Unit of Humidity

We often hear people talking about humidity especially when it is a hot day. While we experience this phenomenon more frequently in the coastal regions. Humidity is defined as the ratio between the vapour pressure of water vapour in the air to the vapour pressure at saturation. It is expressed at different levels depending on certain factors. However, there is no official or special SI unit for humidity measurement.

Having said that, we will look at some of the units of measurement for humidity below.

Relative Humidity

The relative humidity is used for finding the water vapour content in the air. The relative humidity is defined as the ratio between the amount of moisture in the air at a particular temperature to the maximum moisture air can withstand at the same temperature. The relative humidity is 100% during rainy seasons.

RH = \((\frac{actual\;vapoour\;density}{saturation\;vapour\;density})\times 100\)%

Absolute Humidity

Absolute humidity which describes the water content of air is used to measure the weight of water vapour per unit volume of air. The absolute humidity unit is given as g.m-3 which is units of grams of water vapour per cubic meter of air. Since other factors constantly affect absolute humidity, the results are less useful.

Learn more about a unit of pressure here.

Specific Humidity

Specific humidity unit is the most reliable unit of measurement of humidity. This measures the weight of water vapour per unit weight of air and it is expressed as grams of water vapour per kilogram of air g.kg-1 is the specific humidity unit.


Absolute humidity unit g.m-3
Specific humidity unit g.kg-1

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