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Article

A Bibliometric Analysis of the Outdoor Thermal Environment Based on CiteSpace

by
Chao Xie
1,*,
Tangjun Feng
1 and
Li Hu
2
1
School of Architecture and Urban Planning, Guangdong University of Technology, Yuexiu District, Guangzhou 510062, China
2
School of Architecture, Tsinghua University, Haidian District, Beijing 10084, China
*
Author to whom correspondence should be addressed.
Buildings 2024, 14(5), 1384; https://doi.org/10.3390/buildings14051384
Submission received: 24 March 2024 / Revised: 3 May 2024 / Accepted: 7 May 2024 / Published: 12 May 2024
(This article belongs to the Section Building Energy, Physics, Environment, and Systems)
Browse Figures
Versions Notes

Abstract

:
The outdoor thermal environment (OTE) is closely related to sustainable urban development and human living, and related research has attracted widespread attention. The research hotspots and research frontiers were obtained using CiteSpace to analyze 4473 relevant studies published in English from the Web of Science (WOS) core database from 1998 to 2023. The results show that (1) Hong Kong Polytechnic University, National University of Singapore, Chinese Academy of Sciences, Tsinghua University, and Harbin Institute of Technology are important in OTE research. China has the largest number of publications in the field of OTE, but the United States has the greatest centrality and significant influence. (2) The focus of OTE keyword clustering research is divided into four main categories: thermal environment perception, the thermal environment index, thermal environment quality, and thermal environment optimization. (3) The frontiers of OTE research have changed from focusing on environmental quality, thermal perception, numerical simulation, urban space, and thermal adaptation to thermal mitigation, energy conservation, energy consumption, and optimization strategies. Visualization research in the field of OTE helps to provide references for the direction of future research on improving climate change, human thermal comfort, urban planning, and pre-planning.

1. Introduction

The outdoor environment is an indispensable and important part of human survival, and the constant climatic changes in the outdoor thermal environment (OTE) have always affected the quality of human living. With the continuous expansion of urbanization, the underlying surface of the natural green surface of the Earth is being replaced by an artificial hydrophobic skin material, and the continuous increase in building density has had a serious impact on the atmospheric circulation in cities, thus it is difficult to reduce the near-surface temperature through natural ventilation. The large number of urban population gatherings has further increased the energy consumption load of the city, and a large amount of man-made heat emissions are gathering in cities, resulting in the urban heat island effect, which has exacerbated the greenhouse effect. The IPCC’s AR6 Synthesis Report argues that the greenhouse gas emissions are the cause of global warming and undoubtedly bring a lot of harm to human survival.
As the OTE receives widespread attention, improving its quality to create a comfortable living environment has become an important topic for many researchers and scholars. Most researchers take the local climate [1] and microclimate [2] as the research scale, and choose outdoor environmental spaces, such as parks [3], water bodies [4], streets [5], squares [6], cities [7], villages [8], and courtyards [9], as the OTE research object, and consider meteorological parameters that affect OTE, such as temperature [10], humidity [11], radiation [12], wind speed [13], and precipitation [14], and empirical parameters that affect the human body’s perception of the OTE, for instance, outdoor thermal sensation [15], outdoor thermal comfort [16], thermal preference [17], thermal adaptation [18], and thermal acceptance [19], etc. Field measurements [20], questionnaire surveys [21], software simulations [22], and other methods have been used to analyze the materials affecting the OTE, like vegetation [23], water bodies [4], underlying surface [24], structures [25], human activities [26], and other factors, to evaluate outdoor thermal comfort and explore optimization strategies to enhance the OTE.
Current research on the OTE focuses on several aspects such as the thermal mitigation of the urban heat island effect, microclimate improvement measures, and a reduction in anthropogenic heat emissions like building energy consumption, but fewer reviews are describing the field of OTE research in detail. This review of current OTE research is conducive to clarifying the status of the field, discovering the gaps, and providing new directions and guidance for future exploration, such as climate change improvement, human thermal comfort, urban planning, and pre-planning. CiteSpace 6.2.R2 (64bit) was applied to perform a bibliometric visualization and analysis of relevant literature retrieved from the Web of Science database from 1998 to 2023 to present the historical lineage, research hotspots, and research frontiers of the field of OTE research.
The structure of this review is as follows: First, a literature review of OTE research is conducted. Second, the data collection and analysis methods are introduced. Third, the temporal distribution of publications and the main contributors, institutions, and countries, as well as the literature with the highest citation frequencies, are categorized and analyzed followed by an analysis of research hotspots and frontiers. Finally, a conclusion and outlook of the review is carried out.

