Why does your hair curl in the summer? A chemist explains the science behind hair structure

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.

A diagram showing the three hair layers: medulla (innermost), then cortex, then cuticle (outermost)

Each hair can have three layers – the medulla, the cortex and the cuticle. You can think of each hair like a tiny tree trunk.

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, which makes up most of a hair and is analogous to the wood of a tree. The cortex is made up of spring-shaped protein molecules that lie in parallel rows in a cylindrical bundle. The exact shape of that bundle is determined by the hair follicle, which is a pore on the skin from where the hair grows.

How the hair grows out of the follicle influences the distribution of its proteins. So a straight follicle produces straight hair and a curved follicle produces curly hair. The less evenly distributed the squiggly proteins are, the curlier the hair. Your genetic code also plays a role in the shape of the cortex and, therefore, the shape and thickness of your hair.

Lastly, the outermost layer of a hair is called the cuticle. The cuticle is like the bark of a tree – and it even looks like bark under a microscope.

A microscope image of a hair cuticle, which looks like a long, fraying cylinder.

It’s the cuticle’s job to protect the cortex, but the cuticle is very easily damaged. Imagine lifting or removing the bark from a tree. Doing so would leave the wood inside susceptible to moisture loss, exposure to the environment and damage.

The same is true for each hair. When the cuticle is damaged from brushing, chemicals, wind or heat, the proteins of the cortex have a much more difficult time lying smoothly together. This means they can lose moisture, gain moisture, fray like a rope – this causes split ends – and even break. All these factors can influence how your hair looks at any given moment.