A chemist explains why your hair curls in the summer

If you have curly hair, you know that every day is a new adventure. What will my hair do today? Why does it curl better on some days than others? And even those without naturally curly hair might notice their hair curling – or, let’s be honest, frizzing – a bit on humid summer days.

As a person with curly hair, I’m always looking for the best way to care for and understand my hair. As a chemist, I’m interested in the science behind how my hair behaves at the molecular level. There are different hair types, from straight to curly, and they behave differently depending on their structure. But what hairs are made up of at the molecular level is the same.

Hair structure

Hair begins growing under the skin’s surface, but it’s what happens after it pokes through the skin that determines whether you have a good hair day or a bad one.

The innermost, or core layer, is the medulla. This layer holds moisture, much like the pith in the center of a tree trunk. This layer is also very fragile, but only thick or coarse hairs contain this part – so those with thin or blond hair typically don’t have the medulla layer in their hairs.

Next is the cortex,...

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