2. Materials and Methods

2.1. Data Collection

Relevant literature in the Web of Science (WOS) core database was searched on 9 June 2023. The keyword was “outdoor thermal environment”, the time was “all”, the document type was set as “article”, and the language was “English” [27,28]. A total of 4.473 × 103 OTE research areas were obtained from the relevant literature. The earliest literature on the OTE retrieved from the WOS core database dated back to 1998, so literature published from 1998 to the present was chosen for analysis.

2.2. Research Method

CiteSpace is a Java application for analyzing and visualizing co-citation networks [29]. Its primary goal is to facilitate the analysis of emerging trends in a knowledge domain [30]. It has the typical characteristics of analyzing hotspots and frontiers in one research area. In one study, CiteSpace was used to visually analyze documents published in eight SSCI journals between 2001 and 2020 to explore the evolving trend and research directions in the field of early education [31]. Thus, the software was used to examine and analyze authors, institutions, countries, cited literature, keyword co-occurrences, keywords clustering, and burst keywords in the field of OTE to identify the research hotspots and the trends for future research. The study in this paper will help to provide reference suggestions for research scholars to clarify their research focus and develop future research in the field of OTE.

3. Results

3.1. Analysis of Published Literature

Figure 1 illustrates the distribution of 4.473 × 103 OTE research papers from 1998 to 2023, and the total number of studies represents an increasing trend year by year. Scholars have shown a great interest in OTE research. Based on the papers we found, the progress in research on the OTE was categorized into the following three phases: (1) The initial phase (1998–2006): the number of papers published in this phase was generally low, with an average of no more than 20 papers per year in the first five years, and only 174 papers were included, accounting for 3.89% of all papers. (2) The development phase (2007–2013): the papers published in this period revealed a fluctuating and increasing stage, with a total of 519 papers being published, indicating that scholars had begun to pay attention to relevant research on the OTE. However, the level of research in this stage was still shallow, and the maximum number of annual publications did not exceed 100. (3) The rapid development phase (2014–2023): the number of annual publications of papers exceeded 100 in 2014 and entered a sustained rapid development phase, peaking in 2022, indicating that the study of OTE had begun to become an important topic. As of 9 June 2023, 229 papers have been published, and the annual number of publications is predicted to have continued to increase in 2023.

3.2. Author Analysis

Using CiteSpace to analyze the author collaboration network in the literature, the node type “author” and the slicing pruning type “pathfinder” were selected and a network map (Figure 2) with a total of 431 nodes and 361 connections was gained. The network density is 3.9 × 10−3. Figure 2 explores the core authors in the field and the closeness of their collaboration, and some nodes can be clearly seen in the network map, which indicates that the core authors of OTE research have already appeared. The low network density represents the fact that there is less collaboration between authors, and the relationship between the number of node connections shown in the network mapping also suggests that the collaborative relationships between research authors need to be strengthened. Table 1 lists the top 10 authors with the highest publication frequency, among which, Andreas, Matzarakis has published 44 papers, followed by, Ariane, Middel (27), Richard, de dear (26), Dayi Lai (24), and Zhaosong Fang (24).

