Mean radiant temperature

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

Technically, MRT is defined as the uniform temperature of a surrounding surface giving off blackbody radiation (emissivity $\epsilon$ = 1) which results in the same radiation energy gain on a human body as the prevailing radiation fluxes which are usually very varied under open space conditions.

MRT is the most important parameter governing human energy balance, especially on hot sunny days. MRT also has the strongest influence on thermophysiological comfort indexes such as PET (Physiological Equivalent Temperature) or PMV (Predicted Mean Vote) which are derived from heat exchange models. Physiologists have discovered that living human skin has extraordinarily high absorptivity and emissivity ($\epsilon$ = α = 0.98), greater than almost any other known substance, matte-black metals included^[1] Consequently, we are highly responsive to changes in mean radiant temperature.

Radiant temperature is usually measured with what it known as a globe thermometer. This is simply a normal dry bulb thermometer encased in a 150mm diameter matte-black copper sphere whose absorptivity approaches that of the skin. Hence MRT is sometimes referred to as globe temperature (GT, or T_g) as they are nearly equivalent.

The measured globe temperature depends on both convection and radiation transfer, however by effectively increasing the size of the thermometer bulb the convection transfer coefficient is reduced and the effect of radiation is proportionally increased. Because of local convective air currents the globe temperature T_g typically lies between the air temperature T_a and the true mean radiant temperature T_r. The faster the air moves over the globe thermometer the closer T_g approaches T_a. If there is zero air movement, T_g = T_r.

See also

• Thermal comfort
• http://ottp.fme.vutbr.cz/predmety/EnvEng/EnvironmentalEngineering.pdf
• http://ergo.human.cornell.edu/DEA3500allnotes.html

References

1. ^ Thomas, R, Understanding Emissivity - It’s Only Skin Deep!, Think Thermally, Winter 2007