How to calculate water vapour density

Written by nicole schmoll
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How to calculate water vapour density
Mist is often mistaken for water vapour, which is actually invisible. (mist image by Stuart Corlett from

People are most used to seeing water in its liquid state. Probably the next most common way people observe water is in a solid state, when it appears as ice or snowflakes. When water is in its gaseous state, it is called water vapour and is invisible. Clouds, steam and mist are commonly misinterpreted as water vapour, but these are actually forms of water in its liquid state. Although we can't see water vapour, we can measure it.

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  1. 1

    Use a calculator to determine the density of water vapour under a given set of circumstances. Find the variables for the following formula, which is based on the ideal gas law:

    Pw = 0.0022pw / T

  2. 2

    Find the value of pw, which is equal to the partial pressure of water vapour and is measured by Pa, N/m x m. Determine the value for T, which is equal to the absolute dry bulb temperature and is measured in Kelvins. Use these figures to find the density of water vapour, which is measured in kg/m cubed. Write these values down on a piece of paper and use them in the next step.

  3. 3

    Use a calculator to multiply the value you found for pw by 0.0022. Enter the result into your calculator and press the divide ( / ) button on your calculator to divide the result by the value that you found for T. Write down the result, which will equal the water vapour density given the variables you entered.

Tips and warnings

  • Use your calculations to solve other problems like a measurement for relative humidity at a given point in time, which is expressed as the result of saturation vapour density divided by water vapour density multiplied by 100.

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