Mean Radiant Temperature (MRT) emerges as a central determinant in shaping human comfort in outdoor settings. It refers to the uniform temperature of an imaginary enclosure wherein the radiant energy exchange with the human body mirrors that of the actual, non-uniform outdoor environment. This parameter significantly influences thermal perception, thus holding great importance in outdoor environments where human comfort is paramount.

Outdoor Mean Radiant Temperature stands as a pivotal parameter when evaluating the thermal conditions of outdoor spaces. It serves as an indicator of the average temperature of all surrounding urban surfaces, combined with the radiant temperature and sky temperature, as perceived by individuals exposed to the outdoor environment.

Unlike indoor environments, outdoor MRT is dynamically influenced by several key factors, with solar radiation and sky conditions playing a particularly crucial role. These factors, combined with atmospheric conditions, ground surface characteristics, and the presence of nearby objects, collectively shape the MRT outdoors.

Understanding these multifaceted influences on MRT is imperative for assessing thermal comfort levels, identifying potential heat stress risks, and maintaining an equilibrium in the energy exchange within outdoor settings.

MRT assumes a critical role in understanding and managing urban microclimates. By capturing the intricate radiation flux between individuals and their surroundings, MRT provides a reliable measure of outdoor thermal comfort and potential heat stress, particularly under sunlit conditions. This makes it a crucial tool for urban planners, architects, and environmental engineers striving to design sustainable and comfortable outdoor spaces in densely populated urban areas.

Moreover, MRT serves as a fundamental parameter in the assessment and mitigation of urban heat islands, aiding in the development of strategies to mitigate heat-related health risks and enhance the livability of urban environments. Its incorporation into urban planning frameworks enables the creation of resilient and adaptive cities capable of withstanding the challenges posed by climate change and urbanization.

By comprehensively understanding and leveraging MRT, stakeholders can enhance the quality of outdoor environments, promote human well-being, and foster sustainable urban development.

 

MRT AND THERMAL STRESS

 

Outdoor Mean Radiant Temperature is a pivotal factor in the assessment of human thermal comfort and thermal stress. Elevated MRT values, particularly when coupled with high air temperature and humidity, can induce heat stress and discomfort. Conversely, diminished MRT values can evoke a sensation of coolness, notably in shaded locales or during the nocturnal hours.

The monitoring of MRT is not only critical for the prevention of heat-related illnesses but also instrumental in the design of outdoor spaces that foster comfort and well-being. The correlation between MRT and thermal comfort is further emphasized in thermal comfort indicators such as the Universal Thermal Climate Index (UTCI) and other topical indicators of outdoor thermal stress, which extensively utilize MRT in their computation. This underscores the significance of MRT in understanding and mitigating outdoor thermal stress, thereby enhancing human comfort and health in various climatic conditions.

Moreover, studies have highlighted a correlation between MRT and heat-related health risks. It is reported that heat-related mortality risk in adults aged 45-79 may increase by 5% for MRT values exceeding 58.8°C. For adults over 80, the risk may rise by 5% for MRT values over 55.5°C and by 10% for values over 59.4°C. These findings further emphasize the importance of monitoring MRT in various environments to ensure human health and safety.

In addition, MRT has a profound influence on thermos-physiological comfort indexes such as the Physiological Equivalent Temperature (PET) and the Predicted Mean Vote (PMV). The PET is an index that represents the equivalent ambient temperature at which, in a typical indoor setting, the heat budget of the human body is balanced with the same core and skin temperatures as those under the actual conditions.

On the other hand, the PMV is an index that predicts the mean value of the votes of a large group of people on a seven-point thermal sensation scale (from cold to hot) based on the heat balance of the human body. It is used in the field of HVAC (Heating, Ventilation, and Air Conditioning) and it’s a standard index in many building codes.

Both PET and PMV are derived from the human energy balance and are used to assess indoor and outdoor thermal comfort. They both take into account the MRT, along with other environmental factors, including air temperature, humidity, wind speed, and personal characteristics such as metabolic rate and clothing insulation.