3.3. Institutional Analysis

The institutional collaboration network in the literature was analyzed by CiteSpace, with the node type “institution” and the slicing pruning “pathfinder”, obtaining network mapping (Figure 3) with 480 nodes, 891 connections, and a network density of 7.8 × 10−3. Figure 3 presents the collaborative relationship between institutions, forming a core research group of well-known institutions like the Hong Kong Polytechnic University, the National University of Singapore, the Chinese Academy of Sciences, etc. Table 2 lists the top 10 institutions with a high frequency of publications, from which we can see that most of the institutions are universities, and Chinese universities account for a higher number of publications, indicating that universities are the main research force in studying the OTE, and Chinese institutions have made a significant contribution to the field. Hong Kong Polytechnic University, as one of the earliest institutions to study the field, has a total of 119 publications, and other institutions with a higher number of publications are the National University of Singapore (98), the Chinese Academy of Sciences (90), Tsinghua University (90), and the Harbin Institute of Technology (90).

3.4. Country Analysis

Using CiteSpace with the node type “country” and the slice pruning effect “pathfinder” to analyze the number and centrality of literature published in different countries, a network map (Figure 4) was acquired with a total of 112 nodes and 600 connections, and a network density of 9.65 × 10−2. In Figure 4, key nodes can be seen, and China has the largest number of publications, reaching 1.533 × 103 and accounting for 24.07%, followed by the United States, the United Kingdom, Italy, Japan, and Australia. Centrality indicates the importance of a node in the network mapping, the higher the centrality, the more important the node is, which indicates that the literature published in that country has an important influence at this stage. Table 3 gives information about the centrality of the USA (0.21), which is higher than other countries and has an extremely important influence, followed by the UK (0.15), Australia (0.13), and Germany (0.13).

3.5. Co-Citation Analysis

Using CiteSpace to analyze the literature co-citations, a total of 372 nodes and 969 connections were generated in the network mapping stage (Figure 5), with a network density of 1.4 × 10−2. The top 50 most-cited pieces of literature included thirteen reviews, thirty quantitative validation studies, and seven qualitative validation studies. The reviews mainly focus on three aspects: human outdoor thermal perception, outdoor thermal comfort assessment, and urban heat island mitigation strategies. The quantitative validation studies are mainly related to the following two aspects: (1) using environmental monitoring instruments and questionnaires to obtain data, combined with analysis software like Rayman 3.1, SPSS 26.0, and selecting the thermal index for outdoor thermal comfort evaluation; (2) using simulation software such as ENVI-met 4.4.5, CFD 2023 to simulate the environmental variables affecting OTE under different working conditions to determine the range of values for the environmental variables that can create a comfortable OTE. The qualitative validation study focuses mainly on the selection of thermal comfort indices, updating the parameters of the ENVI-met 4.4.5, and the thermal adaptation of the human body to the OTE.
Table 4 lists the top ten papers with a high co-citation frequency, including three reviews and seven quantitative validation studies, all of which were cited 60 times or more, and two of which were cited more than 100 times. The reviews investigated thermal mitigation strategies for reducing the urban heat island [32,33] and applicable indicators for evaluating outdoor thermal comfort [34], while the seven quantitative validation studies were mainly based on environmental measurements, questionnaire surveys, and software simulations. In outdoor shade research, meteorological observations and field surveys of pedestrian streets were conducted to explore the effects of photovoltaic shade and tree shade on thermal comfort [35]. The ENVI-met model was used to better understand the interaction of two forms of shade, trees and buildings, on the thermal comfort of pedestrians in Hong Kong [36]. In terms of outdoor thermal comfort studies, outdoor thermal comfort in a park in Tianjin [37] and a typical public space in Changsha City [38] under different climatic conditions was investigated, and the results showed that there are seasonal and geographical differences in outdoor neutral temperatures. A subjective questionnaire survey with field measurements was carried out on a university campus in Guangzhou in southern China, and the results revealed strong linear relationships between operative temperature and mean radiant temperature (TMRT), WBGT, PET, SET*, UTCI, as well as PMV [39]. The daytime and nighttime contributions of trees and grasses to the mitigation of human heat stress at different spatial scales were quantified using the ENVI-met model simulation of four different green cover types [40]. The effects of five types of urban forms in the Netherlands on urban thermal comfort were also investigated, and the results showed that sunshine hours and mean radiant temperature, which are influenced by urban form, have the greatest impact on urban thermal comfort [41].

