Question

Which instrument is used to measure the relative humidity of air?

Hint:

Think of the wet bulb and dry bulb thermometer setup used in weather stations.

Answer 1

Step 1. Relative humidity is a measure of how much water vapour the air holds compared to the maximum it could hold at that temperature, given as a percentage.
Step 2. The instrument built to measure this quantity is called a hygrometer.
Step 3. A common type is the wet and dry bulb hygrometer, which uses two thermometers. One bulb is kept dry and the other is wrapped in a wet cloth. Evaporation from the wet bulb cools it, and the size of the temperature difference between the two thermometers is used, with standard tables, to find the relative humidity.
Step 4. Modern versions use electronic humidity sensors instead of two thermometers, but they are still classed as hygrometers because they measure the same quantity.
Answer. Relative humidity is measured using a hygrometer.