Relative Humidity (RH) is defined as the ratio of the actual water vapor content in the air to the maximum possible water vapor content at a given temperature. To calculate RH:
* Dry bulb temperature (DBT): the ambient air temperature.
* Wet bulb temperature (WBT): the temperature measured by a thermometer covered with a wet wick, influenced by evaporative cooling.
Using DBT and WBT, the humidity ratio and dew point can be determined with a psychrometric chart or Mollier diagram. From there, RH is calculated as:
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Options A and B are incomplete, while option C is an experimental method, not standard practice. The recognized method is option D.
References: ASHRAE Fundamentals Handbook - Psychrometrics, ISO 7726 (Measurement of Physical Quantities in Thermal Environments).