4. Research HotSpots and Frontiers

4.1. Research HotSpots: Keyword Co-Occurrences and Cluster Analysis

The keywords in the OTE domain were analyzed by using CiteSpace with the node type “keyword”, time slice “2”, threshold “Top45”, and slicing pruning “pathfinder”, resulting in 444 nodes and 1.258 × 103 connections being obtained, and the network density was 1.28 × 10−2. The high-frequency keywords, such as thermal comfort, environment, climate, design, adaptation, outdoor thermal comfort, temperature, vegetation, index, impact, microclimate, environments, climate change, spaces, etc., are densely distributed in the center of the map, and the clusters are closely connected internally and show complex intertwined connectivity externally, which reflects the concentration and connectivity of research hotspots in the field of OTE. Table 5 shows the frequency of the top 10 keywords and their centrality statistics according to Figure 6. Thermal comfort, temperature, performance, and outdoor thermal comfort all have a high frequency of more than 600, which makes them hot research topics in the field of the OTE. In terms of centrality, climate (0.14) and outdoor thermal comfort (0.12) are greater than 0.1, meaning that they have a significant influence on research in the field of the OTE.
Based on the analysis of keywords co-occurrences in the field of OTE, to further understand whether the research hotspots in this field have a degree of commonality, the keywords were clustered according to the “log-likelihood ratio (LLR)” algorithm using CiteSpace, and the results are shown in Figure 7. The clustering module value Q score was 8.999 × 10−1, in the interval of [0, 1] and Q > 0.3, indicating that the depicted association structure is significant. The average profile value of the clusters in this atlas is 9.657 × 10−1 (S > 0.7), indicating that the clustering is effective and persuasive. The network mapping generated a total of 59 clusters, from which we selected the top 12 clusters, including #0 thermal comfort, #1 indoor air quality, #2 environment, #3 street canyon, #4 wet bulb globe temperature, #5 warning coloration, #6 heat island, #7 outdoor thermal comfort, #8 elemental carbon, #10 experiment, #11 thermal environment, and #12 urban heat island, which cover the major research topics in the OTE field.
Integrating the clustering results in Figure 7 yields Table 6, which divides the research priorities within the OTE field into four major categories: thermal environment perception, thermal environment index, thermal environment quality, and thermal environment optimization.

4.1.1. Thermal Environment Perception Research

The first category is thermal environment perception research, which focuses on #0 thermal comfort, #5 warning color, and #7 outdoor thermal comfort. Keywords involved are thermal comfort, temperature, outdoor thermal comfort, hot, adaptation, climate change, natural ventilation, vegetation, urban microclimate, body temperature, clothing insulation, and thermal sensitivity. Outdoor thermal comfort is a complex issue influenced by several factors [42,43], of which the most important factors affecting outdoor thermal comfort are air temperature and solar radiation [18,44,45,46,47,48]. To avoid excessive solar radiation, trees and buildings should provide enough shade to improve thermal comfort in summer [49], whereas if a location has little shade in winter, thermal comfort is improved [50]. Air temperature significantly impacts people’s assessments of the weather, place perceptions, and place-related attendance [51], and people adapt to the outdoor spaces by adjusting their clothing, activity spaces, and activity times in different seasons [52]. Studies have also explored comfort in the OTE by combining empirical measurements with simulations and adjusting for other controllable factors affecting the OTE, such as urban geometry [53], the shallowness and depth of urban streets [54], and different orientations [55].

