Humidity Ratio Calculator and Formula Explained

This page will walk you through understanding and calculating the humidity ratio, a crucial concept in understanding the moisture content of air. We’ll cover both the definition and the formulas, along with calculators to make things easier.

What is Humidity Ratio?

Simply put, the humidity ratio tells you how much water vapor is present in the air compared to the amount of dry air. Think of it as a way to measure the “wetness” of the air. It can be calculated in two main ways:

• By Mass: Comparing the mass of water vapor to the mass of dry air.
• By Vapor Partial Pressure: Comparing the partial pressure of water vapor to the partial pressure of dry air.

Let’s explore each method in detail.

Humidity Ratio Calculator by Mass

This calculator uses the first method, comparing the masses of water vapor and dry air.

Formula: The humidity ratio by mass is expressed as:

Where:

• Humidity Ratio = Mass of water vapor / Mass of dry air

Calculator:

To use the calculator, you need to provide two inputs:

1. Mass of water vapor (in Kg)
2. Mass of dry air (in Kg)

The calculator will then output the Humidity ratio by mass (in Kg/Kg).

Example:

• Input 1 (mass of water vapor): 100 Kg
• Input 2 (mass of dry air): 100 Kg
• Output (Humidity ratio by mass): 1 Kg/Kg

This means for every 1 kg of dry air, there is 1 kg of water vapor.

Humidity Ratio Formula by Mass

As shown above, the formula for humidity ratio by mass is:

Humidity Ratio = Mass of Water Vapor / Mass of Dry Air

This is a straightforward calculation, dividing the mass of water vapor present by the mass of the dry air.

Humidity Ratio Calculator by Vapor Partial Pressure

This calculator utilizes the second method, using partial pressures.

Formula:

The humidity ratio by vapor partial pressure is expressed as:

Where:

• Humidity Ratio = (0.622 * Partial Pressure of Water Vapor) / (Atmospheric Pressure - Partial Pressure of Water Vapor)

Calculator:

To use the calculator, provide these inputs:

1. Partial pressure of water vapor in moist air (in Pascals, Pa)
2. Atmospheric pressure of moist air (in Pascals, Pa)

The calculator then gives the Humidity ratio (in Kg/Kg).

Example:

• Input 1 (Partial Pressure of Water Vapor, Pw): 2222 Pa
• Input 2 (Atmospheric Pressure, Pa): 102325 Pa
• Output (Humidity ratio by vapor partial pressure): 0.0138 Kg/Kg

This result indicates there’s about 0.0138 kg of water vapor for every 1 kg of dry air.

Humidity Ratio Formula by Vapor Partial Pressure

As shown above, the formula for humidity ratio by vapor partial pressure is:

Humidity Ratio = (0.622 * Partial Pressure of Water Vapor) / (Atmospheric Pressure - Partial Pressure of Water Vapor)

This formula takes into account the partial pressures of water vapor and the overall atmospheric pressure to calculate the humidity ratio.

Conclusion

Understanding the humidity ratio, whether calculated by mass or partial pressure, is fundamental in various fields like meteorology, engineering, and HVAC systems. These calculators and formulas should help you easily determine the humidity ratio of air and further your understanding of this important concept.