Sometimes decoding the weather forecast
is more than just the highs and lows,
especially when it comes to humidity.
Buzzy weather words like dew point or
relative humidity often leave forecast
lay people high and dry when it comes to
actually knowing what to wear. So stay
tuned for our special report on how to
make sense of the chemistry behind muggy
weather. Humidity is a measure of how
much water is dissolved into the air.
It's a pretty straightforward concept.
Where it gets tricky is that there's an
upper limit to the amount of water that
can be dissolved in the air. That changes
based on the temperature. The hotter the
air, the more water can be dissolved in
it. So for example if the temperature is
41 degrees Fahrenheit,
you can dissolve up to five grams of
water into a kilogram of air. But if the
temperature is 77 degrees you can
dissolve up to 20 grams. When it comes to
back sweat you would assume that 20
grams per kilogram is going to feel much
much stickier than 5 grams per kilogram,
but it might surprise you to know that
your local weather forecast would report
these two examples of humidity as the
exact same: 100%. When you see this
percentage of humidity on the local
forecast, the meteorologist is referring
to something called relative humidity.
This is a measure of how much water is
dissolved in the air divided by how much
water could be dissolved in it at a
specific temperature. In other words,
relative humidity shows how close the
air is to full saturation. 85 percent
relative humidity means the air is 85
percent saturated. Both of our examples
are at a hundred percent saturation, or a
hundred percent relative humidity. So why
is it that the second example would feel
so much more sticky? Because of how your
body cools itself with sweat. Sweat cools
your body down because water molecules
in it absorb thermal energy from your
skin, evaporate, and take some of the heat
energy with them. If the air is at a
hundred percent relative humidity
there's literally
nowhere for that sweat to evaporate off
to so it stays stuck dripping down your
lower back or in your armpits. At lower
temperatures your body won't produce as
much sweat, so it's less of a problem. If
you live in a place with consistent
year-round temperatures, you can rely on
relative humidity to inform your
wardrobe choices, but anywhere else -- and
we cannot all be so blessed as to live
in San Diego -- it alone isn't going to
give you the info you need to be
prepared for the day. This is why
meteorologists also use heat index and
dew point to clue us in. Heat index
supposedly tells you how hot it feels
with any given temperature and humidity.
So, for example, if it's 92 degrees
Fahrenheit with a relative humidity of
85 percent, the heat index is a hundred
and twenty six degrees Fahrenheit, and
the National Weather Service would tell
you that heatstroke is likely, so stay
inside. This can be useful, but how it
feels is pretty subjective. Dew point, on
the other hand, is objective. It's the
temperature that the air needs to be
cooled to to reach 100% relative humidity.
You can imagine that if the dew point is
high, it's going to be a pretty muggy day.
To understand how it works, take a look
at this little box here. Let's say that
inside the box it's 55 degrees
Fahrenheit with the relative humidity of
90%. At this temperature, the air is
capable of having more water dissolved
in it than is physically inside of the
box. If we lower the temperature slightly
to 52 degrees Fahrenheit, the upper limit
of the air decreases slightly to an
amount equal to that of the water
trapped inside of the box. This means
that we've reached 100% relative
humidity. This 52 degrees sweet spot
represents the dew point. If you get
colder than the dew point, the air would
be unable to dissolve all the water and
condensation or dew would begin forming
inside. That means the dew point can't be
higher than the temperature of the air
because when the two temperatures meet
dew starts to form. To stay crisp, we want
to stay away from high relative humidity.
So keeping an eye on dew point helps you know
the temperature to look out for. You can
use this to your advantage when watching
the weather. As a general rule, most
people will feel comfy and crisp
if the dew point is below 60 regardless
of the temperature. Dew points in the 70s
often feel sticky and gross.
Pro tip: the best way to work with dew
points is to keep an eye on them in the
forecast and then see how you feel
outside. You'll develop a sense of your
favorite dew point range -- as nerdy as it
is to have a favorite dew point range --
and then you can start completely
ignoring relative humidity. You're
welcome.
Thanks for watching our special report
on the weather report. For more, remember
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more great chemistry content. We'll see
you next week.