Mean radiant temperature

The concept of mean radiant temperature (MRT) is used to quantify the exchange of radiant heat between a human and their surrounding environment, with a view to understanding the influence of surface temperatures on personal comfort. Mean radiant temperature has been both qualitatively defined and quantitatively evaluated for both indoor and outdoor environments.

MRT has been defined as the uniform temperature of an imaginary enclosure in which the radiant heat transfer from the human body is equal to the radiant heat transfer in the actual non-uniform enclosure.

MRT is a useful concept as the net exchange of radiant energy between two objects is approximately proportional to the product of their temperature difference multiplied by their emissivity (ability to emit and absorb heat). The MRT is simply the area weighted mean temperature of all the objects surrounding the body. This is meaningful as long as the temperature differences of the objects are small compared to their absolute temperatures, allowing linearization of the Stefan-Boltzmann Law in the relevant temperature range.

MRT also has a strong influence on thermophysiological comfort indexes such as physiological equivalent temperature (PET) or predicted mean vote (PMV).

What we experience and feel relating to thermal comfort in a building is related to the influence of both the air temperature and the temperature of surfaces in that space, represented by the mean radiant temperature. The MRT is controlled by enclosure performances.

The operative temperature, which is a more functional measure of thermal comfort in a building, is calculated from air temperature, mean radiant temperature and air speed. Maintaining a balance between the operative temperature and the mean radiant temperature can create a more comfortable space. This is done with effective design of the building, interior and with the use of high temperature radiant cooling and low temperature radiant heating.

In outdoor settings, mean radiant temperature is affected by air temperature but also by the radiation of absorbed heat from the materials used in sidewalks, streets, and buildings. It can be mitigated by tree cover and green space, which act as sources of shade and promote evaporative cooling. The experienced mean radiant temperature outdoors can vary widely depending on local conditions. For example, measurements taken across Chapel Hill, North Carolina to examine urban heat island exposure ranged from 93 to 108 °F (34 to 42 °C).