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You know what they say: It's not the heat, it's the humidity.

And that's extra true if your hair is the frizzy type.

The higher the humidity, the more prone some of us are to a frizz-tastrophe, and it all has to do with how water interacts with the proteins in our hair.

The really weird thing, though, is what hair has been able to teach us about atmospheric water.

Frizz is what you get when individual strands of hair change shape and stop matching the neighboring strands.

This can mean getting curly, wavy, or otherwise infuriatingly irregular.

Frizzing has to do with hair's composition.

Hair is made up of a long, fibrous protein called keratin, bundled layer upon layer in an elaborate structure.

The keratin molecules stay together through a combination of strong and weak chemical interactions.

The real beefy chemical bonds are called disulfide bonds, in which sulfur atoms from neighboring protein strands join together in a pretty tight hold.

And the weaker interactions are what's known as hydrogen bonds.

Hydrogen bonds aren't true chemical bonds.

Rather, they're a weak attraction between a slight positive charge and a slight negative charge.

For hair, the hydrogen bonding happens between keratin molecules and water molecules.

When a water molecule bonds to two different keratins, it can help hold them together—to an extent, but that hold can be easily broken or changed.

And when more water enters the picture, it can change how keratin sticks together.

Getting your hair wet can break previous hydrogen bonds and form new ones, resulting in a new shape to your hair.

And when your hair dries in a certain shape, it tends to stay—which is why going to bed with your hair wet is asking for epic bedhead.

When there's lots of water in the air in the form of high humidity, the changes to our keratin molecules are more unpredictable.

Frizz occurs when water is absorbed into dry hair in different areas at different rates.

Some strands of hair may absorb water into the center, while others may absorb less, or none at all, causing differences in swelling that affect hair shape.

Alternatively, the higher moisture may cause some strands to form so many hydrogen bonds around the outer layers that the hair folds back on itself, forming curls.

And the water absorption is pretty random.

No two adjacent hairs respond to airborne moisture the same way, resulting in the frizzy mess that we find so frustrating.

They all get bent out of shape differently.

But the amount of water a hair absorbs is ultimately still proportionate to the relative humidity.

Like, it's predictable enough to be useful for making measurements.

Measuring humidity was a goal of scientists back in the day who wanted to know more about what caused rain, and how predictable various weather phenomena could be.

Enter the hygrometer, a device for measuring relative humidity.

One of the first hygrometers was made in 1783 by Horace Bénédict de Saussure.

And his worked using human hair.

This may seem weird and kinda gross, but it's both effective and something you can try for yourself!

Because dry hair will absorb atmospheric moisture, you can reliably observe a long strand of hair to change shape as humidity changes.

All you have to do is fix one end of a clean, de-oiled strand of hair in place and hang a weight from the other end to keep it stretched.

Then you can observe the length of the hair changing as humidity increases or decreases.

The proteins coil and uncoil as hydrogen bonds form and break, all as a function of how much water is in the air.

So the resulting variation in the strand's length is proportionate to the humidity in the air.

You can even calibrate relative humidity by using a hair dryer to simulate 0% and a wet rag for 100%, allowing you to get a sense of how far between those two extremes the air is on any given day.

So while it might be a seasonal annoyance, at least frizz has some practical applications!

Thanks for watching this episode of SciShow, which was sponsored by Raycon.

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