4.1.2. Thermal Environment Index Research

The second category is thermal environment index research, which consists mainly of #4 wet bulb globe temperature (WBGT) and #10 experiment. Keywords involved are health, index, physiologically equivalent temperature, mortality, simulation, and comfort. Over the past century, more than 100 indices have been developed and used to assess bioclimatic conditions for human beings [56], of which physiological equivalent temperature (PET), predicted mean vote (PMV), universal thermal climate index (UTCI), and new standard effective temperature (SET*) are the four most commonly used thermal comfort indices [57]. Since 2012, the PET has become the dominant index used for evaluating the OTE [34]. Steady-state models such as the PMV index may not be appropriate for the assessment of short-term outdoor thermal comfort, mainly because they are unable to analyze transient exposure [58]. The full-coverage neutral range of the SET* thermal range demonstrates its suitability for hot climates [59], the UTCI reflects temporal changes in thermal conditions better than other indices [56] and also provides a good prediction of thermal sensation based on meteorological conditions [60]. However, some studies do not recommend using the UTCI as an index for evaluating outdoor thermal comfort in winter in cold and bitterly cold regions [61], and the human energy balance model does not fully account for human thermal sensation and thermal preference, and psychological adaptations and behavioral factors also play an important role in outdoor thermal comfort [18,62,63]. Therefore, there is no a perfect model or software to quantify outdoor human comfort: the user needs to understand the basic equations under each model and choose the best one according to their study’s needs [64].

4.1.3. Thermal Environment Quality Research

The third category is thermal environment quality research, which mainly includes #1 indoor air quality, #3 street canyon, and #8 elemental carbon. Keywords involved are performance, indoor air quality, ventilation, street canyon, wind environment, quality, air, thermal desorption, pollutants, and air pollution. The quality of the OTE will have a significant impact on human health and the choice and use of indoor and outdoor spaces. As atmospheric CO2 levels increase, outdoor airflow rates must also increase to maintain acceptable indoor CO2 levels [65]. Due to the higher number of photochemical reactions, O3 concentrations are higher in medium canyons, and wide canyons were favorable for O3 removal [66]. Trees can “pollute” air quality, and the presence of trees increases coarse particulate matter in very hot summer conditions and in hot autumn conditions; thermal comfort can be improved by increasing the leaf area index (LAI) or tree planting density (Ptree), but a higher LAI or Ptree causes a greater accumulation of pollutants, thus the placement of trees in urban street canyons should consider the trade-off between outdoor thermal comfort and air quality improvement [67,68].

4.1.4. Thermal Environment Optimization Research

The fourth category is thermal environment optimization research, which mainly includes #2 environments, #6 heat island, #11 thermal environment, and #12 urban heat island. Keywords involved are summer, exposure, design, energy, heat island, optimization, mitigation, urban heat island, mean radiant temperature, and air temperature. An urban heat island worsens a city’s thermal environment, increases building cooling loads, and reduces the thermal comfort of open spaces [33]. Therefore, researchers usually choose different software according to different environmental scenarios, such as the ENVI-met model [69], which has a good accuracy in microclimate prediction with different variables, and the Weather Research and Forecasting (WRF) model [33], which has the ability to carry out land–atmosphere calculations to study the urban thermal environment at different scales in order to analyze and simulate the OTE to investigate optimization strategies that can reduce the urban heat island phenomenon. Among the four strategies to mitigate the urban heat island, changing the urban geometry provided the greatest improvement, with the largest reductions in PET in summer [33,70]. The most popular technique was urban greening, such as planting trees or creating parks or green roofs [71,72,73], while a combination of strategies increased the mitigation potential [32].

