How to Figure Out Relative Humidity With Vapor Pressure

Humidity is relative to heat.

Everyone has suffered through a hot, "muggy" day, when humidity feels like an added weight on your shoulders. But humidity is affected by more than just the amount of moisture in the air. Relative humidity calculates the amount of water vapor in the air as compared with the maximum amount the air could hold at the current temperature, then expresses the result as a percentage. As a result, if the temperature rises, but the amount of water vapor remains the same, the relative humidity drops. There are two ways to calculate this information: using the actual amount of water vapor, or the actual vapor pressure.

Instructions

1. Collect the data you need for the calculation: the current temperature, the saturated vapor pressure at that temperature and the actual vapor pressure.

2. Divide the actual vapor pressure by the saturated vapor pressure. For instance, if you have an actual vapor pressure of 10 grams per cubic meter, and the saturated vapor pressure at your current temperature is 30 grams per cubic meter, you would get 0.33 -- 10 divided by 30.

3. Multiply the resulting number by 100 to determine the percentage. For instance, multiplying 0.33 by 100 gives you a percentage of 33. That is your relative humidity at the current temperature.

References

• Australian Government Bureau of Meteorology: Humidity Calculator
• Transcat: The Many Faces of Water Vapor (PDF)
• USA Today: Getting a Handle on Humidity

Resources

• CSG Network: Vapor Pressure Calculator