What Is Relative Humidity?

Relative humidity can make pleasant temperatures unbearable (and vice versa). Learn why -- and what the dew point has to do with it -- in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff production of I Heart Radio. Hey
brain Stuff, I'm Lauren vocal Bam. And if you've ever
been in the American South on a hot August day,
you're familiar with the incredible mugginess of 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

(00:22):
gross feeling than simply glancing at a weather forecaster's humidity reading.
To really get a grasp on how humidity affects your health,
your home, and your sanity, you need insights on the
types of humidity, as well as the concept of dew point.
Humidity can be measured in several ways, but relative humidity
is the most common. But in order to understand relative humidity,

(00:44):
it's helpful to first understand absolute humidity. Absolute humidity is
the mass of the water vapor divided by the mass
of the dry air in a given volume of air
at a given temperature. The hotter the air is, the
more water it can contain, which is why it's never
quite so humid during cold winters. Relative humidity is the

(01:04):
ratio of the current absolute humidity to the highest possible
absolute humidity at the given air temperature. A reading of
one percent relative humidity means that the air is totally
saturated with water vapor and cannot hold anymore, creating the
possibility of rain. This doesn't mean that the relative humidity
must be one in order for it to rain. It

(01:25):
must be one percent where the clouds are forming, but
the relative humidity near the ground could be much lower.
Humans are very sensitive to humidity, as our 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 because evaporation uses some of the
thermal energy in the air to drive the transition of

(01:46):
liquid sweat to water vapor, making it cooler around you.
But if the air is already totally saturated at one
percent relative humidity, sweat will not evaporate into the air.
As a result, we feel much hotter than the actual temperature.
When the relative humidity is high, your clothes may become
saturated with perspiration that can't go anywhere, prompting the feeling

(02:07):
of being a swampy bog monster of revolting proportions. If
the relative humidity is low, However, we can feel much
cooler than the actual temperature because our sweat evaporates, easily,
cooling us off. For example, if the air temperature is
seventy five degrees fahrenheit, that's twenty four celsius and the
relative humidity is zero percent, the air temperature will feel

(02:28):
like sixty nine fahrenheit or twenty one celsius to our bodies.
But if the air temperature is seventy five degrees fahrenheit
and the relative humidity is one, we feel like it's
eighty degrees fahrenheit or twenty seven celsius. People tend to
feel most comfortable at a relative humidity between thirty and fifty.
Humidifiers and de humidifiers helped to keep indoor humidity at

(02:49):
a comfortable level. They also vitally helped dry interior structures
like drywall and lumber to prevent them from deteriorating due
to moisture and subsequent mold. If you and out doorsy person,
or just particularly sensitive to that clammy damp feeling you
detect outside, it's vitally important to understand the difference between
relative humidity and dew point, because the latter will actually

(03:10):
give you a better idea of just how quickly you'll
become uncomfortable with any exertion, and it's perhaps more often
reported in weather forecasts. The dew point is, in short,
the point at which dew droplets form on objects like grass.
In other words, the temperature which a relative humidity of
one is achieved. The higher the dew point, the muggier

(03:31):
it will be, and the more uncomfortable you'll become. A
dew point around fifty five is pretty comfortable, but higher
than sixty five and you'll quickly realize how oppressive the
situation really is. For example, a temperature of eighty degrees
fahrenheit that's twenty seven celsius and a dew point of
sixty degrees fahrenheit or fifteen celsius would mean a relative
humidity of fifty percent. Humidity is calculated using a formula

(03:55):
combining vapor pressure, temperature, dew point, and other factors. That
relative is 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 forecasters say it's eighty
five degrees outside, but with the humidity it feels like
ninety two degrees, that ninety two is a combination of

(04:15):
the temperature and the dew point, also known as the
heat index. As a 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
uncomfortably humid. Today's episode was written by Nathan Chandler and
produced by Tyler Clang. Brain Stuff is a production of

(04:36):
iHeart Radio's How Stuff Works. For more on this and
lots of other dewey topics, visit our home planet, how
stuff Works dot com. And for more podcasts for my
heart Radio, visit the iHeart Radio app, Apple Podcasts, or
wherever you listen to your favorite shows.