Weather Notebook
Bryan Yeaton
 


 
The Hair Hygrometer
Mon Aug 29, 2005

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Although in summertime, the living may be easy, you may have noticed that combing your hair isn’t. And if you are wondering what this hirsute matter has to do with weather—hold on. Hi, I’m Bryan Yeaton for The Weather Notebook.

If you’ve ever struggled to keep your hair under control on a hot, humid day, you can take heart in the fact that this became the basis for one of meteorology’s classic instruments: the hair hygrometer. It’s a device in which a bundle of specially treated human hair is stretched out and held under constant pressure. As a matter of general practice the hair is first removed from the human. As the relative humidity of the air rises, these hairs, and those on your head, absorb moisture and stretch out a tiny bit. A single lock will stretch about two and a half percent as the humidity goes from 0 to 100 percent. While that’s only a smidgen of length, the rate of this change is very dependable, so it’s possible to obtain highly accurate humidity readings by measuring these tiny shifts. Hair hygrometers are best suited for keeping track of humidity in closed settings such as office buildings, where the variations are small and the goal is to keep humidity constant.

Long ago, even weirder things were tried: the outer membrane of an ox’s intestine, the urinary bladder of a rat, and even strips of whale bone. Fortunately for oxen, rats, whales, and recovering hippies, progress has arrived; you can but commercial hygrometers that operate based on evaporation of water from a wet bulb thermometer (also called a psychrometer).

But if you want to see if you have what it takes to make your own hygrometer (about 9 inches of hair), try our website: www.weathernotebook.org. We are funded by Subaru of America.




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