Outdoor thermal comfort benchmarks and optimization design for children in open parks of hot summer and cold winter region

Abstract

This study investigates the thermal comfort characteristics of children in open parks in China’s hot summer and cold winter regions, using Zizhou Park in Guilin as a case study. Through meteorological measurements and surveys, the study assesses children’s thermal perceptions across different seasons. The Universal Thermal Climate Index (UTCI) is used to evaluate the thermal environment, and the surface temperatures of various materials in the park are measured to analyze their impact on children’s thermal comfort and safety. The results indicate that children’s perception of the thermal environment significantly differs from that of adults, with a higher sensitivity to temperature fluctuations, particularly under extreme climate conditions. Based on the research data, this study proposes thermal comfort benchmarks for children, covering key factors such as temperature, humidity, and wind speed, and suggests three optimization strategies: (1) adjusting plant configurations and the layout of artificial facilities according to the specific climate characteristics of Guilin, particularly enhancing shading effects in the summer; (2) using low-temperature absorbing materials to reduce the negative impact of solar radiation on children’s thermal comfort; (3) improving the structural design of activity spaces to enhance ventilation, ensuring children’s thermal comfort across different seasons. The study provides landscape designers and urban planners with thermal comfort benchmarks and optimization design solutions for children’s activity spaces, aiming to create safer and more comfortable outdoor environments for children in hot summer and cold winter regions.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-95979-8.

Introduction

Open parks serve as critical spaces for children to engage with nature and participate in outdoor activities. The spatial characteristics of these environments not only influence children’s health and safety but also directly impact the frequency and quality of their exploratory behaviors¹. A thermally comfortable environment can significantly prolong children’s outdoor activity duration, thereby facilitating their physical, psychological, and social development². However, substantial differences exist between children and adults in terms of thermal comfort perception³. Due to their reduced thermoregulatory capacity and higher activity levels, children are more vulnerable to the adverse effects of elevated temperatures, leading to discomfort⁴. Additionally, children’s larger surface-area-to-body-mass ratio results in greater heat absorption, and their lack of self-protective awareness in comparison to adults further exacerbates the risk of discomfort, irritability, or fatigue in hot conditions^5,6. Given these physiological and behavioral disparities, research on thermal comfort in open parks for children is particularly pertinent. Such research can guide environmental design improvements, safeguard children’s health and comfort, and encourage greater engagement in outdoor activities.

In recent years, the field of children’s outdoor thermal comfort has garnered increasing attention, particularly in relation to the effects of climate change and urban heat island phenomena on children’s health⁷. While thermal comfort standards for adults are well-established⁸, research focused specifically on children remains limited, especially with regard to the development of thermal comfort guidelines and the application of such standards to environmental design⁹. Existing literature primarily explores the influence of environmental factors such as temperature, humidity, wind speed, and radiation on children, emphasizing the physiological differences between children and adults¹⁰. However, the design of traditional open park spaces for children has predominantly focused on activity zone configurations, plant and facility layouts, and safety considerations, with relatively little attention paid to issues of thermal comfort and safety¹¹, particularly the potential risks associated with the surface temperatures of materials under varying climatic conditions¹².

In the design of thermal environments in open parks, the adoption of active regulation strategies based on microclimatic simulations—such as adjusting spatial elements, including vegetation, materials, and artificial structures—can facilitate the creation of localized microclimates, thereby enhancing the thermal comfort of the park^12,13. This approach not only helps mitigate the thermal stress caused by high ambient temperatures but also contributes to the optimization of the overall thermal environment, providing safer and more comfortable outdoor spaces for children. Moreover, the surface temperatures of ground materials, such as pavements, grass, and water bodies, significantly influence local thermal conditions and children’s thermal perceptions^14,15. This perspective offers crucial insights for the design of child-friendly outdoor spaces.

Thermal comfort indices are employed to quantitatively assess human comfort and adaptability under varying environmental conditions¹⁶. Common indices include PMV (Predicted Mean Vote), which is primarily applicable to indoor environments¹⁷, and PET (Physiological Equivalent Temperature), which accounts for factors such as temperature, humidity, wind speed, and radiation, and is widely used in outdoor thermal comfort studies¹⁸. However, these indices typically focus on specific environments and may not fully capture the complexity of outdoor thermal conditions. In contrast, the UTCI (Universal Thermal Climate Index) stands out due to its comprehensive nature and broad applicability. By integrating multiple environmental variables, such as temperature, humidity, wind speed, and radiation, alongside human physiological responses, UTCI provides a holistic assessment of the effects of outdoor thermal environments across diverse climatic conditions¹⁹. Furthermore, UTCI enables more precise predictions of thermal adaptability for different population groups, especially when combined with material surface temperature data. This comprehensive approach allows for a more thorough evaluation of the thermal comfort and adaptability of vulnerable groups, such as children.

The climate in Hot Summer, Cold Winter (HSCW) regions is characterized by significant seasonal extremes. Taking Guilin as a representative example, summer temperatures exceed 35 °C with relative humidity above 70%, while winter temperatures typically fall below 5 °C, accompanied by frequent frost. Such extreme climatic conditions pose considerable challenges to children’s participation in outdoor activities and their thermal comfort. Moreover, local park designs often prioritize aesthetics and safety, neglecting essential microclimatic regulation. This study investigates four representative open spaces in Zizhou Park, Guilin, employing meteorological measurements and surveys to assess children’s thermal comfort. In addition, it combines UTCI indices with material surface temperatures to provide a comprehensive evaluation of the park’s thermal comfort and safety.