4.2. Research HotSpots: Keyword Co-Occurrences and Cluster Analysis

By using the “burstness” option in CiteSpace to analyze the keywords, the network map (Figure 8) was obtained. In the burst keyword mapping, the dark blue line indicates the entire research cycle, the light blue line indicates the research in the state of not being covered, and the red line indicates the duration of citation bursting. The longer the duration of the burst keyword and the higher the value of burst intensity, the more influential the topic is at a certain stage, and it can be considered as a continuous hotspot and research frontier. The top 25 keywords with the highest research frequency in the field of OTE in different periods are shown in Figure 8. In terms of burst intensity, all keywords have an intensity of 7 or more, with “strategy” having the highest burst intensity (17.75) and “exposure” having the lowest (7.6). In terms of burst duration, thermal environment is the keyword with the longest duration (16 years). The development of the OTE field can be divided into four sections, indicating that the OTE field has undergone a major shift from focusing on environmental quality, thermal sensation, numerical simulation, urban space and thermal adaptation to thermal mitigation, energy saving, energy consumption, and optimization strategies.
(1) 1998–2003: This period is in the initial stage of exploring the field of OTE, mainly focusing on the quality of the thermal environment, including the indoor thermal environment. The field of the OTE was explored from the aspects of indoor air, heat adsorption, thermal environment, particulate matter, etc., with the keywords having a high burst intensity and burst duration of 10 years or more. The IPCC has pointed out that human activities have had a significant impact on the OTE since the establishment of the IPCC in 1988. The increasing amount of greenhouse gases in the atmosphere has led to a continuous rise in global temperatures, threatening human health and living spaces. The quality of the thermal environment due to the greenhouse effect has attracted a great deal of attention from researchers and scholars during this period, and the relevant research has been aimed at improving the quality of the indoor and outdoor thermal environment to provide a comfortable living environment for human beings.
(2) 2004–2015: During this period, OTE research began to diversify and entered a rapid growth stage, and mainly conducted in terms of thermal sensation, exposure, radiation, numerical simulation, urban space, ventilation, thermal adaptation, and urban planning. Some scholars started from the perspective of human thermal sensation, combining various parameters of the outdoor environment to objectively evaluate the OTE, and the research scale mainly focused on the evaluation of human perceptions of the microclimate thermal environment, and the most used evaluation index in the research is PET. Other researchers started from the perspective of numerical simulation and explored design factors for creating a comfortable OTE by regulating the parameter variables affecting the OTE. The research scale was wide, and the ENVI-met software was mainly used to simulate the microclimate thermal environment, and the WRF software was mainly used to simulate the urban heat island effect and other multi-scale aspects.
(3) 2016–2019: In this phase, the research on the OTE mainly focused on the impact of energy conservation on the OTE, and it was carried out from the aspects of energy conservation, mitigation, residential buildings, office buildings, and air. Residential buildings and office buildings are the gathering places for the largest number of human living and production activities, respectively. Energy consumption also increases accordingly with the expansion of the scope of human activities, and the massive release of anthropogenic heat emission pollutes the atmospheric environment, seriously threatening the survival of human beings. Energy conservation and emission reduction are of great significance for improving the outdoor thermal environment.
(4) 2020–2023: This stage of research in the field of the OTE shifted to focus on optimization strategies to improve the OTE, from strategy, energy consumption, street canyon, heat, etc. The keyword burst duration at this stage is shorter, but some keywords burst intensity is greater, including strategy (17.75) and consumption (15.52). The keyword strategy emerged in 2020, and its burst duration highlights that it has been used up to the present. The field of the OTE reveals new trends for research.

5. Conclusions and Outlook

The study of the OTE is complex and diverse, involving meteorology, architecture, geography, biometeorology, and other disciplines, and not only considering the many factors affecting the OTE, but also taking into account the human physiological and psychological perceptual experience and the behavior patterns of human activities in the outdoor environment. Based on the Web of Science core database and CiteSpace visualization software, we conducted a series of analyses on OTE research, creating an overview and describing the trends in the field.
First, from 1998 to 2023, the number of published OTE studies shows an overall upward trend and starts to increase significantly in 2014, indicating that researchers have started to pay more attention to OTE research. Universities have become the main force among the many publishing organizations, with the Hong Kong Polytechnic University contributing the most, followed by the National University of Singapore, the Chinese Academy of Sciences, Tsinghua University, and the Harbin Institute of Technology. China has the largest number of publications in the field of OTE, followed by the United States and the United Kingdom, while the United States has the greatest centrality and significant influence.
Second, OTE research hotspots include thermal comfort, temperature, performance, outdoor thermal comfort, climate, impact, design, comfort, environment, and models. The research focus of OTE keyword clustering is divided into four categories: environmental perception, environmental index, environmental quality, and environmental optimization.
Third, the evolution of research topics can be divided into four parts: environmental quality, urban thermal environment, urban energy conservation, and environmental optimization strategies. The OTE research frontier has shifted from focusing on environmental quality, thermal sensation, numerical simulation, urban space, and thermal adaptation to thermal mitigation, energy conservation, energy consumption, and optimization strategies.
As for the future research trends for the OTE, exploring optimization strategies to improve the OTE will be an important starting point for research, mainly including the following: (1) The impact of urban planning and architectural design and other human-controlled factors on the OTE should be considered in advance, and the corresponding pre-planning and optimization of the design of urban areas need to be carried out, such as the effective implementation of government policies and the rational planning of urban forms and urban street tree layouts, to meet the needs of human production and life, and at the same time, to maximize the realization of the possibility of reducing warming phenomena, to effectively inhibit the intensification of the deterioration of the urban heat island effect. (2) Through the reduction in anthropogenic production emissions, such as reducing the energy consumption of building production and reducing CO2 and other greenhouse gases, as well as through the use of bodies of water, green plants, and other blue-green spaces to realize the reduction and the substitution and sequestration of carbon, the global greenhouse effect will be mitigated and near-surface temperatures will be lowered, leading to the ultimate goal of improving the OTE. (3) OTE varies with geographical changes, and optimization strategies to improve the OTE should also be adapted to local conditions; for example, how to reduce the temperature and humidity in hot and humid areas should be comprehensively considered, and cooling in conjunction with appropriate humidification should be considered for dry and hot areas to provide a relative and comfortable OTE for the occupants.

