# Test and Measurement World

## Test and Measurement World Terminology Section

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## Absolute Humidity vs Specific Humidity vs Relative Humidity | Difference between Absolute Humidity, Specific Humidity, Relative Humidity

This page compares Absolute Humidity vs Specific Humidity vs Relative Humidity and mentions difference between Absolute Humidity, Specific Humidity and Relative Humidity.

➤The amount of water in the air is known as Humidity.
➤There are different ways to measure humidity.
➤Humidity are categorized into absolute, specific and relative.

### Absolute Humidity (AH)

• The mass of water vapor in a unit volume of air is known as absolute humidity.
• It can be expressed as follows.
➤ AH = Mass of water vapor/Volume of air.

Example:
For Parcel size = 2 m2
and Mass of H2O vapor =10 g
Absolute humidity = 5 g/m2

### Specific Humidity (SH)

• The mass of water vapor per unit mass of the dry air is known as specific humidity.
• It is ratio of water vapor mass (mv) to air parcel's total (i.e. including dry) mass (ma).
• It can be expressed as follows.
➤ SH = Mass of water vapor/Total mass of air

Example:
For Mass of Parcel = 1 Kg and Mass of H2O vapor = 1 g
Specific Humidity = 1 g/Kg

➤Specific humidity is concerned with mass of vapor to mass of air.
➤It is not affected by changes in parcel volume. SH is not affected due to temperature.
➤Specific humidity and mixing ratio of an air parcel remains constant as long as water vapor is not added to or removed from the parcel.

### Relative Humidity (RH)

•  It measures water vapor relative to the temperature of the air.
•  It is expressed as a percent.
•  In other words, it is a measure of actual amount of water vapor in the air compare to the total amount of vapor which can exist in the air at its current temperature.

➤Warm air can hold more water vapor than cold air.
➤Hence for same amount of absolute/specific humidity, cooler air will have higher relative humidity and warmer air will have lower relative humidity.