The primary objectives of this study are: (1) to assess children’s thermal perception and establish a thermal comfort baseline for children’s outdoor activities in open parks in Guilin; (2) to analyze the relationship between material surface temperature data and children’s thermal comfort and safety; (3) to propose optimized design strategies for enhancing microclimatic regulation in open parks based on the established outdoor thermal comfort baseline and material surface temperature data. This research highlights the often-overlooked thermal health risks associated with children in open park design, offering scientific evidence for assessing the thermal comfort of children’s activity spaces, and providing both theoretical guidance and practical recommendations for urban planners and landscape designers in optimizing children’s activity spaces and improving thermal environments.

Research methodology

Microclimate field measurements

Guilin (109°–111°E, 24°–26°23’N) experiences a subtropical monsoon climate. The annual average temperature ranges from 16.2 °C to 19.9 °C, with an extreme minimum temperature of -8.4 °C and maximum temperatures ranging from 38.3 °C to 40.4 °C. The annual average relative humidity (RH) is between 67% and 87%, slightly lower in autumn and winter. In March and April, southerly winds with humidity levels close to 100% are common, while dry weather with humidity levels at or below 10% prevails from September to December. The annual average wind speed ranges from 1.3 m/s to 2.9 m/s, with maximum wind speeds reaching 8–9 on the Beaufort scale, especially during late spring, early summer, and early autumn due to the interaction of cold and warm air masses. This study was conducted in Zizhou Park, located in the city center of Guilin. The park is notable for its strong local characteristics and is easily accessible. It features a harmonious blend of natural and cultural landscapes, combining mountain scenery with cultural and leisure elements. The park offers a variety of spatial layouts, making it a popular destination for children who engage in outdoor activities. For this research, four representative activity spaces within the park were chosen based on the varying proportions of landscape elements: the Central Lawn (CL), the Entrance Plaza (EP), the Waterfront (WF), and the Tree Plaza (TP).(Fig. 1).

Fig. 1.

Research Location and Characteristics of Measurement Point Spaces(Satellite imagery sourced from Google Earth, https://www.google.com/earth/).

Experimental design

Meteorological measures

According to the building climate zoning standards, Guilin belongs to the subtropical monsoon climate zone. The climate in September is relatively mild, making it suitable for outdoor activities for children and conducive to helping children better perceive subtle changes in comfort. January, on the other hand, corresponds to the coldest period in Guilin. The measurements were conducted from September 25 to 27, 2023, and from January 18 to 20, 2024. The meteorological parameters recorded during the experiment included air temperature (T_a), RH, wind velocity (V_a), mean radiant temperature (T_mrt), solar radiation (G), and black globe temperature (T_g). T_a and RH were monitored and recorded using a HOBO Onset U23-001 data logger (ONSET, USA) placed within a solar radiation shield. V_a was recorded using a Kestrel 5500 (Nielsen-Kellerman Co., USA). The T_g was recorded using a Delta HD 32.2 black globe sensor (Delta OHM, Italy) (model TP3276.2), and G was measured using an automatic pyranometer (Pyranometer TBQ-2). During the experimental period, no wind, rain, or snow affected the conditions, with measurements taken daily from 9:00 AM to 5:00 PM. Weather conditions remained stable throughout the study period, ensuring reliable data collection.

Questionnaire surveys

A questionnaire survey on children’s thermal perception and thermal adaptation behaviors was conducted, and children and their guardians jointly completed the questionnaire. However, to avoid potential bias, we ensured that children’s perceptions were assessed independently unless they required assistance due to age or understanding. In such cases, guardians helped articulate the children’s responses, but children’s views remained the primary focus. Additionally, research assistants were present to assist in filling out the survey. The age range of the participating children was explicitly limited to those under 14 years old. The questionnaire was divided into three sections:

The first section collected demographic information about the participants, including gender, age, height, duration of residence in the local area, and the activities they had engaged in during the past 15 min.

The second section investigated children’s subjective perceptions of the thermal environment and their preferences. The thermal sensation was assessed using a 7-point scale (ASHRAE’s 7-degree scale), which included the following options: Very hot (-3), Hot (-2), Warm(-1), Neutral (0), Cool (+ 1), Cold (+ 2), and Very cold (+ 3). Preferences for meteorological factors were recorded using a 3-point scale: Lower (-1), No change (0), and Higher (+ 1). In addition, the acceptable thermal range was clearly defined using a 2-point scale: “Acceptable” and “Unacceptable.”

The third section investigated potential thermal adaptation behaviors children might adopt in different seasons and environments. In winter, children were likely to choose behaviors such as moving to the sunshine, adding clothes, exercising, or drinking hot water to maintain body temperature. In summer, they were more inclined to engage in behaviors like moving to tree shade, using umbrellas, putting hats on, removing clothes, or drinking water to regulate their body temperature. The content of the questionnaire is shown in Appendix Fig. 1.

The entire questionnaire survey process was conducted simultaneously with the meteorological measurements, with research assistants at all times to ensure that any questions raised by the children were answered promptly and comprehensively. Written informed consent was obtained from all participants or their legal guardians prior to the study. This consent allowed for the publication of identifying information and images in an open-access publication.

Thermal index

In assessing the thermal environment of open park spaces, the UTCI is commonly used as an evaluation tool²⁰ This index is based on the “Fiala” multi-node human physiological model, which simulates the effects of temperature regulation and clothing on outdoor activities and applies to various weather conditions, seasons, and spatial scales²¹. This study used the UTCI as the thermal comfort evaluation indicator. The UTCI values were calculated based on the data collected from the questionnaires, allowing for a comprehensive assessment of the thermal comfort in open park areas.

