What two values are required to measure relative humidity?

To measure relative humidity, the two values needed are dry bulb temperature (DB) and wet bulb temperature (WB). Dry bulb temperature is the air temperature measured by a standard thermometer, while wet bulb temperature is the lowest temperature that can be achieved through evaporative cooling. The wet bulb temperature reflects the moisture content in the air and is critical for calculating the relative humidity.

The relationship between these two temperatures is essential because, as moisture is added to the air (increasing humidity), the wet bulb temperature increases. This allows technicians to use psychrometric charts or formulas to determine the relative humidity based on the difference between these two temperatures. For instance, the closer the wet bulb temperature is to the dry bulb temperature, the higher the humidity.

Relative humidity is defined as the current amount of water vapor in the air, compared to the maximum amount of water vapor that air can hold at that temperature. Thus, the combination of DB and WB provides the necessary information to quantify how humid the air is relative to its maximum water-holding capacity.

Other options, while they contain important parameters in various physics and HVAC contexts, do not provide the required combination needed to calculate relative humidity directly.