​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.

While I was researching the "extinction of blondes" I came across similar claims about redheads.

Over the years many news organizations have reported that redheads or "gingers," would eventually become extinct.  Other news outlets and blogs picked up the story, citing the "Oxford Hair Foundation" or "genetic scientists" who claimed that there would be no more redheads by as early as 2060.  It turns out that all those people were wrong. Redheads are here to stay and should be around well beyond 2060.

These articles work on the mistaken assumption that recessive genes -- like the one for red hair -- can "die out." Recessive genes can become rare but don't disappear completely unless everyone carrying that gene dies or fails to reproduce. So while red hair may remain rare, enough people carry the gene that, barring global catastrophe, redheads should continue to appear for some time.

Some of the articles discussing redhead extinction referred to the Oxford Hair Foundation as an "independent" institute or research­ foundation, but a Google search shows that the Oxford Hair Foundation is funded by Proctor & Gamble, makers of numerous beauty products -- including red hair dye.

In the most recent wave of redhead extinction warnings, some news outlets incorrectly cited the September 2007 issue of National Geographic as the source of the extinction claims. Others, correctly, cited that issue of National Geographic for the statistics it presented in a short piece on redheads. In fact, the National Geographic story provided some data about red hair in the world population, but it only said that "news reports" have claimed that redheads were going extinct. The piece did not explicitly back the claim. Instead, the article stated that "while redheads may decline, the potential for red isn't going away". Unfortunately the misconception about disappearing redheads is now widespread.

Experts who have been interviewed agree that the redhead extinction claim is bogus. David Pearce from the University of Rochester Medical Center told the Rochester Democrat and Chronicle in 2005 -- after the last round of redhead extinction news -- that the scientists behind the original claim should "check their calculator".  Rick Sturm, a researcher in hair and skin genetics at the University of Queensland, told the Australian Broadcasting Company that "there's no shortage of red-heads" and that the Oxford Hair Foundation didn't provide sufficient scientific evidence to prove its findings.

Red hair is caused by a mutation in the MC1R gene. It's also a recessive trait, so it takes both parents passing on a mutated version of the MC1R gene to produce a redheaded child. Because it's a recessive trait, red hair can easily skip a generation. It can then reappear after skipping one or more generations if both parents, no matter their hair color, carry the red hair gene.

(source: Science: HowStuffWorks)

I was hanging out with a few friends earlier today and we were talking a hair (of course we were) and someone mentioned that there wouldn't be any more blondes in a few hundred years.   I'd never heard this before so I thought I'd research their comment.   

The Claim:  A study undertaken by the World Health Organization concluded that natural blondes are likely to be extinct within 200 years.

Well, this is completely FALSE.  

The problem is that blonde hair is caused by a recessive gene.

In order for a child to have blonde hair, it must have the gene on both sides of the family in the grandparents' generation.  

The researchers also believe that so-called bottle blondes may be to blame for the demise of their natural rivals.
 
They suggest that dyed-blondes are more attractive to men who choose them as partners over true blondes.
 
But Jonathan Rees, professor of dermatology at the University of Edinburgh said it was unlikely blondes would die out completely.  "Genes don't die out unless there is a disadvantage of having that gene or by chance. They don't disappear," he told BBC News Online.  "The only reason blondes would disappear is if having the gene was a disadvantage and I do not think that is the case.
 
"The frequency of blondes may drop but they won't disappear."  [source: BBC News].

(Source: Science: HowStuffWorks)

SNOPES.com also debunked this claim.