What instrument is used to measure water vapor?

The instrument known as a psychrometer is specifically designed to measure the amount of moisture in the air, or water vapor. It typically consists of two thermometers: one that measures the ambient air temperature (the dry bulb) and another that measures the temperature of air in contact with a wet wick (the wet bulb). By comparing these two temperatures, the psychrometer can determine the humidity level in the air, which is a direct indicator of the amount of water vapor present.

The principle behind the psychrometer is based on the fact that evaporation has a cooling effect; thus, the wet bulb thermometer will show a lower temperature than the dry bulb thermometer when moisture is evaporating from the wick. The difference in temperature readings can be used to calculate relative humidity, which correlates to the water vapor content in the air.

Other instruments listed, such as a barometer, pyranometer, and anemometer, are designed to measure different atmospheric parameters. A barometer measures atmospheric pressure, a pyranometer measures solar radiation, and an anemometer measures wind speed. None of these instruments is focused on quantifying water vapor, making the psychrometer the appropriate choice for this measurement.