Assessment of Biometeorological Conditions in the Ancient Olive Grove Campus of the University of West Attica ^†

Abstract

This study assesses the biometeorological conditions in the Ancient Olive Grove Campus of the University of West Attica, near central Athens, to evaluate human thermal comfort in an urban green space. Hourly calculations of the Universal Thermal Climate Index (UTCI) and HUMIDEX were performed using BioKlima 2.6, based on data from the campus meteorological station (DAVIS Vantage Pro 2), covering July 2022 to April 2024. Results show diverse thermal comfort-discomfort conditions, with more extremes in warmer months. The study highlights how microclimatic factors influence thermal perception, supporting efforts to design climate-adaptive, user-friendly urban environments with historical and ecological value.

1. Introduction

The phenomenon known as human thermal comfort/discomfort refers to the condition in which there is an absence of personal dissatisfaction regarding the prevailing thermal conditions in living spaces. Thermal discomfort occurs when a person is exposed to temperatures that disrupt their thermal equilibrium. Depending on the individual’s sensitivity, discomfort can be perceived even with small or large temperature variations. Thermal comfort/discomfort plays a significant role in various studies, as it is considered an indicator of human quality of living. It is relevant in research on climate change, optimization of heating and cooling systems, and energy upgrading of buildings. It also concerns architectural design, where natural lighting, ventilation, and shading are used to achieve thermal comfort while reducing energy costs and utilizing environmentally friendly materials.

In more modern applications, thermal comfort indices are also used as predictive tools during the design phase of buildings (e.g., hospitals, theaters), recreational parks, and even work environments exposed to weather conditions—especially when these conditions significantly affect the planning of large-scale infrastructure (e.g., Olympic venues, wind farms, telecommunications projects, remote area interconnections, or public transportation systems). These forecasts arise from the need to protect human resources and function as preventive safety measures for workers and, by extension, for the general population, through civil protection strategies. A notable example is the Heat Health Warning System (HHWS) established in Germany in 2005, which, in 2018, issued an alert for a three-week heatwave during which average daytime temperatures would reach 35.9 °C and maximums would hit 39 °C. Greece, as a preventive model, uses average and maximum temperatures, whereas the German system applies perceived temperature [1,2]. In recent years, thermal comfort indices, like pollution indices before them, have begun to appear in new sectors, such as finance, serving as risk indicators for investments in tourism, entertainment, and real estate, which need protection from projected thermal fluctuations.

At the top of the environmental and meteorological parameters affecting thermal comfort/discomfort are temperature, humidity, exposure to sunlight, wind speed, and the temperature of surrounding objects [3].

In this study, thermal comfort/discomfort conditions at the Ancient Olive Grove Campus of the University of West Attica are examined through the calculation of appropriate human thermal comfort/discomfort indices. For this purpose, the free software tool known as BioKlima 2.6 was used [4].

2. Materials and Methods

To study thermal comfort in the area under investigation, hourly values of two thermal comfort/discomfort indices were calculated. Specifically, the hourly values of the Universal Thermal Climate Index (UTCI) and the HUMIDEX index.

The Universal Thermal Climate Index (UTCI) is a widely used biometeorological index that evaluates outdoor thermal comfort based on air temperature, wind speed, humidity, and mean radiant temperature. It is designed to reflect the physiological response of the human body under various environmental conditions and is applicable across diverse climatic zones. The UTCI supports urban planning, occupational safety, and climate impact assessments [5,6]. Its relevance has grown in climate-sensitive sectors due to its accuracy and adaptability [7].

The HUMIDEX is a thermal comfort index developed in Canada to express the combined effect of air temperature and humidity on the human body. It estimates the perceived temperature, indicating the level of discomfort during hot and humid conditions. Humidex is commonly used in public health advisories and in occupational safety, especially during heatwaves [8,9]. Recent studies also apply it in climate risk assessments and urban heat stress analyses [10,11].

To calculate the hourly values of the two aforementioned thermal comfort indices, hourly meteorological parameters (air temperature, relative humidity, wind speed and solar irradiation) were used, which were recorded by the automatic meteorological station of the Laboratory of Air Pollution, which is located in the Ancient Olive Grove Campus of the University of West Attica. The automatic meteorological station is known as DAVIS Vantage Pro 2 (Davis Instruments Corporation: Hayward, CA, USA). The meteorological data cover the time period from July 2022 up to April 2024. As is evident, due to the availability of data only for the months of May and June, the values are derived from the corresponding figures for these months in the year 2023. For the calculation of the hourly values of the two specific human thermal comfort/discomfort indices, the free software tool BioKlima 2.6 was utilized.