Results and analysis

Descriptive analysis

Basic information of children

To ensure the authenticity and reliability of the data, the number of respondents for each season and each spatial survey location was statistically analyzed. A total of 2,464 valid questionnaires were collected, of which 60.6% (1,491 questionnaires) were from the summer season, and 39.4% (973 questionnaires) were from the winter. To accurately assess the thermal environment of the spaces, all respondents had lived in Guilin for more than one year, ensuring their adaptation to the local climatic conditions (Table 1).

Table 1.

Distribution of the number of children tested.

Space	Summer			Winter
	2023.9.25	2023.9.26	2023.9.27	2024.1.18	2024.1.19	2024.1.20
CL	138	165	72	87	74	98
EP	134	132	95	81	68	87
WF	112	175	90	65	75	102
TP	108	185	85	79	77	80
Total	492	657	342	312	294	367

Meteorological parameters

There were no significant differences in meteorological parameters, such as air temperature, RH, and wind speed, across the four spaces between the winter and summer seasons. During winter, the average T_a in space CL was the highest at 7.9 °C, while space TP recorded a slightly lower value of 7.4 °C. Differences in RH were minimal, with space WF exhibiting the highest RH at 66.7% and space CL the lowest at 63.7%. Due to Zhi Zhou Park’s proximity to the Li River, the average wind speed across the four spaces was generally favorable. Space TP recorded the highest wind speed at 3.3 m/s, whereas spaces CL and WF experienced the lowest wind speeds at 2.3 m/s, attributed to the shielding effects of surrounding buildings and tree cover. Space EP, being unobstructed, received the highest G (257.1 W/m²) alongside the highest black globe temperature (10.6 °C) and radiant temperature (30.2 °C). In contrast, space TP exhibited the lowest average G (127.9 W/m²) and the lowest black globe temperature (9.2 °C) due to the dense tree canopy that effectively absorbed and blocked intense G. Space CL recorded the lowest average radiant temperature (21.7 °C).

In the summer, space EP recorded the highest average T_a (35.6 °C), while space TP was comparatively cooler at 34.7 °C. Regarding RH, spaces WF and TP exhibited values of 44.8% and 42.4%, respectively, slightly exceeding those of spaces CL and EP. This difference can be attributed to the proximity of space WF to water bodies and the higher plant density in space TP, where lake water evaporation and plant transpiration jointly increased humidity levels. In terms of V_a, space CL recorded the highest value (0.7 m/s), whereas space TP experienced the lowest (0.4 m/s). G levels in the unobstructed spaces CL and EP were relatively high at 579 W/m² and 567 W/m², respectively, accompanied by elevated T_g of 45.8 °C and 45.6 °C. Conversely, space TP, characterized by dense vegetation that nearly obscured the sky, recorded the lowest average radiant temperature (49.9 °C) (Table 2).

Table 2.

Meteorological variables in each measured space.

		Summer				Winter
		CL	EP	WF	TP	CL	EP	WF	TP
Ta (℃)	Max	38.4	39.7	37.8	37.5	8.9	9.2	8.2	8.7
	Min	28.9	29.8	26.1	24.7	5.4	5.3	5.4	5.3
	Mean ± SD	35.2 ± 3.7	35.6 ± 4.1	35.2 ± 3.5	34.7 ± 3.6	7.9 ± 1.2	7.8 ± 1.3	7.5 ± 1.1	7.4 ± 1.2
RH(%)	Max	45.2	49.5	53.2	52.1	73.2	75.8	78.1	77.6
	Min	32.6	33.1	36.7	34.7	51.2	53.4	52.7	55.4
	Mean ± SD	39.9 ± 4.2	39.6 ± 5.5	44.8 ± 6.1	42.4 ± 6.6	63.7 ± 6.1	65.3 ± 6.7	66.7 ± 5.2	64.6 ± 5.8
Va (m/s)	Max	4.7	3.6	3.7	4.2	6.7	7.2	5.8	6.2
	Min	0	0	0	0	0.7	0.4	0	0.3
	Mean ± SD	0.7 ± 0.4	0.6 ± 0.3	0.5 ± 0.3	0.4 ± 0.3	2.3 ± 1.2	3.2 ± 1.2	2.3 ± 0.9	3.3 ± 1.2
Tg (℃)	Max	50.6	51.4	49.7	49.6	15.3	14.6	12.7	13.4
	Min	37.6	39.7	37.5	30.8	7.9	8.9	6.8	7.2
	Mean ± SD	45.8 ± 4.2	45.6 ± 5.2	43.9 ± 5.3	40.2 ± 3.2	9.7 ± 1.2	10.6 ± 1.5	10.1 ± 2.3	9.2 ± 1.2
G(W/m2)	Max	968	957	947	875	398	348	351	243
	Min	67.1	57.8	26.8	19.6	35.2	37.6	34.3	15.7
	Mean ± SD	579 ± 377	567 ± 369	537 ± 287	427 ± 291	217.6 ± 122	257.1 ± 90	213.5 ± 120	127.9 ± 101
Tmrt (℃)	Max	79.5	80.6	76.8	75.7	42.9	37.6	35.4	36.8
	Min	31.6	34.7	32.5	26.8	5.2	5.7	4.5	4.8
	Mean ± SD	62.7 ± 8.2	63.5 ± 7.2	62.6 ± 8.5	49.9 ± 6.2	21.7 ± 5.2	30.2 ± 5.6	26.1 ± 4.2	23.4 ± 4.7

