American Meteorological Society Certification (AMS) Practice

The appropriate instrument for measuring relative humidity is the psychrometer. A psychrometer consists of two thermometers: a dry bulb thermometer that measures the air temperature and a wet bulb thermometer that is covered in a moist cloth. The evaporation of water from the cloth cools the wet bulb thermometer, and the difference in temperature readings between the two thermometers allows for the calculation of relative humidity. This method is based on the principle that the amount of moisture in the air affects the rate of evaporation, which in turn influences the wet bulb temperature.

While hygrometers also measure relative humidity, they do so through different mechanisms, such as using materials that change physical characteristics with moisture levels or electronic sensors, making them different from the psychrometer method.

Barometers are used to measure atmospheric pressure, and anemometers measure wind speed, which are unrelated to humidity measurements. Understanding the specific function of each instrument highlights why the psychrometer is the correct choice for measuring relative humidity.