This may be because impervious building and paved surfaces limit the presence of moisture which plays a key role in moderating local air temperatures. This is consistent with those from previous research that vegetation can reduce air temperature through the combined effect of direct shading and evapotranspiration during daytime  —  , . Although the vegetation does not provide shading and evapotranspiration during the night, the uses of the trees and lawn were still cooler than other kind of man-made surfaces.
In winter, both of the trees and lawn are withered, and they had rather similar cooling effect. In summer, however, the tree cover had stronger cooling effects. This result emphasizes that in places with hot summer, planting more trees in urban area will help to reduce the air temperature so that to provide a pleasant environment and reduce energy consumption.
It was also shown that the relationship between air temperature and the percentage of water area was very poor, the possible reason might be the comparatively few measurement points containing water bodies. Spatial extent significantly influenced the relationship between air temperatures and land cover. The magnitude of the air temperature variation, which was due to the conversion of land cover from rural natural surfaces to man-made surfaces, increased consistently with increasing spatial extent from 20 to m in radius.
The results also indicated that spatial extent scale substantially affected the explanatory power of the PerMS on air temperature. These findings suggest that the air temperature is affected by a certain spatial extent surrounding it. Moreover, different land cover types also had very different spatial extent dependence: with spatial extent expansion from 20 to m in radius , the correlation coefficient of different land cover types varied differently, and more interestingly, their relative impacts also varied among different times and seasons.
At noon in summer, the highest correlation coefficient between air temperature and the PerTC occurred at the smallest spatial extent of 20 m in radius, which provided evidence of the influence of tree canopy shading on daytime air temperature. This is also consistent with previous studies that have discussed the importance of urban canopy structure i.
However, on summer night, the highest correlation coefficient between air temperature and the PerTC occurred at the spatial extent of m in radius. This result quantitatively confirmed previous studies that the cooling effect of green areas could extend for several hundred meters . In suburban Tokyo, for example, Yokohari et al. The results of the multiple regression models indicate that the PerBA and PerPA could best explain variation in air temperature for winter night.
Especially, the PerBA was the most important variable on winter night, as a rule of thumb, a 1. This result indicates that a more densely clustered arrangement of the buildings is favorable for thermal environment and energy conservation during winter night. At noon in summer, air temperature could best be explained by the PerTC at the spatial extent of 20 m in radius. At this time, the significant negative correlation between urban tree cover and ambient air temperature suggests that the trees shading plays a major role in determining the cooling effect of the site during the day.
This is consistent with the investigations of the cooling effect of small vegetated areas by Shashua-Bar and Hoffman . Therefore, during nighttime in summer, increasing percent cover of building could significantly increase air temperature, while the increase of tree cover would significantly decrease air temperature, and thus help to mitigate excess heat in urban areas. These findings emphasizes the key role of tree cover in mitigating urban air temperature during hot summer night, further highlights that increasing urban vegetation cover could be one of effective way to ameliorate the urban thermal environment.
The relationships between urban air temperature and the composition of land cover conditions at a neighborhood scale in Beijing were investigated through a field measurement campaign and statistical analysis. The results showed that the increase of man-made surfaces due to urbanization has a significant positive effect on the magnitude of intra-urban air temperature variation, however, the effect varied among different times and seasons. Moreover, the influence of man-made surfaces on the magnitude of the intra-urban heat island effect increased with spatial extent. The different land cover types would have different effect on air temperature, but also indicated that the relative impacts of land cover on air temperature differed with time.
Among the five land cover types, the PerBA was the mostly important factor increasing local air temperature, while the PerTC was the mostly important factor decreasing local air temperature. In addition, different land cover types had very different spatial extent dependence: with increasing spatial scale the buffer zone size from 20 to in radius , the correlation coefficient of different land cover types varied differently, and more interestingly, their relative impacts also varied among different times and seasons.