Author Contributions

Conceptualization, C.X. and T.F.; methodology, C.X. and T.F.; formal analysis, C.X. and T.F.; investigation, T.F. and L.H.; resources, C.X. and L.H.; writing—original draft preparation, C.X. and T.F.; writing—review and editing, C.X. and T.F.; visualization, T.F.; supervision, C.X.; funding acquisition, C.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by grants from the Humanity and Social Science Youth Foundation of the Ministry of Education of China (grant number 20YJCZH186), and the 2023 Youth Project of Philosophy and Social Science of Guangdong Province (grant number GD23YSH04), the Youth Fund of the National Natural Science Foundation of China (grant number 52008114) , the University-level Major Certification and Evaluation Project for Continuous Improvement in Education Reform in 2022 (Guang Dong University of Technology Educational Official Document, grant number 59).

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors also extend special thanks to the anonymous reviewers and editor for their valuable comments and recommendations for publishing this paper.

Conflicts of Interest

The authors declare no conflicts of interest.

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Buildings 14 01384 g001
Figure 1. The annual number of OTE-related studies published from 2000 to 2023.
Figure 1. The annual number of OTE-related studies published from 2000 to 2023.
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Figure 2. A network map showing authors’ collaborations in OTE research.
Figure 2. A network map showing authors’ collaborations in OTE research.
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Figure 3. A network map showing institutional collaborations in OTE research.
Figure 3. A network map showing institutional collaborations in OTE research.
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Figure 4. A network map showing national collaborations in OTE research.
Figure 4. A network map showing national collaborations in OTE research.
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Figure 5. A network map showing reference co-citations in OTE research.
Figure 5. A network map showing reference co-citations in OTE research.
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Figure 6. A network map showing keyword co-occurrences in OTE research.
Figure 6. A network map showing keyword co-occurrences in OTE research.
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Figure 7. A network map showing keyword clustering in OTE research.
Figure 7. A network map showing keyword clustering in OTE research.
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Figure 8. Top 25 keywords with the strongest citation bursts.
Figure 8. Top 25 keywords with the strongest citation bursts.
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Table 1. Top 10 productive authors in OTE research.
Table 1. Top 10 productive authors in OTE research.
NO.AuthorQuantityNO.AuthorQuantity
1Andreas Matzarakis446Brown, Robert D24
2Ariane Middel277Pisello, Anna Laura22
3Richard de dear268Niu, Jianlei22
4Dayi Lai249Lin, Tzu-Ping22
5Zhaosong Fang2410Lin, Jing21
Table 2. Top 10 institutions in OTE research.
Table 2. Top 10 institutions in OTE research.
NO.QuantityProportionInstitutionYear
11192.21Hong Kong Polytechnic University1999
2981.82National University of Singapore2002
3901.67Chinese Academy of Sciences2004
4901.67Tsinghua University2003
5901.67Harbin Institute of Technology2006
6871.61University of California System2006
7741.37Chongqing University2006
8681.26Tianjin University2014
9631.17Arizona State University2006
