​Humid air causes hydrogen bonds to form between water molecules and the proteins in your hair, triggering curls and frizz
 
By Joseph Stromberg  - smithsonian.com
 
If you have long hair, you probably don’t need to look up a weather report to get an idea of how much humidity’s in the air: You can simply grab a fistful of hair and see how it feels. Human hair is extremely sensitive to humidity—so much that some hygrometers (devices that indicate humidity) use a hair as the measuring mechanism, because it changes in length based on the amount of moisture in the air.
 
Straight hair goes wavy. If you have curly hair, humidity turns it frizzy or even curlier. Taming the frizz has become a mega industry, with different hair smoothing serums promising to “transform” and nourish hair “without weighing hair down.” But just why does humidity have this strange effect on human hair?
 
Hair’s chemical structure, it turns out, makes it unusually susceptible to changes in the amount of hydrogen present in the air, which is directly linked to humidity.  Most of a hair’s bulk is made up of bundles of long keratin proteins, represented as the middle layer of black dots tightly packed together in the cross-section below.

​Bundles of keratin proteins (the middle layer of black dots above) are susceptible to changing shape on a humid day. Image from Gray’s Anatomy

These keratin proteins can be chemically bonded together in two different ways. Molecules on neighboring keratin strands can form a disulfide bond, in which two sulfur atoms are covalently bonded together. This type of bond is permanent—it’s responsible for the hair’s strength—and isn’t affected by the level of humidity in the air.
 
But the other type of connection that can form between adjacent keratin proteins, a hydrogen bond, is much weaker and temporary, with hydrogen bonds breaking and new ones forming each time your hair gets wet and dries again. (This is the reason why, if your hair dries in one shape, it tends to remain in roughly that same shape over time.)
 
Hydrogen bonds occur when molecules on neighboring keratin strands each form a weak attraction with the same water molecule, thereby indirectly bonding the two keratin proteins together. Because humid air has much higher numbers of water molecules than dry air, a given strand of hair can form much higher numbers of hydrogen bonds on a humid day. When many such bonds are formed between the keratin proteins in a strand of hair, it causes the hair to fold back on itself at the molecular level at a greater rate.
 
On the macro level, this means that naturally curly hair as a whole becomes curlier or frizzier due to humidity. As an analogy, imagine the metal coil of a spring. If you straighten and dry your hair, it’ll be like the metal spring, completely straightened out into a rod. But if it’s a humid day, and your hair is prone to curling, water molecules will steadily be absorbed and incorporated into hydrogen bonds, inevitably pulling the metal rod back into a coiled shape.

Why Face Shape and Hair Texture Matters Most

By Julyne Derrick
​
In this article, you'll find out what hairstyles work best for thin hair, thick hair and curly hair. You'll figure out how to pick the perfect haircut by face shape and hair texture.
 
Check out my picture down on the bottom of this page.  I have a long face and a high forehead (I was once dubbed 'IMAX' by a frat guy in college). In my lifetime, I've had short hair, shoulder-length hair and long hair.
 
Of all the hairstyles I've had, a shoulder-length cut with side-swept bangs looks best on me. It works with my face shape AND my hair texture, which is the key to all great haircuts.

Consider Your Face Shape
The secret to great haircuts is two-fold: first you must consider your face shape and secondly, you need to choose a cut that works with your hair's texture. Find out what your face shape is.
 
Just as there are flattering styles for all face shapes, there are also unflattering styles. Get the scoop on which cuts work and which ones don't with your face shape in The Best & Worst Styles for Every Face Shape.
 
Not everyone should worry about her face shape, find out if you should in Does Your Face Shape Really Matter?

Choose a Cut That Works With Your Hair Texture
Each one of us can classify our hair into one or more hair texture types: Dry, coarse, curly, wavy, straight, fine, stringy, limp, oily. If you have thin, stringy hair that tends to fall flat, your hair will look fuller the shorter it is.
 
Coarse, curly girls need to weigh hair down with length: The shorter you go, the higher the frizz and fluff factor. Same thing with bangs. If you have curly, coarse hair, bangs are an invitation to frizziness. Straight hair can fall flat, but cutting in layers adds body. And no matter your hair type, damaged hair never looks good. Cut it off.

The Most-Flattering Haircut on Everyone
I was struck a couple years ago by a passage in my favorite beauty bible, "Confessions of a Beauty Editor". The writer, Linda Wells, stated that almost everyone at Allure Magazine where she works has shoulder-length hair. Turns out shoulder-length hairstyles are universally the most flattering on nearly everyone.
 
Long hair can make a long face appear longer. Short hair can make a round face appear rounder. Short hair is especially dangerous on super curly hair. If you are petite, you could look like a 15-year-old with super long hair. But shoulder-length hair looks good on all women no matter their size, face shape or age.

Who Can, and Can't, Go Long?
Long hair generally means below the shoulders. Women who generally shouldn't go long are super-short people (you'll forever look age 12) and women with long, narrow faces (like me, wahhh).
 
Women who look fabulous in long cuts have oval or square faces. That said, anyone with hair below their breasts is taking a risk of looking outdated.

There is One Long Cut That's Universally Flattering
The 'v-cut' is long in the back and shorter along the sides. This cut works because the length is in the back and not around the face. Lip and chin-grazing layers on the side add to the sexy allure.

The Basic Dos and Don'ts For Short Hair
Short hair comes in a variety of lengths. In order to figure out which length works for you, consider your best and worst assets and your height. Super short hair on very tall women can look too masculine or it can look very powerful (depending on how you see it).
 
Short hair on women with round faces can make the round face appear fuller, however rules are meant to be broken, as my popular gallery, The Best Short Hairstyles for Round Face Shapes, proves. Typically, you'll want to grow your hair longer than your chin if you have a round face.
 
If you have a pointy chin, avoid hair that falls right at the chin. If you have great eyes, consider short hair with side-swept bangs that hit right at the eye.

Remember, a good hair cut masks your downfalls and promotes your best features.

Why You Should Consider Upkeep
If you're like me and you hate going to the salon or if you tend to put it off for months on end, ask your stylist for a cut that will grow out nicely. Eva Scrivo of Eva Scrivo Salon in NYC told me a great haircut should last 3 months.

Uh-Oh, What About That Forehead?
Small forehead? Consider bangs that start further back on the head and that are as long as possible
 
Big forehead? Bangs are flattering because they cover up the 'IMAX' screen. Side-sweeping bangs are the most flattering look for you. According to 'Confessions,' bangs are flattering when they are longer at the temples than in the middle.
 
Get the full article with links here