How to Calculate Relative Humidity Using a Formula

Introduction

In this post, you will learn how to calculate relative humidity using a formula.

The formula for calculating relative humidity is

Relative Humidity = (Vapor pressure/Saturation vapor pressure) × 100%.

we can equally say that

Relative Humidity = (specific humidity/saturation vapor pressure) x 100%

The final answer for relative humidity is in percentage (%)

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Examples of How to Calculate Relative Humidity Using a Formula

Here are a few examples of how to calculate relative humidity using a formula

Example 1

The mass of water vapor in a given volume of air is 0.05g at 20⁰c, while the mass of water vapor required to saturate it at the same temperature is 0.15g. Calculate the relative humidity of the air.

Solution

Data

Mass of water vapor in a given volume of air, M_a = 0.05g

Mass of water vapor required to saturate it at the same temperature, M_w = 0.15g

Relative humidity of air, R_a = ?

and

R_a = ( M_a / M_w ) x 100%

R_a = ( 0.05 / 0.15 ) x 100% = 33.33%

Example 2

Assuming the actual vapor density is 20g/m³ at 30⁰c compared to the saturation vapor density at the same temperature of 25g/m³. Calculate the relative humidity of the air.

Solution

Data:

Actual vapor density = 20g/m³

saturation vapor density = 25g/m³

Relative humidity = ?

And the formula is

Relative Humidity = (specific humidity/saturation vapor pressure) x 100%

Relative Humidity = (20/25) x 100% = 0.8 x 100% = 80%

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