Absolute Humidity vs. Relative Humidity: Formulas & Conversion

Instructor: Raghav Mahalingam

Raghav has a graduate degree in Engineering and 20 years of professional experience.

In this lesson, you will learn the difference between absolute humidity and relative humidity. You will understand how humidity (or moisture content) relates to the composition of air, and the relationship between different ways of representing the humidity level of air.

What is Humidity?

Before we talk about absolute or relative humidity, let's understand what humidity itself means. The term humidity is derived from the Latin word humere, which means be moist. This should immediately conjure up an image of something that has moisture, or water content, in it. So humidity is used to describe the water content in air.

Absolutely and relative humidity are just ways of representing the same thing, i.e, the moisture content in air.

Composition of Dry Air

Let's first consider dry air without any moisture in it. As a fraction, dry air is composed of approximately 78% Nitrogen (N2) and 21% Oxygen(O2) and small amounts of many other gases on a volume basis.

There is another way to represent the composition of dry air: in terms of mass density, or mass per unit volume of air. If you consider the mass density of the two main components of air, N2 has a density of 1.165 kg/m3 and O2 1.331 kg/m3. These values are defined at NTP (Normal Temperature and Pressure) conditions of 20° C and 1 atm, respectively.

Wet (or Humid) Air

Now let's take the dry air and add some water vapor to it. Water vapor is nothing but water, except in gaseous form. When you start heating up a pot of water, the steam that you see rising is water vapor.

We can define a density for water vapor as well, the same way we defined a density for nitrogen and oxygen. Absolute humidity is nothing but the density of water vapor in air (sometimes called vapor density). It has a units of kg/m3 and represents the actual mass of water vapor per unit volume of air.

Absolute humidity(AH) can be calculated as,

AH = m(watervapor)/V(air)

where, m(watervapor) is the mass of the water vapor and V(air) is the volume of air.

Saturation Vapor Density and Relative Humidity

Air does not have an infinite capacity to absorb moisture. The ability of air to absorb moisture is a function of temperature. At any given temperature the maximum density of water vapor in air is called the saturation vapor density. At this point the air is termed as being saturated (with water vapor). The maximum amount of water vapor (by volume) that can reside in air before condensation begins to occur varies from much less than 1% near the poles to around 5% near the equator. Compare this to oxygen and nitrogen in dry air, which are 21% and 78%, respectively.

The relative humidity is calculated as

RH = vapor density(actual)/vapor density(saturation) x 100

or

RH = AH/vapor density(saturation) x 100

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