Relative humidity

Relative humidity (abbreviated RH and measured as a percentage %) is the ratio of the partial pressure of water vapor to the vapour pressure of water at the same temperature. This means relative humidity depends on temperature and the pressure of the system of interest.

liquid state vs gas state produces a vapour pressure. image: wikipedia

It is typically measured/estimated by comparing temperatures of a wet bulb thermometer with a dry bulb thermometer and then comparing with a chart like the one shown below.  

The red curved lines indicate relative humidity showing 100%  curving down to 0%. Notie though at cold temperatures the relative humidity changes very rapidly. Green indicates dry bulb and blue indicates wet bulb temperatures.  image: ArthurOgawa /wikipedia

Electronic hygrometers are also often used that rely on changes in resistance or capacitance due to humidity.

Adapted from Relative humidity. (2015, January 28). In Wikipedia, The Free Encyclopedia. Retrieved 05:40, February 13, 2015, from http://en.wikipedia.org/w/index.php?title=Relative_humidity&oldid=644549987


Commentary

Relative humidity can sometimes be a misleading measurement when talking about the amount of water vapour in an area (humidity).  During winter time for example around 1-2°C it is possible to have very high relative humidity of around 95% yet the atmosphere can have very little water vapour present. In very cold climates almost all the water vapour can be frozen. 

Relative humidity is mostly relevant to comfort levels in environments. It also can give an approximate indication of whether equipment is likely to end up with a lot or little condensation.