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| 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. |
Relative Humidity
What
Is Relative Humidity and How Does it Affect How I Feel Outside?
BY Nathan Chandler
If
you've ever been to southern Arkansas on a hot July day, you're familiar with
incredible mugginess, a borderline hallucinatory experience where you feel like
you're walking 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.
To
really get a grasp on how humidity affects your health, 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 (RH) is the most common.
In
order to understand RH, it is helpful to first understand absolute 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
are very sensitive to 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 the relative
humidity is high. Your shirt may become saturated with perspiration that
doesn't go anywhere, leaving you feeling like a swampy bog monster of revolting
proportions.
If
the relative humidity is low, 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 Fahrenheit (21 C) to our bodies.
If
the air temperature is 75 degrees Fahrenheit (24 C) and the relative humidity
is 100 percent, we feel like it's 80 degrees (27 C) out, and you start praying
that you had the air conditioner serviced last fall.
The Whole (Dew)
Point of the Matter
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 —
vitally — help to dry interior structures like drywall and lumber to
prevent them from deteriorating due 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 vitally important 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, 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.
For
example, a temperature of 80 degrees F (27 C) and a dew point of 60 degrees F
(15 degrees C) would mean a 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 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 35-degree F (1.6 C) winter day, if the RH is 94 percent, the dew
point would be 34 degrees F.
But
because the air is frigid, 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.
Nathan Chandler, Contributing Writer
Nathan Chandler is a freelance writer and photographer based in Lincoln, Neb. He earned his B.A. from the University of Northern Iowa, and has researched and written about consumer tech for more than 10 years.
Nathan Chandler is a freelance writer and photographer based in Lincoln, Neb. He earned his B.A. from the University of Northern Iowa, and has researched and written about consumer tech for more than 10 years.
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