What comprises a psychrometer?

A psychrometer is an instrument specifically designed to measure humidity in the air. It consists of two ordinary thermometers: one measures the ambient air temperature, while the other has a wetted bulb. As water evaporates from the wetted bulb, it cools down, and the temperature difference between the two thermometers can be used to calculate the relative humidity. This method is based on the principle that moisture in the air affects the rate of evaporation, allowing for a more accurate assessment of humidity levels based on the temperatures recorded.

The other choices do not reflect the characteristics of a psychrometer. For instance, wind measuring devices are used for tracking wind speed and direction, barometers measure atmospheric pressure, and humidity gauges are typically single instruments focused on measuring moisture in the air rather than using comparative thermometry like the psychrometer does. Therefore, having two ordinary thermometers is a defining feature of this specific instrument.