Differences in meteorological factors

In thermal comfort studies, the Tukey test in one-way analysis of variance (ANOVA) is widely employed to identify significant differences in meteorological variables across various spaces. The analysis reveals significant seasonal variation in meteorological parameters across the four spaces studied. During the summer, all parameters show notable differences, with T_g and T_mrt displaying the most significant variation, while RH shows the least. Space TP demonstrates the most pronounced differences among the four spaces, followed by space WF. In contrast, during the winter, three key parameters T_a, RH, and V_a do not exhibit significant differences across the spaces. However, other meteorological variables still show varying degrees of difference. Notably, the differences in space TP remain the most significant, while space CL shows the least variation across the four spaces. Overall, V_a experiences the most substantial changes between seasons, followed by T_a. T_g and T_mrt exhibit relatively small fluctuations throughout the year. The variability in these meteorological parameters across the four spaces is likely influenced by a combination of factors, such as shading effects, ventilation conditions, and shortwave radiation, which differ based on spatial configurations. These factors contribute to the observed differences in meteorological parameters across the various spaces, highlighting the complex interplay between environmental conditions and spatial design (Fig. 2).

Fig. 2.

A post-hoc Tukey’s test for different pairs of measurement points for the meteorological variables (summer (a), winter (b)).

Space use and attendance

To examine the relationship between the outdoor thermal environment and children’s activities, the number of children in various spaces and their activities were recorded hourly. Based on this data, a chart was created illustrating the types of activities children performed across different spaces and periods (Fig. 3).

Fig. 3.

The spatial and temporal distribution of children among spaces.

In summer, the number of children participating in activities in spatial CL was the lowest, at 1,230, while the number of children participating in activities in spatial TP was the highest, at 1,372. This difference may be attributed to the higher plant density and increased understory space in spatial TP, which enhanced the thermal environment of the area, thereby facilitating outdoor activities and rest for children. Three primary activity types were identified by analyzing the types of activities across different spaces and periods: slow walking (0.9 m/s, approximately 3.2 km/h), fast walking (1.2 m/s, approximately 4.3 km/h), and sitting. Physical exercise and standing accounted for a smaller proportion of activities. This is likely because, during high summer temperatures, children struggle to engage in intense physical activities for extended periods and tend to rest and recover to restore thermal comfort after exercising. Between 9:00 AM and 2:00 PM, the number of children in the open park initially increased, peaking at 10:00 AM and gradually declining. This trend is likely due to the rising G, which elevates T_a and deteriorates the park’s thermal environment, reducing comfort levels. From 2:00 PM to 5:00 PM, the number of children peaked at 3:00 PM before gradually decreasing again. This trend may reflect the park’s thermal environment improvement as G intensity diminishes, attracting more children to participate in activities. However, at 9:00 AM and 5:00 PM, the number of children was relatively low, possibly because most children were still at home.

In winter, the lowest number of children participating in activities was observed in spatial EP, with only 891 participants. In contrast, spatial CL had the highest participation, with 1,002 children involved. This difference may be attributed to the limited artificial facilities and vegetation in spatial CL, along with high sky visibility, maximum G, and elevated temperatures. These factors contributed to improved thermal comfort in that area. In response to the low air temperature, children primarily engaged in vigorous physical activities to generate heat and maintain thermal comfort. These activities mainly included slow walking, fast walking, and exercise, with fewer instances of sitting or standing. Between 9:00 AM and 2:00 PM, the number of children participating in outdoor activities steadily increased, peaking at 2:00 PM, after which it gradually declined.

This study found that the thermal environment significantly impacts the frequency and types of children’s activities in different spaces and that the number of activities within the same space at different times is closely related to the thermal environment. In summer, the hotter the weather, the stronger the G; the higher the temperature, the fewer children engage in activities. In winter, as temperatures rise and G increases, the number of children participating in outdoor activities increases. At the same time, children’s activities are also influenced by daily routines such as school attendance and meal times. However, compared to adults, children’s activity patterns are less constrained by daily routines and are more influenced by thermal environmental factors, primarily because they have winter and summer vacations. Therefore, the design of thermally suitable environments for children’s activity spaces is crucial, as it significantly affects the quality and duration of their activities.

TSV

This survey employed a seven-point thermal sensation scale to measure children’s subjective perceptions of temperature accurately. The questionnaire used descriptors that were easily understood by children, including Very Cold, Cold, Slightly Cold, Neutral, Slightly Warm, Warm, and Very Warm. These descriptors correspond to the seven-point scale defined by international standards: Very hot (-3), Hot (-2), Warm(-1), Neutral (0), Cool (+ 1), Cold (+ 2), and Very cold (+ 3).

In winter, children’s perceptions of temperature varied significantly. Approximately 46% of children reported feeling “Neutral” ,21% felt “Cool” ,15% felt “Warm” ,2% felt “Very cold” and only 1% felt “Very hot”. These results indicate that children generally do not perceive cold temperatures as significant during winter activities. In contrast, during summer, 31% of children felt “Warm” and 29% felt “Neutral” .Significantly, few children reported feeling “Cold” (2%) or “Very cold” (1%), reflecting a higher sensitivity to heat among children. Overall, children’s thermal sensations primarily clustered around “Neutral” (39%), “Warm” (22%), “Hot” (10%), “Very hot” (8%), and “Cool” (16%). The percentages of children feeling “Cool” (4%) and “Very cold” (2%) were relatively low, suggesting that children exhibit a lower sensitivity to cold and a more substantial tolerance for cold temperatures. While winter clothing provides additional warmth, the limited clothing options in summer restrict children’s ability to mitigate heat stress. Therefore, proper site design in summer is essential to improve the thermal environment of activity spaces and enhance children’s thermal comfort (Fig. 4).

Fig. 4.

TSV distributions.

