Measuring Humidity
There is water vapour in the air at all times. Humidity is the amount of water vapour present in the air and we can measure this using a hygrometer.

Other Types of Hygrometer
The best hygrometer is one which has two thermometers - one of which has its bulb wetted continuously by a wet cloth. This is called a Mason's or a 'wet-and dry-bulb' hygrometer. Air passing over this wet bulb evaporates some water and lowers the wet bulb temperature by an amount that depends on the humidity of the air. To demonstrate this cooling, wet your finger and blow in it. The humidity can be found by noting the difference in temperature between the wet and dry bulb and looking up the RH value in a set of tables. The relative humidity (RH value) is the amount as a percentage of that required to saturate the air completely.