10621.15Arizona State University-Tempe2006
Table 3. Top 10 countries in OTE research.
Table 3. Top 10 countries in OTE research.
NO.QuantityProportionCountryCentrality
1153324.07PEOPLES R CHINA0.03
26109.45USA0.21
33515.44ENGLAND0.15
42874.44ITALY0.07
52674.14JAPAN0.1
62654.11AUSTRALIA0.13
72183.38GERMANY0.13
81862.88SOUTH KOREA0.04
91662.57CANADA0.1
101592.46SPAIN0.09
Table 4. Top 10 most highly cited references in OTE research.
Table 4. Top 10 most highly cited references in OTE research.
FrequencyAuthorTitleSource Year
119DY Lai [33]A review of mitigating strategies to improve the thermal environment and thermal comfort in urban outdoor spaces.Science of The Total Environment2019
105O Potchter [34]Outdoor human thermal perception in various climates: A comprehensive review of approaches, methods and quantification.Science of The Total Environment2018
76H Lee [40]Contribution of trees and grasslands to the mitigation of human heat stress in a residential district of Freiburg, Southwest Germany. Landscape and Urban Planning2016
74S Tsoka [32]Analyzing the ENVI-met microclimate model’s performance and assessing cool materials and urban vegetation applications—a review. Sustainable Cities and Society2018
72DY Lai [37]Studies of outdoor thermal comfort in northern China.Building and Environment2014
70M Taleghani [41]Outdoor thermal comfort within five different urban forms in the Netherlands.Building and Environment2015
70A Middel [35]Impact of shade on outdoor thermal comfort—a seasonal field study in Tempe, Arizona.International Journal of Biometeorology2016
65TE Morakinyo [36]A study on the impact of shadow-cast and tree species on in-canyon and neighborhood’s thermal comfort. Building and Environment2017
62ZS Fang [39]Investigation into the differences among several outdoor thermal comfort indices against field survey in subtropics. Sustainable Cities and Society2019
60WW Liu [38]The effects of urban microclimate on outdoor thermal sensation and neutral temperature in hot-summer and cold-winter climate. Energy and Buildings2016
Table 5. Top 10 keywords in OTE research.
Table 5. Top 10 keywords in OTE research.
NO.KeywordFrequencyCentrality
1thermal comfort11310.08
2temperature6960.01
3performance6630.06
4outdoor thermal comfort6060.12
5climate5190.14
6impact5110.04
7design4590.03
8comfort4290.02
9environment4080.02
10model3910
Table 6. Keyword cluster analysis in OTE research.
Table 6. Keyword cluster analysis in OTE research.
Research TypeNO.Cluster NameSize
thermal environment perception#0thermal comfort47
#5warning coloration24
#7outdoor thermal comfort20
thermal environment index#4wet bulb globe temperature 26
#10experiment14
thermal environment quality#1Indoor air quality34
#3Street canyon26
#8elemental carbon19
thermal environment optimization#2environments21
#6heat island 21
#11thermal environment13
#12urban heat island11
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Xie, C.; Feng, T.; Hu, L. A Bibliometric Analysis of the Outdoor Thermal Environment Based on CiteSpace. Buildings 2024, 14, 1384. https://doi.org/10.3390/buildings14051384

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Xie C, Feng T, Hu L. A Bibliometric Analysis of the Outdoor Thermal Environment Based on CiteSpace. Buildings. 2024; 14(5):1384. https://doi.org/10.3390/buildings14051384

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Xie, Chao, Tangjun Feng, and Li Hu. 2024. "A Bibliometric Analysis of the Outdoor Thermal Environment Based on CiteSpace" Buildings 14, no. 5: 1384. https://doi.org/10.3390/buildings14051384

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