Unit of Humidity

You may have often heard people remark that the humidity is unbearable. Have you ever wondered what humidity means? Humidity is nothing but the measure of water vapour in the air. Humidity, combined with high temperatures, can increase the distress caused due to heat. Since humidity is a measurable entity, there is a SI unit of humidity.

Nevertheless, before learning the humidity unit, you must learn about the various ways in which humans measure humidity.

Absolute Humidity

In this form of measurement, you can consider the mass of water vapour present in a particular volume of air. Due to such a direct and specific approach, this humidity measurement unit is the most scientifically accurate method.

Absolute humidity unit is expressed in grams per cubic metre of air. 

Example 1 

Your measurements reveal that 7 grams of water vapour exist in 2 cubic metres of air. In such a case, you can say that the absolute humidity is equal to 3.5 grams/m3

While scientifically accurate, this unit of measurement is difficult to understand for the general population. Therefore, other forms of unit for humidity are more popular.

Relative Humidity

Relative humidity refers to the amount of water vapour present in the air relative to the maximum water vapour that it can hold. In most cases, relative humidity unit is expressed in percentage.

Example 2

Suppose the air is capable of holding 10g/m3 of water vapour. Now consider, weather analysts determine that at present the air holds 8g/m3. From this, they can easily derive the relative humidity (RH), through the following formula – 

RH = 8/10 x 100

Therefore, RH = 80%

This is the most common method used as a measure of humidity. Weather forecasts in the news also tend to report humidity predictions in the relative humidity format. Doing so eases comprehension significantly.

Quick Exercise

Q. 22 grams of water vapour exist in 6 m3 of air. This volume of air can hold a maximum water vapour content of 25 grams. Calculate relative humidity and absolute humidity.


Absolute Humidity (AH)

Relative Humidity (RH)

AH = Grams of water vapour/ cubic metres of air

AH = 22/6

RH = Existing water vapour/Maximum water vapour x 100

RH = 22/25 x 100

AH = 3.66 g/m3

RH = 88%

Specific Humidity

Specific humidity is a measurement method, which is similar to the absolute method. The only difference is that instead of considering air volume, this calculation considers the total air mass. Thus, the specific humidity unit is g/kg. This means that such humidity is expressed as the amount of water vapour (in grams) present in each kilogram of air.

Specific humidity remains constants as long as moisture in the air is not removed or added. Therefore, the figure remains constant, even with changes in pressure and temperature. 

True or False

Relative humidity can never be 100%.

Ans. False

Relative humidity can be 100% in many cases, especially during the monsoons when air is saturated with moisture.

Dew Point

Apart from understanding the various units for humidity, you should also learn about dew point. Dew point is the temperature at which water vapour content in air reaches the maximum point. At a dew point, the rate of water condensation is equal to the rate of evaporation. 

You can also define it as the temperature when air reaches its saturation point when it comes to holding water. Temperature below dew point will result in condensation, in the form of clouds, fog or dew (you can see it on leaves and blades of grass). 

Humidity is a complex topic, which determines weather, temperature and even precipitation. Vedantu’s unique online classes and material can help students gauge the concept better. Download our app on your phones and begin learning in a more interactive and convenient way!

FAQ (Frequently Asked Questions)

1. How to Measure Relative Humidity?

Ans. You can measure relative humidity by determining existing humidity and relating it to maximum humidity that the air can hold. 

2. What is the Unit of Absolute Humidity?

Ans. Absolute humidity is expressed in grams per cubic metre. The unit is g/m3.

3. Why is Relative Humidity the Preferred Mode for Measuring Humidity?

Ans. Relative humidity is expressed in percentage, which is easier to comprehend by average people. The other units for it are g/m3 and g/kg, both of which are difficult to understand.