urban areas to nearby rural areas. Results show the presence of both the UHI effect, from mid-afternoon until sunrise, and the UCI after sunrise, both being more intense under the dense midrise urban context and during the summer. Urban Green Spaces had an impact on both UHI and UCI, with an important role in cooling areas of the city during daytime in the summer. Other LCZs had less impact on local thermal conditions. Despite the small size of this city, both effects (UHI and UCI) had a relevant intensity with an impact on local climate conditions. Both effects tend to decrease in intensity with increasing wind speed and precipitation.
Highlights
As over half of the global population lives nowadays in urban environments [1], the changes in land use and land cover, from rural to urban, induced modifications in physical processes which generate the urban climate, described by Oke [2] as the particular meteorological processes and atmospheric changes that take place at the urban level
Starting from sunrise, soon after the Urban Heat Island (UHI) effect reaches its highest values, there is a quick shift in the UHI intensity, as surface air temperature increases faster in the rural context when compared with the urban Local Climate Zones (LCZ). These results suggest the existence of a Urban Cold Island (UCI) effect, as in the early hours of the day, the solar radiation hits the ground of the open rural areas earlier, warming up the surfaces and causing air temperature to rise quickly, while the shadows cast by the buildings and trees partially intersect the solar radiation reaching the urban environment
When analyzing the relationship between UHI intensity and both wind speed and precipitation, there is a clear reduction in the dispersion of results, with both UHI and UCI effects showing less intensity (Figure 7). These results show the importance of wind in reducing the heat storage by promoting convection, while the rain conditions are associated with a lower intake of shortwave radiation, as a consequence of cloud covering, and with the cooling effect of water evaporating on surfaces
Summary
As over half of the global population lives nowadays in urban environments [1], the changes in land use and land cover, from rural to urban, induced modifications in physical processes which generate the urban climate, described by Oke [2] as the particular meteorological processes and atmospheric changes that take place at the urban level. Many urban patterns can influence the local climate, including the distribution of land use, the design of streets, buildings and open spaces, as well as the choice of materials; such urban design options generate local changes in wind speed and direction, humidity, precipitation, air temperature, air pollutants dispersion and deposition [3,4]. Back in 1981, an increase in the amount of urban climate studies was already identified by Landsberg [5], who associated this trend with the growing interest by the scientific community on the human impact on the environment. Urban climate studies increased over the last decades as a consequence of the growing concern with human thermal comfort, energy savings, and climate change [6,7].
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