This is a statement of basic meteorological and thermodynamic fact, defining how a psychrometer (wet-and-dry bulb thermometer) measures relative humidity, and requires no citation.
The claim describes the standard operating principle of a psychrometer (whirling hygrometer), which uses the psychrometric difference between wet and dry bulb thermometers to determine relative humidity. This is a foundational scientific definition found in basic physics and meteorology textbooks, making it common knowledge that does not require empirical citation.