BioKlima 2.6 is a free software tool that utilizes input meteorological data to calculate thermal comfort indices and their related parameters. It was developed by the team of Professor Krzysztof Blazejczyk at the Institute of Geography and Spatial Organization of the Polish Academy of Sciences [4].

3. Results and Discussion

HUMIDEX is a thermal comfort index that has specific threshold values referring exclusively to thermal comfort or discomfort due to high temperatures. In other words, it describes thermal comfort during the warm period of the year. In contrast, the UTCI describes thermal comfort or discomfort both due to low temperatures (cold) and high temperatures (heat), thus covering the entire year.

Based on this reasoning, the first result of this study was the establishment of threshold values/ranges for the HUMIDEX index so that it could also describe thermal comfort or discomfort sensations throughout the entire year. This was achieved by applying the linear interpolation method between the hourly values of the two indices. As a result, the linear equation y = ax + b was obtained, where (y) represents the HUMIDEX values and (x) the corresponding UTCI values.

Table 1. Human thermal comfort/discomfort sensation and the corresponding values of UTCI and HUMIDEX.

Human Thermal Comfort/Discomfort Sensation	UTCI (°C)	HUMIDEX (°C)
Extreme heat stress	≥46	≥47
Very strong heat stress	[38, 46)	[40, 47)
Strong heat stress	[32, 38)	[35, 40)
Moderate heat stress	[26, 32)	[30, 35)
No thermal stress	[9, 26)	[14, 30)
Slight cold stress	[0, 9)	[6, 14)
Moderate cold stress	[−13, 0)	[−5, 6)
Strong cold stress	[−27, −13)	[−18, −5)
Very strong cold stress	[−40, −27)	[−29, −18)
Extreme cold stress	<−40	<−29

presents the UTCI values and the corresponding HUMIDEX values derived through linear interpolation. The equation derived through linear interpolation and the least squares method is the following, with a coefficient of determination equal to R² = 0.851.

$$\mathrm{H}\mathrm{U}\mathrm{M}\mathrm{I}\mathrm{D}\mathrm{E}\mathrm{X}=0.8937·\mathrm{U}\mathrm{T}\mathrm{C}\mathrm{I}+6.3153$$

(1)

Based on the above classification of HUMIDEX and UTCI values (Table 1), Figure 1a,b were produced. Figure 1a illustrates the number of hours with different human thermal comfort/discomfort sensations according to UTCI values and ranges. Figure 1b illustrates the corresponding number of hours with different human thermal comfort/discomfort sensations according to HUMIDEX values and ranges. Taking into account the UTCI hourly values, it seems that 90.7% of the hours during the year appear to bring no human thermal discomfort stress, 5.6% presents high levels of heat stress, and 3.7% presents high levels of cold stress. Concerning the HUMIDEX hourly values, it appears that 89.6% of the hours during the year bring no human thermal discomfort stress, 6.7% presents high levels of heat stress, and 3.8% presents high levels of cold stress. In any case, for both human thermal comfort/discomfort indices, there are no hours with extreme heat or cold stress during the year.

Taking into account the hourly values of both indices for each month of the year, Figure 2 was developed. More concretely, Figure 2a shows the UTCI intraday variation contour chart for each month of the year and Figure 2b represents the corresponding intraday variation contour chart for the HUMIDEX.

According to Figure 2a and concerning the UTCI hourly values, it seems that between January and February, significant cold stress appears during almost all day. Also, during mid-June up to the end of July, significant heat stress appears too. Especially, it was found that between 08:00 until 20:00, the heat stress is consecutive hours, with a peak between noon and early afternoon hours. About the same conclusions arise from the HUMIDEX hourly values. According to HUMIDEX, there is no cold stress during the day for all the months of the year. Also, it was found that between 08:00 until 20:00, the heat stress presents a lot of consecutive hours with a peak between noon and early afternoon hours. According to the HUMIDEX, the heat stress hours continue during July through August, even during the late-night hours up to 04:00.

4. Conclusions

In this study, the hourly values of two thermal comfort/discomfort indices were calculated—specifically, the UTCI and the HUMIDEX index—for the period from July 2022 to April 2024, at the Ancient Olive Grove Campus of the University of West Attica, near central Athens, Greece. The results showed that throughout the year, a variety of different thermal conditions for humans were observed in the study area. The majority of the hours—approximately 90.0%—corresponded to conditions of near-neutral thermal comfort. Around 5.00% to 6.00% of the hours indicated strong thermal stress due to high temperatures, while approximately 3.00% to 4.00% indicated strong thermal stress due to low temperatures. During the warm period of the year (June–August), very strong thermal stress due to heat was observed, lasting for several consecutive hours during the day, and persisting for several consecutive days. These conditions can become hazardous, especially for vulnerable populations. Further research is required to draw more reliable conclusions, taking into account additional years and a broader dataset.