In the case of PET, it represents the thermal environment through a single value (the equivalent temperature), making it easier to understand for non-experts. PMV, on the other hand, provides a quantitative prediction of the average thermal sensation of a large group of people, which can be useful in the design and assessment of buildings.

 

MEASURE OF OUTDOOR MRT

 

The measurement of Outdoor Mean Radiant Temperature (MRT) is a complex process due to the various factors that influence it. There are several methods available that can accurately measure outdoor MRT, each with its own advantages and limitations. Common techniques and tools include using globe thermometers, radiometers, infrared thermometers, and thermal imaging cameras.

Globe Thermometers. These are common tools used in the measurement of MRT. They work by measuring the radiant heat exchange between a blackened (copper) globe and its surroundings. The black color of the globe absorbs all incident radiation, ensuring a comprehensive measurement of the radiant heat. However, the readings from globe thermometers can be influenced by wind speed and direction, which may require additional measurements or corrections. A globe thermometer does not directly measure the mean radiant temperature (MRT). Instead, it measures the globe temperature, which reflects the balance of heat from various sources, including the air around it and the radiation it receives from nearby surfaces. Knowing the globe temperature allows the mean radiant temperature (MRT) to be determined according to different approaches. Besides globe thermometers, acrylic globe thermometers (AGT) and cylindrical radiation thermometers (CRT) can be employed for estimating MRT.

Integral Radiation Measurements: One of the most accurate methods for determining MRT in real-world settings involves using three net radiometers to measure integral radiation. This method provides a detailed assessment of radiant energy exchange, resulting in a more precise MRT calculation. Net radiometers gauge the balance of radiation from all directions, offering a comprehensive view of the radiant surroundings. They account for incoming and reflected short-wave radiation, as well as the long-wave radiation emitted by the sky and the ground. This thorough measurement enhances our understanding of environmental energy dynamics. However, the high cost of these sensors may restrict their widespread use.

A simpler and more accessible method entails using only two devices: a pyranometer and a pyrometer. These devices offer flexibility, as they can be adjusted and directed in six different directions.

Infrared Thermometers: Infrared thermometers are another tool used to measure MRT. They work by detecting the thermal radiation emitted by surfaces. By averaging measurements from multiple surfaces, an estimate of MRT can be obtained. However, as infrared thermometers only measure surface temperatures, they may not accurately represent the radiant temperature of the entire environment outdoors. So, while infrared thermometers are a tool used in the measurement of MRT, they should be used in conjunction with other methods or tools for a more comprehensive and accurate measurement of MRT.

Thermal Imaging Cameras: These cameras do indeed offer a more comprehensive view of surface temperatures, with respect to infrared thermometers, and can provide spatial variations in MRT across outdoor environments. They work by detecting the infrared radiation emitted by objects, allowing for the visualization of temperature distributions. This can be particularly useful in complex environments with significant temperature variations. However, like infrared thermometers, thermal imaging cameras only measure surface temperatures. Hence, they should be used in conjunction with other methods or tools for a more comprehensive and accurate measurement of MRT.

 

APPROACHES AND TOOLS TO SIMULATE OUTDOOR MRT

 

Mean Radiant Temperature is a crucial factor in determining thermal comfort in outdoor environments. It represents the average temperature of the surroundings, factoring in the effect of both direct and indirect radiation on an individual. Accurate assessment of MRT is essential for urban planning, environmental studies, and climate research.

There are several methodologies and tools available for assessing outdoor MRT. Each of these methodologies and tools has its unique approach, catering to different needs and applications. They range from complex mathematical models to advanced computer simulations, and from large-scale remote sensing techniques to empirical models based on meteorological variables.

These methodologies and tools are designed to provide accurate, detailed, and comprehensive estimations of MRT, thereby aiding in the creation of comfortable, sustainable, and climate-resilient urban spaces.

