1. Introduction
Urban areas modify the materials, the structure and the energy balance of the surface of the Earth compared with the surrounding rural areas. It is most important to recognize the thermal behavior of various urban surfaces and landscape components such as pavements, vegetation, and water bodies, because this is directly integrated with the Urban Heat Island (UHI) phenomenon and environmental aspects such as heat stress and public health.
A number of studies have been carried out to investigate the thermal performance of different types of pavements. Field experiments in Singapore showed that granite slab, terracotta bricks, and interlocking blocks gave lower surface temperatures and heat output to the environment than conventional asphalt concrete pavements [
1]. Some studies demonstrated that the solar reflectance of a paved surface is a critical factor affecting its surface temperature [
2,
3]. A field measurement conducted in Tel-Aviv showed that asphalt paved roads and rooftops were the warmest urban elements during the daytime while exterior walls and trees had the highest surface temperatures at night [
4]. Observation in subtropical Taiwan revealed that there were significant differences in air temperature among several pavements at noon time during hot summer weather [
5]. In addition to studies on traditional pavements, several innovative and effective strategies for mitigating the UHI phenomenon and improving outdoor thermal environments have been proposed: the application of cool materials for roofs, facades, and pavements [
6,
7,
8,
9,
10,
11], the urban form and design [
12], other novel solutions such as suspending white sails in the square [
13]. The latest review synthesized the influence of cool pavements, such as reflective pavements and evaporative pavements, on the urban thermal environment [
14].
The thermal effects of vegetation have been widely studied. Taha
et al. [
15] measured the micro-meteorological parameters in and around an isolated orchard in a warm climate zone. They found that, compared to the air temperature in an open field, the area beneath trees showed lower temperatures during the daytime but higher temperatures at night. Souch and Souch [
16] investigated the effects of trees on air temperature and humidity in a hot-humid climate in terms of different factors such as tree species, environments, and physical characteristics of trees. The results demonstrated that temperatures were reduced and humidities were elevated under the canopies of all the trees; however, the difference between species was insignificant, and the clumps had no greater effect than the individual tree. In another project, Giridharan
et al. [
17] noted that, in high-rise high density urban environments, the influence of vegetation in lowering outdoor temperature was affected by the on-site variables such as sky view factor and altitude. Chow
et al. [
18] examined the horizontal and vertical nocturnal cooling influence of a small park with an irrigated lawn and xeric surfaces in a hot-arid city. A distinct park cool island, with mean observed magnitudes of 0.7–3.6 °C, was documented. The impacts of xeriscaping on near-surface temperatures and outdoor thermal comfort in a desert city were examined [
19]. Park
et al. [
20] performed the outdoor measurements at a scale model site consisting of an array of concrete cubes each 1.5 m high. The quantitative effects of vegetation along the sidewalk and in median strips on the thermal environment in the summer have been extensively examined. Morille
et al. evaluated the influence of various greenery types on building cooling needs and demonstrated that green walls and trees appear to be the most efficient to reduce cooling needs [
21]. Tan
et al. delineated two tree planting strategies for mitigating daytime UHI effects in a high-density urban environment [
22]. In addition to the greenery at the street level, the environmental benefits of green roofs, such as alleviating UHI effects, reducing energy use, and improving microclimate and air quality, have also been extensively investigated [
23,
24,
25,
26].
Several studies have focused on the effects of the water features in urban space on outdoor thermal environment. Nishimura
et al. [
27] reported that the artificial water facilities, including a shallow pond, a small waterfall, and a spray fountain created an air temperature decline area on the leeward side. Another field measurement performed by Xu
et al. [
28] showed that the urban lake significantly reduced air temperature and increased humidity in the littoral zone, and such effects were gradually decreased as the distance from water increases. The investigation conducted by Tominaga
et al. [
29] showed that the evaporative cooling effect induced by the water surface lowered the temperature by approximately 2 °C at the pedestrian level and propagated downwind over an unobstructed distance of 100 m. Nevertheless, based on weather observations by Dutch hobby meteorologists and a station network in Rotterdam (The Netherlands), Steeneveld
et al. concluded that water bodies increased rather than decreased the daily maximum UHI. That could be attributed greatly to the fact that the high heat capacity of water suppresses the diurnal and annual cycle over water, and water temperatures remain relatively high after evening and season transitions [
30].
Apart from the method of field measurement, the technique of numerical simulation has also been widely used in urban microclimate study and continues to grow in popularity. Several researchers employed numerical models to conduct parametric and quantitative analyses by focusing on single environmental factor such as trees [
31,
32,
33], the integrated effects of vegetation with other elements [
34,
35], and urban form and design [
36,
37,
38]. Such studies have shown that numerical simulation is an effective and powerful method to deal with the complexities and non-linearities of urban climate systems. However, the present study is focused on the field experiment, and therefore the literature review of numerical studies is not presented in detail.
Previous studies provided deep insights into the thermal effects of pavements, vegetation, and water bodies in urban areas under different climatic conditions. However, we noticed that pavements and trees have received the most attention while water ponds, short vegetation, as well as different greenery configurations have been less studied. Besides, most studies focused on air temperature and fewer studies addressed both air temperature and humidity. In addition, most field data were merely available during the daytime, and relatively fewer data were measured during both daytime and nighttime.
The objective of the study is to perform a field measurement to investigate the effects of selected urban surfaces and landscape elements, including pavements, vegetation, and a water pond, on air temperature and humidity in a hot-humid city. The temperature of ground surfaces and grass leaves and the air temperature and humidity from 0.1 to 1.5 m heights were measured for a period of 24 h under hot summer conditions. This study aims to increase our understanding on the microclimatic effects of typical urban surfaces and landscape components in the hot and humid climate zone.
5. Conclusions
Based on the results of the field measurement, some characteristics of the thermal performances of the selected urban surfaces and landscape components under hot summer conditions can be summarized as below.
The concrete and granite slab pavements exposed to direct solar radiation elevated the temperature of the atmosphere above them throughout the hot summer day. That indicates such impervious artificial surfaces can be regarded as “heat sources” to the summer urban environment and the usage of impervious pavements should be reduced. Compared to the open lawn, the grass shaded by trees produced a greater cooling effect during the daytime but showed a slight warming effect during the nighttime; and the mixture of shrub and grass showed a higher cooling efficiency during the nighttime. The results imply that multi-layer greenery composed of grass, shrub, and tree can efficiently enhance the cooling effect of greenery. That also implies that the configurations of different greenery species are quite important for the outdoor thermal environment and are worth being further studied. The water pond lowered the air temperature during the daytime but decreased the cooling rates of the atmosphere above it during the nighttime, and the humidity above pond were elevated throughout the day. That indicates that the effects of artificial water facilities need to be comprehensively evaluated, especially for hot and humid climatic regions.
The knowledge of thermal behavior of various urban surfaces and landscape components is an important tool for planners and designers. If utilized properly, it can lead to climatic rehabilitation in urban areas and an improvement of the outdoor thermal environment.