The key difference between absolute and relative humidity is that absolute humidity is a fraction, while relative humidity is a percentage.

Relative humidity and absolute humidity are two important topics we discuss under psychrometrics. These theories are highly important in fields such as meteorology, chemical and process engineering and many more.

CONTENTS

1. Overview and Key Difference
2. What is Absolute Humidity
3. What is Relative Humidity
4. Side by Side Comparison – Absolute vs Relative Humidity in Tabular Form
5. Summary

What is Absolute Humidity?

Absolute humidity is an important factor when it comes to the study of psychrometrics. Psychrometrics is the study of gas-vapour systems. In thermodynamics, we define absolute humidity as the mass of water vapour per unit volume of moist air. It can take values ranging from zero to saturated water vapour density. The saturated water vapour density depends on the pressure of the gas; therefore, the maximum mass of vapour per unit volume also depends on the air pressure.

As pressure and temperature affect the absolute humidity, it is inconvenient to use this as an engineering quantity. It is because most of the engineering systems have variable temperatures and pressures. Therefore, we need to give a new definition for absolute humidity. The new definition says absolute humidity is the mass of the water vapour in a volume divided by the mass of dry air in the said volume. Thus, this definition is much convenient when dealing with pressure changes. However, to avoid confusion, we need to rename the first definition as the volumetric humidity.

What is Relative Humidity?

Relative humidity is important when we consider the real effect of humidity. To understand the concept of relative humidity, there are two concepts we need to understand first. First one is partial pressure. Imagine a gaseous system where there are A1 molecules of gas G1 generating pressure P1, and A2 molecules of gas G2 generating pressure P2. The partial pressure of G1 in the mixture is P1/ (P1+P2). For an ideal gas, this is also equal to A1/ (A1+A2). The second concept that has to be understood is the saturated vapour pressure. The vapour pressure is the pressure vapour in equilibrium in a system creates.

Now let us assume that there is still liquid water (however infinitesimal) in a closed system. That means; the system is saturated with water vapour. If we decrease the temperature of the system, the system will surely remain saturated, but if we do not increase it, we may have to calculate the result.

Figure 01: A Graph showing the Variations in Relative Humidity

Now let us see the definition of relative humidity. Relative humidity is the percentage of the partial pressure of the vapour divided by the saturated vapour pressure at the given temperature. Thus, this is in the form of a percentage. It is a useful quantity in conveying the real feeling of humidity. If the relative humidity is high, we feel sweaty; if it is low, we feel dehydrated. An air-conditioned room is a good example of a low relative humid environment. A beach on a hot day is a high relative humid area.

What is the Difference Between Absolute and Relative Humidity?

Absolute humidity is an important factor when it comes to the study of psychrometrics while relative humidity is the percentage of the partial pressure of the vapour divided by the saturated vapour pressure at the given temperature. So, the key difference between absolute and relative humidity is that absolute humidity is a fraction, while relative humidity is a percentage. Furthermore, absolute humidity is a measure of water vapour in the air regardless of temperature, while relative humidity is a measure of water vapour we measure relative to the temperature of the air.

Moreover, absolute humidity cannot give any measure of the real condition since it is independent of temperature. However, the relative humidity gives a good view of the condition since the saturated pressure depends on the temperature. Therefore, this too is a notable difference between absolute humidity and relative humidity.

Summary – Absolute vs Relative Humidity

Absolute humidity is an important factor when it comes to the study of psychrometrics. It is a measure of water vapour in the air regardless of temperature. Relative humidity, on the other hand, is the percentage of the partial pressure of the vapour divided by the saturated vapour pressure at the given temperature. The key difference between absolute and relative humidity is that absolute humidity is a fraction, while the relative humidity is a percentage.

Reference:

1.“Relative Humidity.” Wikipedia, Wikimedia Foundation, 14 Mar. 2019, Available here.

Image Courtesy:

1. “Relative Humidity” By English Wikipedia user GregBenson (CC BY-SA 3.0) via Commons Wikimedia

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