**Core Concept**
The mean radiant temperature is a measure of the temperature of the surfaces in an environment that are emitting radiant heat. It is an important factor in heat stress and thermal comfort assessment.

**Why the Correct Answer is Right**
The mean radiant temperature is measured using a device called a globe thermometer or a radiative thermometer. This device is a hollow, black-painted metal sphere with a thermometer inside. The temperature reading on the thermometer is the mean radiant temperature. The principle behind this device is that it absorbs and equilibrates with the radiant heat from the surrounding surfaces, providing an accurate measurement of the mean radiant temperature.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a mercury thermometer is used to measure liquid temperature, not radiant temperature.
**Option B:** This option is incorrect because a hygrometer measures humidity, not temperature.
**Option C:** This option is incorrect because a barometer measures atmospheric pressure, not temperature.

**Clinical Pearl / High-Yield Fact**
In hot environments, the mean radiant temperature is a significant contributor to heat stress, and it can be higher than the air temperature. This is especially true in urban areas with dark-colored surfaces, which can absorb and re-emit radiant heat.

**Correct Answer: C. Globe thermometer.**