Numerical simulation methods of outdoor urban microclimate, based on simplified models of the physical environment, have been particularly promoted to overcome restrictions in estimating MRT.

Radiative Transfer Models: These models, such as the Radiative Transfer Equation (RTE), simulate the exchange of radiation between surfaces. The RTE can be used to compute MRT using parameters like surface temperatures, solar radiation, and atmospheric conditions.

Remote Sensing Techniques: These techniques, based on satellite and aerial imagery, can be used to estimate MRT over large spatial scales. Google Street View has also been used for MRT estimation, leveraging image segmentation and deep learning to calculate view factors and convert panoramas into fisheye images.

Empirical Models: Empirical Models provide simplified MRT estimates based on meteorological variables such as air temperature, humidity, and wind speed.

Numerical Simulation Methods: These methods, which have gained prominence in recent years, simulate outdoor urban microclimates based on simplified models of the physical environment. They address the limitations of traditional approaches and offer enhanced precision and detail.

Advanced Tools for MRT Simulation: The exploration of computer techniques has led to the development of advanced tools for MRT simulation, including:

• SOLWEIG (SOlar LongWave Environmental Irradiance Geometry): SOLWEIG is a model used to estimate spatial variations of 3D radiation fluxes and mean radiant temperature in complex urban settings. It follows the same approach commonly adopted to observe MRT, with shortwave and longwave radiation fluxes from six directions being individually calculated to derive MRT. The model requires a limited number of inputs, such as direct, diffuse, and global shortwave radiation, air temperature, relative humidity, urban geometry, and geographical information. It provides daily and hourly shading maps, Sky View Factor (SVF), and thermal indices (PET, UTCI).
• ENVI-met: A pioneer of holistic 3D modeling software committed to placing environmental planning at the center of urban development worldwide. It offers comprehensive tools for accurate microclimate modeling and environmental analysis, including features for wind simulation in urban microclimates. It provides accurate analysis and simulation of microclimatic elements.
• RayMan: Model developed for the calculation of radiation fluxes in simple and complex environments. It is used to assess outdoor thermal comfort and mean radiant temperature. The model considers topographic inputs, meteorological data, and different types of surfaces (like grass or asphalt) to provide detailed information about the distribution of solar radiation.
• Ladybug: Part of the Ladybug Tools suite, which is a collection of free computer applications that support environmental design and education. It allows visualizing and analyzing weather data in Grasshopper, providing a variety of 2D and 3D interactive climate graphics that support decision-making during the early stages of design.
• CitySim Pro: A simulation software designed for energy demand modeling in urban areas. It allows for the evaluation of the impact of different energy strategies at the district scale. It is particularly useful for simulating energy flows in cities and assessing the performance of various energy systems in complex urban environments. This tool, along with ENVI-met, RayMan, and Ladybug Tools, uses different assumptions and equations to calculate MRT.

Mean Radiant Temperature finds extensive applications across various domains, including urban planning, urban design, and environmental studies. Its significance lies in its ability to characterize thermal comfort, guide design decisions, and inform environmental assessments in outdoor settings. Overall, MRT serves as a valuable tool for enhancing the livability, sustainability, and resilience of urban environments, guiding decision-making processes in urban planning, design, and environmental management. By incorporating MRT considerations into planning and design strategies, stakeholders can create healthier, more comfortable, and more sustainable outdoor environments for current and future generations.

• Urban Planning and Design: MRT plays a crucial role in urban planning and design by informing decisions related to the layout, orientation, and configuration of outdoor spaces. Understanding MRT variations within urban environments helps identify areas prone to thermal discomfort or heat stress, allowing planners to prioritize interventions such as green spaces, shading structures, and cool roofs. By optimizing MRT distributions, urban planners can create more livable and sustainable cities that enhance residents’ well-being and mitigate the urban heat island effect.

