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What is Relative Humidity?

Relative humidity measures the amount of water vapor in the air compared to the maximum it can hold at a given temperature. It's crucial for understanding comfort levels and weather patterns. High relative humidity can make warm days feel hotter. How does this affect your daily life, and what can you do to stay comfortable? Join us to uncover more.
Tricia Christensen
Tricia Christensen
Tricia Christensen
Tricia Christensen

Many people notice that hot muggy days seem much warmer somehow than days with dry heat. One of the reasons for this is humidity, the amount of water the air holds. When the air holds more water, the basic act of perspiration or sweating is less efficient in cooling down the body. More water in the air means less water can evaporate from the skin, and most people will end up feeling warmer. Some of the ways that humidity is measured is by measuring absolute humidity, and relative humidity, which is often how people tell how “wet” a day will be.

First, it’s important to understand that air can only hold a certain amount of water at any given time. This measurement is absolute humidity, and absolute humidity is dependent on the temperature of air. Under many circumstances, the actual air holds far less water than it technically could, so the term relative comes into play. When meteorologists discuss relative humidity they usually do so in percentage amounts, and this percentage is a ratio of how much water the air does hold in comparison to how much water it could hold. The actual formula is water amount (actual vapor density) divided by total possible water amount (saturation vapor density) times 100%. Most people will see the expression of this formula often in relative humidity counts when they watch or read weather reports.

High humidity can make people feel colder than they otherwise would.
High humidity can make people feel colder than they otherwise would.

That percentage or the relative humidity of a day can tell people how warm they may feel under given circumstances. Air that is drier may not feel as hot in warmer temperatures. Air at about 45% relative humidity is going to feel most like the temperature that is outside. Anything above this level may make the day feel warmer than it truly is at certain temperatures.

When meteorologists discuss relative humidity they usually do so in percentage amounts.
When meteorologists discuss relative humidity they usually do so in percentage amounts.

Temperature perception may be affected by relative humidity in the reverse. On cold days, usually those below 53 degrees F (11.67 degrees C), higher humidity can actually make people feel colder than they normally would. Though other determinants like wind chill may affect “temperature feel” and perception, relative humidity in cold weather may also be an important factor. Freezing weather with a near 100% relative humidity may be much chillier in feel than freezing weather with a lower level of humidity.

Relative humidity is determined by the amount of moisture in the air versus how much the air is capable of holding at that specific temperature.
Relative humidity is determined by the amount of moisture in the air versus how much the air is capable of holding at that specific temperature.

The relative measurement of humidity cannot be the only determination of how weather will feel; the amount of wind, especially in colder temperatures and other factors come into play. Moreover, individuals can be more or less sensitive to certain temperatures. However, it is one good way of determining just how hot or cold a day might feel.

Tricia Christensen
Tricia Christensen

Tricia has a Literature degree from Sonoma State University and has been a frequent AllTheScience contributor for many years. She is especially passionate about reading and writing, although her other interests include medicine, art, film, history, politics, ethics, and religion. Tricia lives in Northern California and is currently working on her first novel.

Learn more...
Tricia Christensen
Tricia Christensen

Tricia has a Literature degree from Sonoma State University and has been a frequent AllTheScience contributor for many years. She is especially passionate about reading and writing, although her other interests include medicine, art, film, history, politics, ethics, and religion. Tricia lives in Northern California and is currently working on her first novel.

Learn more...

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Discussion Comments

Lostnfound

@Grivusangel -- That meteorologist was probably one who looks more at dewpoints. That's the "second" temperature you see that indicates how saturated the air is for the temperature. The closer the dewpoint is to the actual air temperature the more humid it is -- and often, the more unstable the air is. Dewpoint is a big factor in determining the possibility of severe weather in a given area.

By the same token, during the winter, the dewpoint is often a good predictor of how low the temperature will drop overnight. The lower the dewpoint, the lower the temperature will likely go, since dry air doesn't hold heat nearly as well.

Still, relative humidity is, as you say, a good indicator for the average person of how muggy the day may be.

Grivusangel

One local meteorologist says relative humidity is "relatively useless." I guess that's from a scientific point of view. From a layman's way of looking at it, relative humidity is at least a point of reference for how miserable the heat will make you on a given day.

It's pretty much a given that a humidity reading below 50 percent is going to mean the air is drier and more comfortable, even if the air itself is warm.

I've been in 95-degree temperatures that felt fairly comfortable because the humidity was low. But I've been outside when the temperature was 80 degrees, but humidity was really high and it was miserable.

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    • High humidity can make people feel colder than they otherwise would.
      By: visivasnc
      High humidity can make people feel colder than they otherwise would.
    • When meteorologists discuss relative humidity they usually do so in percentage amounts.
      By: Innovated Captures
      When meteorologists discuss relative humidity they usually do so in percentage amounts.
    • Relative humidity is determined by the amount of moisture in the air versus how much the air is capable of holding at that specific temperature.
      By: Are Barstad
      Relative humidity is determined by the amount of moisture in the air versus how much the air is capable of holding at that specific temperature.
    • Perspiration is less efficient at cooling the body on humid days.
      By: Dirima
      Perspiration is less efficient at cooling the body on humid days.