WEBVTT - What Is Relative Humidity?

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<v Speaker 1>Welcome to brain Stuff production of I Heart Radio. Hey

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<v Speaker 1>brain Stuff, I'm Lauren vocal Bam. And if you've ever

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<v Speaker 1>been in the American South on a hot August day,

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<v Speaker 1>you're familiar with the incredible mugginess of borderline hallucinatory experience,

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<v Speaker 1>where you feel like you're walking in a stew of

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<v Speaker 1>your own perspiration. But there's more to understanding that clammy

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<v Speaker 1>gross feeling than simply glancing at a weather forecaster's humidity reading.

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<v Speaker 1>To really get a grasp on how humidity affects your health,

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<v Speaker 1>your home, and your sanity, you need insights on the

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<v Speaker 1>types of humidity, as well as the concept of dew point.

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<v Speaker 1>Humidity can be measured in several ways, but relative humidity

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<v Speaker 1>is the most common. But in order to understand relative humidity,

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<v Speaker 1>it's helpful to first understand absolute humidity. Absolute humidity is

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<v Speaker 1>the mass of the water vapor divided by the mass

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<v Speaker 1>of the dry air in a given volume of air

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<v Speaker 1>at a given temperature. The hotter the air is, the

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<v Speaker 1>more water it can contain, which is why it's never

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<v Speaker 1>quite so humid during cold winters. Relative humidity is the

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<v Speaker 1>ratio of the current absolute humidity to the highest possible

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<v Speaker 1>absolute humidity at the given air temperature. A reading of

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<v Speaker 1>one percent relative humidity means that the air is totally

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<v Speaker 1>saturated with water vapor and cannot hold anymore, creating the

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<v Speaker 1>possibility of rain. This doesn't mean that the relative humidity

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<v Speaker 1>must be one in order for it to rain. It

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<v Speaker 1>must be one percent where the clouds are forming, but

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<v Speaker 1>the relative humidity near the ground could be much lower.

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<v Speaker 1>Humans are very sensitive to humidity, as our skin relies

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<v Speaker 1>on the air to get rid of moisture. The process

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<v Speaker 1>of sweating is your body's attempt to keep cool and

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<v Speaker 1>maintain its current temperature because evaporation uses some of the

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<v Speaker 1>thermal energy in the air to drive the transition of

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<v Speaker 1>liquid sweat to water vapor, making it cooler around you.

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<v Speaker 1>But if the air is already totally saturated at one

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<v Speaker 1>percent relative humidity, sweat will not evaporate into the air.

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<v Speaker 1>As a result, we feel much hotter than the actual temperature.

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<v Speaker 1>When the relative humidity is high, your clothes may become

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<v Speaker 1>saturated with perspiration that can't go anywhere, prompting the feeling

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<v Speaker 1>of being a swampy bog monster of revolting proportions. If

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<v Speaker 1>the relative humidity is low, However, we can feel much

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<v Speaker 1>cooler than the actual temperature because our sweat evaporates, easily,

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<v Speaker 1>cooling us off. For example, if the air temperature is

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<v Speaker 1>seventy five degrees fahrenheit, that's twenty four celsius and the

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<v Speaker 1>relative humidity is zero percent, the air temperature will feel

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<v Speaker 1>like sixty nine fahrenheit or twenty one celsius to our bodies.

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<v Speaker 1>But if the air temperature is seventy five degrees fahrenheit

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<v Speaker 1>and the relative humidity is one, we feel like it's

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<v Speaker 1>eighty degrees fahrenheit or twenty seven celsius. People tend to

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<v Speaker 1>feel most comfortable at a relative humidity between thirty and fifty.

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<v Speaker 1>Humidifiers and de humidifiers helped to keep indoor humidity at

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<v Speaker 1>a comfortable level. They also vitally helped dry interior structures

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<v Speaker 1>like drywall and lumber to prevent them from deteriorating due

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<v Speaker 1>to moisture and subsequent mold. If you and out doorsy person,

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<v Speaker 1>or just particularly sensitive to that clammy damp feeling you

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<v Speaker 1>detect outside, it's vitally important to understand the difference between

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<v Speaker 1>relative humidity and dew point, because the latter will actually

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<v Speaker 1>give you a better idea of just how quickly you'll

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<v Speaker 1>become uncomfortable with any exertion, and it's perhaps more often

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<v Speaker 1>reported in weather forecasts. The dew point is, in short,

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<v Speaker 1>the point at which dew droplets form on objects like grass.

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<v Speaker 1>In other words, the temperature which a relative humidity of

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<v Speaker 1>one is achieved. The higher the dew point, the muggier

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<v Speaker 1>it will be, and the more uncomfortable you'll become. A

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<v Speaker 1>dew point around fifty five is pretty comfortable, but higher

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<v Speaker 1>than sixty five and you'll quickly realize how oppressive the

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<v Speaker 1>situation really is. For example, a temperature of eighty degrees

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<v Speaker 1>fahrenheit that's twenty seven celsius and a dew point of

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<v Speaker 1>sixty degrees fahrenheit or fifteen celsius would mean a relative

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<v Speaker 1>humidity of fifty percent. Humidity is calculated using a formula

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<v Speaker 1>combining vapor pressure, temperature, dew point, and other factors. That

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<v Speaker 1>relative is a level that most people won't enjoy unless

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<v Speaker 1>they have easy access to a nice cool air conditioner.

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<v Speaker 1>So when you hear a weather forecasters say it's eighty

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<v Speaker 1>five degrees outside, but with the humidity it feels like

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<v Speaker 1>ninety two degrees, that ninety two is a combination of

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<v Speaker 1>the temperature and the dew point, also known as the

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<v Speaker 1>heat index. As a rule of thumb, the closer the

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<v Speaker 1>dew point is to the actual air temperature, the more

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<v Speaker 1>likely it is that you'll feel like the air is

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<v Speaker 1>uncomfortably humid. Today's episode was written by Nathan Chandler and

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<v Speaker 1>produced by Tyler Clang. Brain Stuff is a production of

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<v Speaker 1>iHeart Radio's How Stuff Works. For more on this and

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<v Speaker 1>lots of other dewey topics, visit our home planet, how

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<v Speaker 1>stuff Works dot com. And for more podcasts for my

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