Preference vote

In winter, most children (48%) preferred an increase in temperature, while 40% preferred the temperature to remain the same, and only 12% desired a decrease. Regarding wind speed, 60% of children preferred a reduction, 34% preferred no change, and 6% desired an increase. For solar radiation, 59% of children preferred an increase, 33% were content with no change, and 8% favored a decrease. These findings indicate that, due to the lower temperatures and weaker G typical of winter, most children prefer environments with higher temperatures, increased G, and reduced wind speeds to enhance their thermal comfort.

In summer, most children (72%) preferred a decrease in temperature, while only 6% desired an increase. Regarding wind speed, 68% of children preferred an increase, 25% were satisfied with the current wind speed, and 7% desired a decrease. Additionally, 69% of children favored reduced G, 22% were content with no change, and 9% preferred an increase. These findings suggest that over 70% of children are dissatisfied with the thermal environment in open parks during summer, indicating a heightened sensitivity to heat (Fig. 5).

Fig. 5.

Meteorological parameter preference votes during winter (a) and summer (b).

Thermal benchmark for the activity spaces in open parks

Neutral UTCI (NUTCI) and neutral UTCI range (NUTCIR)

To investigate children’s thermal comfort perceptions and their thermal comfort range, weighted average thermal sensation vote (MTSV) charts for each 1 °C UTCI interval in winter and summer were plotted²².

In winter, the slope of the linear regression equation between the MTSV of females and the UTCI is 0.159, indicating that a 6.3 °C change in UTCI corresponds to a 1-unit change in MTSV. In contrast, the slope for males is 0.105, equating to a 9.5 °C change in UTCI per 1-unit change in MTSV. The average slope for all respondents is 0.13, meaning that a 7.7 °C change in UTCI results in a 1-unit change in MTSV. These results suggest that females are more sensitive to temperature changes than males in cold weather.

In summer, the situation is reversed. The slope for females is 0.08, requiring a 12.5 °C change in UTCI to produce a 1-unit change in MTSV. For males, the slope is 0.115, indicating an 8.7 °C change in UTCI per 1-unit change in MTSV. The average slope for all respondents is 0.098, signifying that a 10.2 °C change in UTCI results in a 1-unit change in MTSV. This demonstrates that males are more heat-sensitive than females in hot environments. Overall, the relationship between MTSV and UTCI across all seasons shows a linear regression slope of 0.071 for females, implying a 14.1 °C change in UTCI for a 1-unit change in MTSV. For males, the slope is 0.067, corresponding to a 14.9 °C change in UTCI for a 1-unit change in MTSV. The average slope for all respondents is 0.069, meaning a 14.5 °C change in UTCI results in a 1-unit change in MTSV.

The neutral temperature (NUTCI), defined as the UTCI at which MTSV equals 0²³, was calculated as follows: for females, 12.3 °C in winter, 19.5 °C in summer, and 15.6 °C overall; for males, 12.3 °C in winter, 27.3 °C in summer, and 15.0 °C overall; and for all respondents, 12.5 °C in winter, 23.3 °C in summer, and 15.2 °C overall. The small differences in NUTCI between genders suggest that gender does not significantly affect neutral temperature perception.

The neutral UTCI range (NUTCIR), determined using thermal sensation votes (TSV) in the range of -0.5 to 0.5²⁴, is 8.5–22.9 °C for female children, 7.5–22.4 °C for male children, and 8.0–22.4 °C for all children. The upper limit of NUTCIR for females is 0.5 °C higher than that for males, while the lower limit is 1 °C higher, indicating that females are more sensitive to cold temperatures, whereas males exhibit greater sensitivity to heat in hot environments (Fig. 6).

Fig. 6.

Correlation between UTCI and MTSV: male (a), female (b) and all respondents (c).

TAR

The thermal acceptable range is employed in thermal comfort studies to evaluate individuals’ comfort within specific thermal conditions^25,26. The international standard ASHRAE 55 defines this range as the acceptable temperature interval to at least 80% of respondents under standard environmental conditions. Under more stringent conditions, the acceptance rate is increased to 90%, implying that at most 20% or 10% of respondents may find the thermal environment unacceptable. To determine this thermal acceptable range, the rate of thermal unacceptability was calculated for each 1 °C UTCI interval, and these data were fitted using a quadratic polynomial equation. The statistical results reveal that the thermal acceptable UTCI range for male children is 4.0 °C to 24.9 °C, while for female children, it is 5.9 °C to 26.5 °C. When all respondents are considered, the acceptable UTCI range extends from 5.4 °C to 25.8 °C. A comparison of male and female thermal acceptable ranges indicates some differences: the lower limit for females is 1.9 °C higher than that for males, and the upper limit is 1.6 °C higher. These findings suggest that while gender-based differences exist in thermal acceptable ranges, the magnitude of the difference is relatively small (Fig. 7).

Fig. 7.

Relationship between the thermal unacceptable rate and UTCI: male(a), female (b)and all respondents (c).

Thermal adaptive behaviors

In winter, children employed two primary strategies to alleviate thermal discomfort: 30% moving to the sunshine, while 29% adding clothes. Additionally, 24% of children exercising to improve their thermal comfort, and 17% drank hot water to reduce discomfort. These findings highlight the significant role of sunlight in mitigating thermal discomfort among children during winter. In summer, the strategies for alleviating heat discomfort shifted. Drinking water and moving to tree shade are the most popular choices for children, accounting for 33% and 27% respectively. Other strategies included putting hats on (16%), using umbrellas (13%), and removing clothes (11%). These results suggest that children primarily seek to avoid direct sunlight in hot environments to mitigate thermal discomfort. Accordingly, enhancing thermal environments in children’s activity spaces during summer should focus on incorporating shading structures and ensuring convenient access to drinking water to meet their needs (Fig. 8).

