A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain. Jose Luis Raota/Getty Images
If you’ve ever been to places with humid climates, like southern Arkansas on a hot July day, you’re familiar with incredible mugginess, a borderline hallucinatory experience where you feel like you’re walking in a stew of your own perspiration.
But there’s more to understanding that clammy, gross feeling than simply glancing at a weather forecaster’s humidity reading. Humidity can be measured in several ways, but relative humidity (RH) is the most common.
To really get a grasp on how humidity affects your health, home and sanity, you need insights on the types of humidity (absolute and relative), as well as the concept of dew point.
Absolute Humidity vs. Relative Humidity
Absolute humidity is the mass of water vapor divided by the mass of dry air in a volume of air at a given temperature. The hotter the air is, the more water it can contain. Absolute humidity is expressed as grams of moisture per cubic meter of air (g/m3).
Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain.
This doesn’t mean that the relative humidity must be 100 percent in order for it to rain — it must be 100 percent where the clouds are forming, but the relative humidity near the ground could be much less [source: University of Illinois].
Humans and Humidity
Humans are sensitive to very high humidity, as the skin relies on the air to get rid of moisture. The process of sweating is your body’s attempt to keep cool and maintain its current temperature.
If the air is at 100 percent relative humidity, sweat will not evaporate into the air. As a result, we feel much hotter than the actual temperature when there is high relative humidity. Your shirt may become saturated with perspiration that doesn’t go anywhere, leaving you feeling like a swampy bog monster of revolting proportions.
When there’s low relative humidity, we can feel much cooler than the actual temperature because our sweat evaporates easily, cooling us off. For example, if the air temperature is 75 degrees Fahrenheit (24 degrees Celsius) and the relative humidity is zero percent, the air temperature feels like 69 degrees F (21 C) to our bodies. If the air temperature is 75 degrees F (24 C) and the relative humidity is 100 percent, we feel like it’s 80 degrees F (27 C) out, and you start wishing that you had the air conditioner serviced last fall.
The Whole (Dew) Point of the Matter
The higher the dew point, the muggier it will be. Antoni Agelet/Getty Images
People tend to feel most comfortable at a relative humidity of between 30 and 50 percent. Humidifiers and dehumidifiers help to keep indoor humidity at a comfortable level. They also help to dry interior structures like drywall and lumber to prevent them from deteriorating due to moisture and subsequent mold [sources: Keefe, EPA].
If you’re an outdoorsy person or just particularly sensitive to that clammy, damp feeling you detect outside, it’s vital to understand the difference between relative humidity (RH) and dew point, because the latter will actually give you a better idea of just how quickly you’ll become uncomfortable with any exertion.
The dew point is, in short, the point at which dew droplets form on objects like grass — in other words, it’s when a relative humidity of 100 percent is achieved [source: National Weather Service].
The higher the dew point, the muggier it will be and the more uncomfortable you’ll become. A dew point around 55 is pretty comfortable, but higher than 65, and you’ll quickly realize how oppressive the situation really is.
Examples to Interpret Weather Reports
For example, a temperature of 80 degrees F (27 C) and a dew point of 60 degrees F (15 C) would mean an RH of 50 percent. (Humidity is calculated using a formula combining vapor pressure, temperature, dew point and other factors [source: WHIO]). That’s a level that most people won’t enjoy unless they have easy access to a nice, cool air conditioner.
So when you hear a weather forecaster say, “It’s 85 degrees outside, but with the humidity, it feels like 92 degrees,” that 92 is the combination of the temperature and the dew point, also known as the heat index.
As a quick rule of thumb, the closer the dew point is to the actual air temperature, the more likely it is that you’ll feel like the air is too humid, but keep in mind that air temperature does affect our perception of humidity.
For example, on a winter day of 35 degrees F (1.6 C), if the RH is 94 percent, the dew point would be 34 degrees F. But because of the cold air, you’re much less likely to complain about the muggy conditions. So if you’re not a fan of tropical jungle conditions, just remember: You can always move to North Dakota.
Humidity FAQ
What is relative humidity?
Relative humidity is a percentage that represents the amount of water vapor in the air at a given temperature compared to the max possible water vapor amount at that same temperature. For instance, relative humidity of 25 percent means that air is at a quarter of its possible water vapor capacity.
What is absolute humidity?
Absolute humidity refers to the density of water vapor in the air and is generally expressed in g/m3. The higher the air temperature is, the more water vapor it can potentially contain.
What’s the difference between absolute humidity and relative humidity?
Both readings measure water vapor levels in the air. However, relative humidity measures how much water vapor the air actually holds by comparing it to the maximum achievable amount at a given temperature. In contrast, absolute humidity simply measures the total amount of water vapor air can hold without considering the temperature.
What is an uncomfortable level of humidity?
Most people are comfortable with a relative humidity level of 30 to 50 percent. Higher levels are uncomfortable because there’s too much moisture in the air, making people sweaty and unable to cool down.
What causes high levels of humidity?
High humidity (which is anything over 50 percent or so) is caused by high temperatures and high dew points.
Lots More Information
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More Great Links
Sources
Environmental Protection Agency. “Mold Course Chapter 2.” (April 19, 2018) https://www.epa.gov/mold/mold-course-chapter-2Geography for Kids. “Humidity.” (April 19, 2018) https://kidsgeo.com/geography-for-kids/humidity/Keefe, Zachary. “Industrial Hygiene: What is Relative Humidity and Why is it Important?” Cashins & Associates. Aug. 15, 2014. (April 19, 2018)National Weather Service. “Dew Point vs. Humidity.” (April 19, 2018) https://www.weather.gov/arx/why_dewpoint_vs_humidityUniversity of Illinois Extension. “Relative Humidity.” (April 19, 2018) https://extension.illinois.edu/treehouse/clouds.cfm?Slide=2WHIO: “Dew point and humidity: What’s the difference?” June 24, 2015 (May 30, 2018) https://www.whio.com/news/dew-point-and-humidity-what-the-difference/GcjsRuIGq6pvHu69M3ZBXM/
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