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What’s the difference between Relative Humidity and Absolute Humidity?

What’s the difference between Relative Humidity and Absolute Humidity?

If you’ve been reading our website or posts for any length of time, you will see that we really focus on attaining the correct humidity in your home or office as a staple condition for good air quality.  Recently a customer asked us to clarify the difference between Relative Humidity and Absolute Humidity, and which one should we be monitoring.  This is a great question!

Water is a very interesting substance because it can exist as a solid, liquid or gas.  When it’s in gaseous state, it’s called water vapor, or moisture. Let’s talk about the presence of water vapor in the air.  If you want to measure the amount of water vapor in the air, you can express it in grams of water per cubic meter volume of air, and you would have absolute humidity.  (weather.gov) Absolute humidity does not depend on any specific temperature; it is just a physical amount of water in a given volume of air.   

Relative humidity has an additional variable thrown in–temperature.  Whenever relative humidity is discussed, it takes into account temperature, because temperature has a big effect on how much water the air can theoretically hold.  As temperature increases, the same volume of air can hold more water, just as hot water can hold more dissolved sugar or salt than cold water.  Relative humidity is always measured in percent.  If you have 50% relative humidity, the air is holding 50% of the water vapor it could theoretically contain at that temperature.  The theoretical maximum is that point where any increase in the water vapor at that temperature would cause it to fall out of the air (rain).   What’s the takeaway of relative humidity?  A certain relative humidity–say fifty percent– should “feel” the same whether it is 60 degrees or 70 degrees in your house, but 70 degree air at 50% relative humidity actually contains more volume of water vapor than 60 degree air at 50% relative humidity.  

As you may note, relative humidity is what we normally reference in all of our discussions, because we like to keep our homes at different temperatures throughout the day and throughout the year, but the target relative humidity (40-60%) should remain the same for good respiratory health, and low bacterial/viral transmission and mold growth.  Check out our portable, economical humidity sensors so that you can easily monitor the relative humidity in different areas of your home!