Fig. 8.

Preferred thermal adaptive behaviors: winter (a) and summer (b).

Evaluation of the surface temperature of Spatial materials

During the experiment, surface temperatures of materials in four spaces that children might contact for more than 5 s were monitored and recorded at half-hour intervals. Subsequently, the safety of these materials under thermal conditions was assessed based on the international standard ISO 13,732, considering burn threshold temperatures for contact durations of 5 s, 1 min, and 10 min. The results revealed that under direct sunlight, for a contact duration of 5 s, the maximum surface temperatures of dark-colored asphalt and dark-coated wooden flooring exceeded the burn threshold by 2.3 °C and 5.3 °C, respectively. For a contact duration of 1 min, dark-colored asphalt exceeded the threshold by 6.3 °C, while light-colored permeable bricks and dark-coated wooden flooring surpassed the threshold by 3.2 °C and 9.3 °C, respectively. For a contact duration of 10 min, all tested materials exceeded the burn threshold. Given that children’s typical contact with ground surfaces in open parks ranges from 1 to 10 min, only light-colored ceramic tiles were found to pose no thermal risk. After 10 min of exposure to sunlight, dark-coated wooden benches exceeded the burn threshold by 8.7 °C, posing a potential risk of thermal injury for children resting on them for extended periods. In shaded areas, however, G and temperature were significantly reduced, and the maximum surface temperatures of all tested materials remained below the burn threshold, posing no thermal safety risks. Regardless of exposure conditions, the surface temperatures of trees, ground cover, and water bodies remained within a safe range. Nevertheless, apart from light-colored ceramic tiles, other materials exceeded their burn threshold values under direct sunlight. These findings underscore the significant influence of material type, color, and shading conditions on surface temperature. While light-colored materials effectively reduce surface temperatures, they may also cause glare, potentially impacting user thermal comfort. Therefore, selecting ground materials should balance surface temperature reduction and user comfort by comprehensively considering material type and color (Table 3).

Table 3.

A summary of the average surface temperatures of various materials in each space in summer.

Materials		In the sun			In the shade
		Max	Mean ±SD	Min	Max	Mean ±SD	Min
Ground	Asphalt(ground)	62.3	55.6 ± 2.4	39.2	43.4	39.9 ± 1.2	34.2
	Ceramics(ground)	52.3	46.7 ± 1.3	37.4	42.9	38.9 ± 1.0	32.1
	Water permeable bricks(ground)	59.2	54.7 ± 2.3	38.8	41.2	35.9 ± 1.3	32.3
	Wood(ground)	65.3	59.1 ± 1.6	34.8	43.5	37.6 ± 0.9	29.6
Artificial facilities	Wood(chair)	56.7	51.6 ± 1.6	38.9	41.6	36.7 ± 0.8	33.6
	Wood(scenic pergola)	/	/	/	41.2	39.9 ± 1.0	32.5
	Granite(strip stone)	46.5	43.3 ± 0.8	40	42.7	36.2 ± 0.5	31.4
	Lime(feature wall)	39.7	37.2 ± 0.5	35.4	40.2	37.2 ± 0.5	33.4
	Light painted metal(Fitness equipment)	45.8	40.3 ± 0.7	33.2	39.7	36.4 ± 0.3	32.1
	Coated metal(sculpture)	49.5	43.8 ± 0.4	33.3	40.6	37.8 ± 0.3	32.6
Natural landscapes	Trees	39.4	34.8 ± 1.1	30.1	/	/	/
	Shrubs and grass	48.9	41.8 ± 1.3	35.6	38.5	35.4 ± 0.3	29.2
	Water	36.2	33.8 ± 0.7	30.2	/	/	/

Bioclimatic design strategies

Thermal benchmarks and thermal stress indices provide reliable frameworks for objectively assessing the thermal conditions of specific spaces. They also offer significant theoretical insights for developing strategies to optimize thermal environments^27,28. Accordingly, the TSV (Thermal Sensation Vote) values corresponding to various levels of thermal stress were substituted into the linear regression equation between UTCI (Universal Thermal Climate Index) and MTSV (Mean Thermal Sensation Vote). This approach enabled the development of a revised UTCI thermal stress evaluation scale tailored to children’s unique thermal comfort requirements. Based on the thermal sensation ranges associated with different UTCI thermal stress categories, a revised UTCI thermal stress classification was explicitly formulated for children²⁹ (Table 4).

Table 4.

Thermal stress assessment scale for children based on UTCI (a sensation obtained through linear regression).

Thermal stress	TSV	UTCI (℃)	UTCI, children(℃)
Extreme cold stress	<-4.5	<-40	<-50.0a
Very strong cold stress	-4.5—-3.5	-40—-27	-50.0—-35.5 a
Strong cold stress	-3.5—-2.5	-27—-13	-35.5—-21.0 a
Moderate cold stress	-2.5—-1.5	-13—0	-21.0—-6.5 a
Slight cold stress	-1.5—-0.5	0—9	-6.5—7.9 a
No thermal stress	-0.5—0.5	9—26	7.9—22.4
Moderate heat stress	0.5—2.5	26—32	22.4—51.4a
Strong heat stress	2.5—3.5	32—38	51.4—65.9 a
Very strong heat stress	3.5—4.5	38—46	65.9—80.4 a
Extreme heat stress	> 4.5	> 46	> 80.4 a

This study found that the UTCI in real-world thermal environments ranged from 4 °C to 47 °C. The linear relationship between UTCI and Mean Thermal Sensation Value (MTSV) is only applicable within this temperature range. Data outside this range should be interpreted with caution, as they are inferred from the linear regression model. According to the collected data, children experience “no thermal stress” when the UTCI temperature is between 7.9 °C and 22.4 °C. Below 7.9 °C, children feel cold stress, while above 22.4 °C, they experience heat stress. For adults, the “no thermal stress” range is between 9 °C and 26 °C, with cold stress occurring below 9 °C and heat stress above 26 °C. The results indicate that children have a lower tolerance for thermal conditions compared to adults (Fig. 9).

