A psychrometer measures relative humidity by comparing the temperature in wet air and dry air. The device typically consists of two thermometers: one with a dry bulb exposed to the air and the other with a wet bulb that has a damp wick around it. The evaporation from the wet bulb cools down its temperature, and the difference in temperature readings between the two thermometers is used to calculate the relative humidity. Therefore, the correct answer is:
It compares the temperature in wet air and dry air.