Relative Humidity: What It Is & How It’s Measured

While relative humidity can be measured with a hygrometer, most modern dehumidifiers use a built-in humidistat to measure relative humidity levels, allowing the user to set a specific desired humidity level for easy, automatic and hands-free operation. But what exactly is relative humidity, you ask? Well, in order to understand relative humidity, you must first understand humidity. Here are some key points:

Humidity vs. Relative Humidity

  • Humidity and relative humidity are not the same thing. Using the term “humidity” in relation to the condition of the air is correct, but refers only to the quantity of water vapor present in the air, which is also known as “absolute humidity”.
  • The amount of water vapor in the air (“absolute humidity”, or commonly “humidity”) is contingent upon air temperature. The higher the air temperature, the more water vapor it can hold, and the higher the potential for humidity.
  • In regards to humidity, water vapor can be measured in terms of pressure or density – the higher the temperature, the higher the vapor pressure and the lower the density.
    1. Water vapor pressure is measured in millibar (mbar, mb), which is a measure of pressure where 1,013 mbar is equal to Earth’s atmospheric pressure at sea level.
    2. Water vapor density is measured in grams per cubic meter (of air) at standard sea-level atmospheric pressure
    3. Note: Elevation factors into atmospheric pressure – the higher the elevation, the lower the atmospheric pressure.

So… How Does This Tie Into Relative Humidity?

Relative humidity (RH) is the actual amount of water vapor present in relation to the capacity that the air has at a particular temperature. While it is a bit misleading, think of the capacity as the amount of water vapor that the air can “hold”, though it is actually incapable of physically holding onto the water vapor (it moves too fast). When a specific water vapor saturation point or capacity is reached, the water vapor in the air will condense as moisture, making the air feel damp.

Just like water vapor, relative humidity can be expressed in terms of pressure or density. In both cases it is expressed as a percentage and is calculated by dividing actual vapor pressure by saturation vapor pressure (or actual vapor density by saturation vapor density), then multiplying that number by 100. In other words, the relative humidity would be measured by the current pressure point in the air divided by the pressure point at which water vapor saturates the air at EdgeStar 50 Pint Dehumidifier control panel that current temperature, multiplied by 100 to give a percentage.

However, with a modern day dehumidifier, there’s no need to crunch numbers, as the built-in humidistat will do all the measurements and calculations for you!

Why Does Relative Humidity Matter?

Relative humidity is important when figuring out dehumidification needs because it allows you to properly assess the exact amount of moisture present in the air at any given point in time. With this information, you can gauge how much humidity you will typically deal with and which dehumidifier will be best suited for you.

While humidity may present an uncomfortable environment, more importantly, it can present many dangers to you and your home. When relative humidity is above 50%, the air can begin to feel moist or damp, and presents the ideal environment for airborne allergens and bacteria, among a host of other issues.

As you may well know, different climates and geographic regions can face varying humidity levels. Proximity to bodies of water and temperature trends, among other factors, can certainly affect how humid the air is going to be. It is important to regularly assess your relative humidity levels to maintain a healthy living environment. A dehumidifier can not only help measure relative humidity, but can also help bring it to a comfortable and healthy level.

Kara Zorn

About Author

Kara Zorn is a wife and working mom who loves life hacks that help her schedule allow for as much quality family time as possible. She enjoys cooking with locally sourced food and produce and recently took up brewing Kombucha.


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