Fig. 9.

UTCI corresponding to different stress categories (a Sensation obtained through linear regression).

Based on thermal benchmarks, thermal stress assessments for children, and thermal safety considerations across four open park spaces, strategies for improving spatial climate suitability were proposed from three perspectives: plant configuration, material selection, and artificial facility layout. The climate-adaptive design was integrated with spatial functionality to enhance the overall quality of parks. This optimization was achieved through carefully designed plant landscapes, selecting suitable ground paving materials, and adding appropriate small-scale facilities, all tailored to meet the recreational needs of children (Fig. 10).

Fig. 10.

Diagrammatic sketches of bioclimatic design of open parks appropriate for children. (Created using SketchUp 2023, https://www.sketchup.com/)

Vegetation

In the Central Lawn (CL) area, to enhance ventilation and cooling, the density of shrubs in the direction of the prevailing wind should be reduced by approximately 30–40%. This density reduction will create ventilation corridors that improve airflow and support cooling. These corridors should ideally be 3–5 m wide, ensuring effective air circulation while still maintaining a natural, aesthetically pleasing landscape. In the areas where shrubs are reduced, there will be relatively open lawn spaces, which can provide optimal conditions for children to enjoy a high-quality green environment. It is recommended to use herbaceous plants that are resistant to trampling, such as certain varieties of grass, to ensure durability and sustainability.

In the Entrance Plaza (EP), where direct sunlight exposure is significant, additional planting pools and trees should be added. The density of trees should be increased by planting species with a high leaf area index (LAI) to provide adequate shading. Local tree species with broad crowns and dense foliage are ideal choices, and these trees should be spaced at intervals of 5–6 m to maximize shading coverage while ensuring sufficient airflow. This will create shaded areas that help reduce short-wave radiation, particularly during the summer months, while also offering aesthetic appeal and comfort throughout the year.

The Waterfront (WF) space, characterized by a large and dense forest area, can be optimized by reducing the shrub density by approximately 40–50% to prevent excessive plant density, which can hinder air circulation. The introduction of vertical greening elements, such as climbing vines along the pathways, will improve shading and help shield against short-wave radiation, increasing the overall green coverage and creating a more comfortable microclimate. The vines should be planted with intervals of 1–2 m to ensure they grow densely enough to provide effective shading.

In the Tree Plaza (TP), a planting pool with trees will be added in the center of the open space. It is recommended to plant trees with wide crowns and dense canopies, spaced 4–5 m apart. These trees, in combination with a green wall, will enhance the vertical greening of the area, creating a cooling and shaded environment. The green wall will provide vertical shading, and the plants’ transpiration will help lower air temperatures, improving thermal comfort both in summer and winter. Additionally, the overall greening strategy should be optimized for year-round effectiveness, balancing shading and ventilation to provide comfort during both hot and cold seasons.

Materials

Based on the measured surface temperature of materials during summer, all materials exceeded their thermal burn thresholds after 10 min of exposure to direct sunlight. To mitigate this issue, infrared-resistant and high-temperature reflective coatings can be applied to the surfaces of ground paving across various spaces. Alternatively, the use of light-colored asphalt materials can effectively lower ground surface temperatures. Additionally, in EP (Entrance plaza) and TP (Tree plaza), incorporating guided paving patterns can direct children toward shaded areas, thereby reducing prolonged exposure to sunlight and mitigating heat-related risks.

Built facilities

According to the surface temperature measurements of materials in summer, the surface temperatures of all materials under shaded conditions remained below their thermal burn thresholds even after 10 min of exposure. However, under direct sunlight, common interactive facilities such as benches, pergolas, and slides exceeded their thermal burn thresholds after 10 min of contact. They should be placed in shaded areas to improve heat safety for children engaging with these facilities in open parks. In WF (Waterfront) and TP (Tree plaza), interactive facilities can be integrated with fountain systems to reduce the surrounding temperature. Furthermore, misting devices can be installed near pergolas in space WF to increase humidity and lower ambient temperatures.

Discussion

Thermal benchmarks

NUTCI

In this study, the neutral Universal Thermal Climate Index (NUTCI) of children in Guilin was found to be similar to that of children in Chongqing³⁰. This similarity may be attributed to both cities being situated in the same climate zone, resulting in comparable thermal perceptions and comfort levels among children. In contrast, a study on thermal comfort in Hong Kong³¹ reported a NUTCI that was 8.3 °C higher than that of children in Guilin. This difference could be explained by the study in Hong Kong being conducted during the hot summer in a region with a warm, year-round climate. Residents in Hong Kong have likely adapted to high temperatures, resulting in a greater heat tolerance. The NUTCI of residents in Wuhan⁶was 4 °C higher than that of children in Guilin. This difference might stem from the Wuhan study including a mixed-age population, while our study focused on children, who generally exhibit a higher tolerance for cold environments. Conversely, the NUTCI of children in Harbin³²was 8.5 °C lower than that of children in Guilin (6.7 °C). This discrepancy may be due to the Harbin study being conducted during the cold winter season, as Harbin experiences extreme winter temperatures. Local children in Harbin may have developed a higher tolerance for cold environments (Table 5).

Table 5.

NUTCI among OTC studies.

