Some well-known types of temperature include air temperature and surface temperature. But another method of measuring temperature - mean radiant temperature, or MRT - measures thermal comfort and heat stress; namely, how people experience heat. Understanding MRT can help us create more comfortable living situations when located in hot places.
Heat can be quantified in various ways. The most common heat metric is air temperature, which simply refers to how hot or cold is the ambient air that surrounds us. The weather forecast usually reports air temperature, and so does the weather app on your phone. Air temperature is measured using a thermometer and is always read in the shade to protect the device from the Sun’s radiation, but air temperature does not vary a lot in the shade and sun, only by a couple of degrees.
Another type of temperature is surface temperature. Surface temperature refers to how hot or cold the “surface” of an object feels to the touch. Nowadays, we can monitor land surface temperature from space using satellite-based remote sensing. On the ground, we can use thermal cameras and infrared guns to measure the surface temperature of various objects.
A less known but still important temperature measure for thermal comfort and heat stress analyses is mean radiant temperature, or MRT. MRT is defined in the ISO 7726 standard 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”. In essence, MRT quantifies the heat load on the human body as the sum of the radiation that hits the body from all directions. This includes the longwave radiation emitted from hot surfaces, such as an asphalt parking lot in the summer, and the direct shortwave radiation from the sun. MRT roughly equals air temperature in the shade but can easily be 30 degrees F/20 degrees C higher in the sun, making a person feel much less comfortable when it is hot.
In hot, dry places, MRT is the temperature that best describes how people experience heat. It can be measured using “MaRTy”, a mobile weather station with three net radiometers that observe the shortwave and longwave radiation in six directions.
MRT can also be measured with a globe thermometer, which usually consists of a hollow copper sphere painted matt black to absorb radiant heat, with a temperature sensor at its center.
Now that we know three types of temperature: air, surface, and mean radiant temperature, which one is the most relevant for urban heat mitigation?
Well, it all depends on the context: who or what would you like to cool? Air temperature is important for lowering building energy use from air-conditioning. Surface temperature is more relevant when it comes to cooling urban surfaces that people touch or walk on with their bare feet, such as playground equipment and swimming pool decks. MRT is the most important temperature metric when it comes to human thermal comfort. It is an important input to calculate thermal comfort indices that describe how you “experience” a thermal environment.