In general, the moderating effects were significantly stronger at night than at noon, although they varied slightly with the seasons.
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These findings provide valuable insights for the analysis of the local air temperature variation and will help to improve the data assimilated in urban climate models and surface energy balance applications. In addition, predictive models can be used to determine the air temperature variation that might result from land cover conversions due to urban expansion. Urban plan and landscape design professionals could also easily apply predictive models to provide guidance in their planning and design process for a comfortable thermal environment.
This work represents the first step in understanding the quantitative relationship between air temperature and land cover composition a two-dimensional urban surface at local scale. The study has also proved the existence of air temperature patterns that might be related to the urban canyon geometry a three-dimensional urban structure.
As Henry and Dicks  said that urban heat island, especially those that develop in the daytime, are perhaps most importantly determined by the urban canopy structure, and that the thermal properties of land cover are of secondary importance. Therefore, to explain the air temperature difference more precisely, both of land cover composition and structure need to be taken into account in future study. Whether this conclusion can be applied to different metropolitan areas and other climatic conditions should be further explored.
The authors also thank the editor and two anonymous reviewers for their helpful comments and suggestions on the original manuscript. Conceived and designed the experiments: HY LD. Wrote the paper: HY LD. Browse Subject Areas? Click through the PLOS taxonomy to find articles in your field. Abstract The effects of land cover on urban-rural and intra-urban temperature differences have been extensively documented. Funding: The authors have no support or funding to report.
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Introduction Humanity is rapidly urbanizing. Materials and Methods Ethics statement The field studies were conducted at an urban public open space in Beijing city. Download: PPT. Figure 1. A map of the study area and location of measurement points. Figure 2. Photographs a and land cover classification b of one example of the measurement point points 1. Air temperature measurements Spatial variations in canopy layer temperature can be observed via mobile traverses, which can accurately depict the spatial distribution of microclimate over the target areas for focused surveys upon small time and space scales.
Land cover classification Satellite imagery Google Earth satellite image, September 15, was used to classify the land cover types in the study area. Statistical analyses For each spatial scale, we first performed simple linear regression to analyze the bivariate relationship between air temperatures and the percentage of man-made surfaces.
Results Effects of man-made surfaces on urban air temperature Figure 3 shows the relationships between air temperatures and the percentage of man-made surfaces PerMS at different temporal and spatial scales. Figure 3. Relationships between air temperature and the percentage of man-made surfaces at different spatial scales. Effects of spatial scale on relationship between temperature and man-made surfaces It shows clearly that spatial scale the buffer zone size significantly influenced the relationship between air temperatures and the PerMS. Figure 4. Changes in the maximum air temperature variation MaxATV values a and explanatory power, R 2 values b of regression models for air temperatures as a function of the percentage of man-made surfaces with spatial scale during daytime and nighttime in winter and summer.
Effects of land cover type on air temperature The correlation coefficients indicate that the relationships between air temperatures and land cover types at various spatial extent scales differed among the four time periods Table 1. Table 1. Correlation coefficients between air temperature and land cover types at different temporal and spatial scales. Multiple regression analysis Stepwise multiple regression analysis was carried out in order to find out how well the observed temperature differences can be explained by the combination of different land cover types Table 2.
Table 2. Regression results with the land cover composition as predictor variables and the air temperature as response variables on winter night.
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Table 3. Regression results with the land cover composition as predictor variables and the air temperature as response variables at summer noon. Table 4. Regression results with the land cover composition as predictor variables and the air temperature as response variables on summer night. Discussion In its essence, urban climate is directly related to the conversion of land cover from rural natural surfaces to urban man-made surfaces.
Conclusions The relationships between urban air temperature and the composition of land cover conditions at a neighborhood scale in Beijing were investigated through a field measurement campaign and statistical analysis. References 1. World Urbanization Prospects, the Revision. Accessed Oct 16 Kalnay E, Cai M Impact of urbanization and land-use change on climate.
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