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Posted at 10:45 AM ET, 07/ 9/2008

How Humid's That Air? Ask Your Hair.

By Capital Weather Gang

Wx and the City

By Ann Posegate, Guest Contributor

A hair hygrometer (date unknown). Courtesy NOAA.

D.C.'s tropical rainforest-like summer weather has begun, and I know I'm not the only one out there with a frizzy hair problem as a result. Not sure how a bad hair day is related to weather? Believe it or not, human (and horse) hair is a great indicator for relative humidity.

Keep reading for more on the connection between hair and humidity. Will today's humid air lead to afternoon and evening storms? See our full forecast, and NatCast for tonight's game at Nationals Park.

Humans have been using hair to test humidity since 1783, when Horace Benedict Saussure discovered that the length of human and animal hair changes according to humidity levels and invented the hair hygrometer. Before that, inventors thought that wool, sponge and even rat bladders could accurately measure moisture in the air and predict rain. I don't know about you, but I'd rather pluck a few hairs from my head than watch the expansion and contraction of a rat bladder filled with mercury.

Why hair? Relative humidity measures the saturation of the air, or how much water vapor air "holds" depending on temperature; the higher the moisture, the higher the humidity. (Check out USA Today for a thorough guide to understanding humidity). In a hair hygrometer, several strands of human or horse hair are stretched out and held at a constant pressure. When hair is exposed to water (i.e., high humidity), the hydrogen bonds in keratin -- a protein found in hair -- break, and the hair lengthens. When the air becomes drier, the bonds form again and the hair shortens.

Spending much of my childhood in Arizona and years of my adult life in northern New England, I didn't have to worry much about humid summers until moving to D.C. Now I have a deeper understanding for the subject. And as an informal weather observer, I can now appreciate a bad hair day.

Want to make your own hair hygrometer? Check out these fun experiments.

By Capital Weather Gang  | July 9, 2008; 10:45 AM ET
Categories:  Education, Posegate, Wx and the City  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   StumbleUpon   Technorati   Google Buzz   Previous: Forecast: After Storms, Forecast Finally Changes
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My naturally curly hair gets even more curly in the humidity and so the higher the humidity, the shorter it looks. But the article says that high humidity makes the hair longer. Please explain....

Posted by: L | July 9, 2008 11:12 AM | Report abuse

humidity definitely makes my hair look awful, long or short!

Posted by: madison | July 9, 2008 11:20 AM | Report abuse

I'm with L, above, but my hair and skin think the humidity is an advantage. In humid weather, the natural curl in my hair keeps it tidy according to its cut, but in dry weather (or in winter when I'm in a dry building all the time) it has to be cut shorter because it lies flat like a pancake!

Posted by: RobinD | July 9, 2008 11:33 AM | Report abuse

L -- If you were to stretch a strand of your hair out straight on a humid day, it would likely measure longer than on a dry day. But the overall look is shorter on a humid day because as your curly hair gets longer it curls up even more -- into a more compact space than before. At least that's my understanding.

Posted by: Dan, Capital Weather Gang | July 9, 2008 11:54 AM | Report abuse

I hope I remember enough to give an educated guess (directed at L's frizz):

Hair curl is different than hair length. Hair consists of fibers of cross-linked proteins surrounded by a protective sheath. The number of cross-links determines whether hair is straight or curly or somewhere in between. (aside: heat can break and re-link proteins, hence the curling iron. Chemicals can add more disulfide bonds, hence a "perm") In addition to the cross-linking, there is a weak, inter-molecular attraction due to the hydrogen in the organic material, called a "hydrogen bond." However, water also has its own hydrogen bond, and high humiditiy will force more water into the hair fiber. The presence of the water alters the nature of the cross-link bonds, affecting both length and curl. My guess is that if you were to pull your curlier hair straight, it would be a bit longer than on a day when it isn't so curly. The fact that it takes a more circuitous route to your shoulders probably makes it appear shorter.

Posted by: I Hated Biology | July 9, 2008 12:15 PM | Report abuse

I don't have very much hair left so I can't say what, if anything, humidity does to my hair!

Posted by: Jim from Annapolis | July 9, 2008 12:42 PM | Report abuse

Why do the specifications for human hair hygrometers specify "blonde" human hair??? Is this a case of "gentleman meteorologists preferring blondes" or is there something about the melanin pigment in darker hair shades which throws off the accuracy of a hair hygrometer reading??? To be frank, I have a decided preference for redheads.

In addition nowadays you had better take a close look at the roots. There are quite a few "dyed blondes" (and even a few dyed redheads!) around these days. It would be quite an awful event if the DCA humidity readings were "off" because the airport's hygrometer was provided its hair by a Clairol blonde!

Posted by: El Bombo | July 9, 2008 12:49 PM | Report abuse

El Bombo raises an interesting question - what, if anything, does hair dye do the hair's reaction to humidity levels?

There seems to be an optimum humidity level for my short, thin, straight, flat, naturally blonde hair...and that level is NEVER reached here in NoVA.

Posted by: ~sg | July 9, 2008 1:35 PM | Report abuse

Ugh. I don't care about the scientific specifics. I look like a redheaded poodle!

Posted by: Sara in Oakton | July 9, 2008 2:40 PM | Report abuse

Interesting idea about hair dye v. humidity. I don't know a whole bunch on the subject, but I did spend some time as a shampoo girl at a local salon while in college.

There are two types of hair coloring out there that I can think of: hair color (put color on) v. bleach (take color out). Stylists may use both to even out a color, or if you're trying to go lighter than you are right now (ie, black to red).

I think (don't quote me) that hair color actually adds moisture or leaves it as is, since you're only adding something to the hair. I'd bet that this doesn't much change how your hair would react to the humidity.

With bleach, since you're taking away from its structure, you're probably taking away its elasticity (as the straw-like texture and potential breakage would show). I'd bet that it makes it so your hair doesn't react at all, since it damages your hair (and the aforementioned molecular connections) a whole bunch in the process.

Posted by: kate | July 9, 2008 2:53 PM | Report abuse

The comments to this entry are closed.

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