References	Location	Climate zone	Seasons	Population	NUTCI(℃)
This study	Guilin, China	Cwa	Summer&winter	Children	15.2
Gu et al.2022	Chongqing, China	Cwa	Summer	Children	19.6
Shao et al.2022	Harbin, China	Dwa	Winter	Children	6.7
Huang et al.,2016	Wuhan, China	Cfa	All	Mixed populations	19.2
Lam et al.,2018	Hong Kong, China	Cwa	Summer	Mixed populations	23.5

Material

Recent advancements in materials science have led to the development of innovative solutions that can simultaneously reduce heat absorption and enhance visual comfort in outdoor environments³³. One effective approach is the use of high-performance reflective coatings, particularly near-infrared (NIR) reflective materials, which reflect G, especially in the NIR spectrum—a primary contributor to heat accumulation^34,35. NIR-reflective materials significantly reduce surface temperatures while minimizing glare, with reflectivity levels ranging from 0.40 to 0.70, making them ideal for park pavements, playgrounds, and other outdoor surfaces³⁶. Another promising solution is thermochromic coatings, which change color in response to temperature changes, transitioning from dark to light as the temperature rises, adjusting reflectivity to reduce heat absorption and prevent glare. These coatings are particularly useful on building facades, pavements, and playground equipment, offering dynamic control over thermal comfort³⁷. Additionally, micro-surface treatments, such as textured or microporous materials, scatter light, reducing direct glare and enhancing overall visual comfort. When combined with vegetation—such as trees, shrubs, and green lawns—these materials can further cool the environment through shading and evapotranspiration, reducing the urban heat island effect. This integrated approach of using advanced materials alongside natural elements provides a balanced solution to managing both heat and glare, ensuring that public park environments remain comfortable, safe, and aesthetically pleasing for visitors¹⁴.

Limitations and future research

This study has several limitations. First, the research was conducted exclusively during the summer and winter in Guilin, a region characterized by hot summers and cold winters, while excluding the spring and autumn transitional seasons. These seasons may introduce diverse climatic conditions that warrant further investigation. Second, the sample size of young children involved in this study was relatively small. Moreover, the specific influences of factors such as gender, family background, and regional culture on children’s thermal comfort remain unexplored, highlighting areas for future research. Third, the surface temperatures of materials vary under different spatial and climatic conditions. Consequently, additional data collection is necessary to evaluate materials’ suitability, safety, and practicality across various environments. Furthermore, some reflective materials may increase ground radiation, suggesting that future studies should examine the interplay between material reflectivity and children’s thermal comfort. Finally, while this study proposed strategies for optimizing children’s activity spaces in open parks, focusing on plant configuration, material selection, and the layout of artificial facilities, their applicability in real-world settings has yet to be thoroughly validated. Future research should employ quantitative numerical simulation methods to assess the practical effectiveness of these proposed design strategies in enhancing children’s thermal comfort in open park spaces.

Conclusions

This study combines meteorological measurements and surveys to examine the thermal sensations of children in Zizhou Park, Guilin. By analyzing the surface temperatures of materials in children’s activity areas within the open park and integrating spatial meteorological parameters with thermal benchmarks, we propose optimization strategies for designing open parks. The key conclusions are as follows:

1. Children’s perception of thermal comfort in a space directly influences their frequency of park use. A comfortable thermal environment significantly enhances children’s willingness to participate in activities, thereby increasing attendance and activity intensity.

2. Children’s thermal sensations are predominantly categorized as “Neutral” (39%), “Warm” (22%), “Hot” (10%), “Very hot” (8%), and “Cool” (16%). The proportions of “Cool” (4%) and “Very cold” (2%) are comparatively low, suggesting that children are less sensitive to low temperatures and exhibit greater tolerance for cold environments.

3. The thermal comfort benchmark (Neutral UTCI, or NUTCI) for children is 15.6 °C for females, 15.0 °C for males, and 15.2 °C overall. The comfort range (Neutral UTCI Range, or NUTCIR) is 8.5–22.9 °C for females, 7.5–22.4 °C for males, and 8.0–22.4 °C overall. The suitable temperature range for activity (TAR) spans 5.9–26.5 °C for females, 4.0–24.9 °C for males, and 5.4–25.8 °C overall.

4. To ensure a safe and comfortable environment for children in open parks, optimization strategies are proposed focusing on three aspects: plant configuration, material selection, and artificial facility layout. These strategies include reducing the sky view factor and minimizing G to improve thermal comfort, planting broad-canopy, densely foliage trees, and increasing vertical greening. Additionally, applying high near-infrared reflective coatings to ground and surface materials can effectively lower surface temperatures and reduce thermal hazards. For open and unobstructed areas, artificial shading facilities are recommended. Furthermore, interactive devices and clear directional signage should be installed to guide children to shaded areas.

Electronic supplementary material

Below is the link to the electronic supplementary material.

Author contributions

Hu Luyao: Writing - review & editing, Validation，Resources, Project administration，Methodology，Investigation，Funding acquisition，Formal analysis. Lu Ling: Writing-review & editing, Visualization，Methodology, Data curation, Conceptualization. Dai Junfeng: Writing - review & editing, Validation, Supervision. Li Xinkai: Validation, Supervision, Resources.

Funding

This study was funded by the National Social Science Foundation of China project “Experience Research on the Harmonious coexistence between terraced people and Nature in Southwest Ethnic Areas” (23XMZ024).

Data availability

The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Ethical approval

All methods were carried out in accordance with relevant guidelines and regulations. The experimental protocols were approved by the Guilin Tourism University Ethics Committee, and informed consent was obtained from all subjects and their legal guardians.