What is Relative Humidity?

Relative Humidity is the most commonly used measurements of moisture content in the air.  The key to understanding relative humidity is to understand that it is a measure of the ‘actual humidity’, relative to the maximum possible humidity at a given temperature.  Let’s explain it a bit further.
Consider the air to be like a big sponge that can soak up moisture. Also consider that this ‘sponge’ can hold more moisture as it gets hotter, and less moisture as it cools.

Then the Relative Humidity is the amount of water vapour (moisture) in the air compared to the maximum amount that the air could hold at a given temperature.

The relative humidity is expressed as a percentage, so the maximum is 100 %. The formula for relative humidity is:

Relative humidity % =Moisture in the air now / Maximum possible moisture air can hold at the current temperature (x100)

The diagrambelow illustrates how the relative humidity changes with temperature. In this example, a volume of air contains a specific, (fixed) amount of moisture. As the volume of air is heated, the relative humidity decreases. As the volume of air is cooled, the relative humidity increases.


In the above example, the actual moisture content stays constant; it is the potential of the air mass to hold moisture that is changing with the temperature. That is why an increase in temperature is reflected as a reduction in the relative humidity.

The diagrambelow shows lines of constant relative humidity. The horizontal axis represents air temperature in degrees Celsius (ºC) and moisture content in g/kg of air on the vertical axis. Note that the same relative humidity value can exist at different temperatures and different moisture contents (follow the up-sloping curves).Relative-Humidity-Plot


Relative Humidity readings are used to determine human and animal comfort levels, predict the likelihood of certain crop diseases, and is used in certain manufacturing environments. The effectiveness of crop spraying is also determined by the relative humidity in conjunction with wind and other climatic variables.

Please refer to our Faq article on “How to read a psychrometric chart” for information on determining relative humidity using wet bulb and dry bulb thermometers, as well as determining a wet bulb reading or Dew Point temperature from relative humidity and air temperature.