• Building Design and Architecture: In building design and architecture, MRT influences decisions regarding building orientation, facade design, and material selection. Designing buildings to minimize MRT exposure can improve occupant comfort and reduce the need for mechanical cooling systems, leading to energy savings and environmental benefits. Additionally, incorporating MRT considerations into building designs enables architects to create spaces that promote thermal comfort and enhance the overall user experience.

• Transportation and Infrastructure: MRT assessments are essential in transportation and infrastructure planning to ensure the comfort and safety of commuters and pedestrians. By analyzing MRT variations along transportation corridors and at transit stops, policymakers can implement strategies such as shade structures, vegetation, and surface materials to mitigate heat stress and improve the overall travel experience. Additionally, incorporating MRT considerations into infrastructure projects helps minimize thermal discomfort and heat-related health risks for vulnerable populations while navigating the urban outdoor environment, mostly in relation to active modes.

• Environmental Studies: In environmental studies, MRT serves as a valuable metric for assessing the impact of land use changes, urbanization, and climate change on outdoor thermal environments. By monitoring MRT variations over time, researchers can identify trends and patterns related to urban heat island formation, microclimate dynamics, and heatwave frequency. This information aids policymakers in developing strategies to mitigate heat-related risks and enhance resilience to climate change impacts.

• Public Health and Well-being: Understanding MRT is crucial for safeguarding public health and well-being, particularly in areas prone to extreme heat events. By identifying hotspots with elevated MRT levels, public health officials can implement targeted interventions such as heat emergency response plans, cooling centers, and heat mitigation strategies to protect vulnerable populations from heat-related illnesses and fatalities.

 

RELATED LINKS AND ADDITIONAL RESOURCES

• Methods for Estimating MRT: This study evaluates the accuracy of different sensors in estimating mean radiant temperature (MRT). It proposes correction algorithms for low-cost sensors, improving MRT estimation accuracy and eliminating dependency on other meteorological factors. Valuable for urban climate and biometeorological research.
• Globe Temperature and its Measurement: This study evaluates globe temperature measurement using different-sized thermometers across various conditions, highlighting the need for longer measurement periods and proposing correction equations for standard globe temperature under different air velocities.
• Simulation of MRT in Outdoor Conditions: This study compares different software tools’ methods for calculating Mean Radiant Temperature (MRT), which is crucial for assessing outdoor human comfort in urban environments.
• Calculation of MRT in Outdoors: This report reviews three conventional methods for calculating outdoor mean radiant temperature: (a) using measured short and long-wave radiation fluxes from six directions, (b) using measured short and long-wave radiation fluxes from two directions, and (c) using measured globe temperature along with air temperature and velocity.
• Runner’s Outdoor Heat Exposure: This study proposes a novel framework to assess human heat exposure using GPS data and urban microclimate modeling. It finds no significant gender-based heat exposure differences among runners but notes higher exposure for females aged 45-54 compared to younger groups.
• On the understanding of the MRT: This review discusses the evolution and challenges of understanding mean radiant temperature, emphasizing the need to address geometric complexities in radiant heat transfer research. It highlights methods for quantifying MRT and its increasing usage in indoor and outdoor environments.
• Infrared thermography for estimating MRT: This study highlights the importance of measuring mean radiant temperature (MRT) in large indoor spaces like stadiums or auditoriums. It proposes a method using an infrared thermal imaging camera (IR camera) for multi-location MRT measurements.
• Outdoor Wellbeing and Quality of Life: This work reviews the existing literature on outdoor thermal comfort to address the gaps in understanding and classification of related subjects. It identifies key research topics and challenges in defining suitable indexes for assessing outdoor thermal comfort. While accurate models and software exist, the subjective nature of human comfort perception remains a central challenge in this area.
• Globe Thermometer and Accuracy of Outdoor MRT: This study assesses the accuracy of the globe thermometer method in outdoor thermal environments and its impact on thermal comfort indicators. Findings show reduced accuracy with increasing shortwave radiation and significant deviations with rapid changes in wind speed.

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https://publications.ibpsa.org/conference/paper/?id=bs2019_210301