Does your hair go crazy when the weather turns damp? Did you know that strands of hair can relax and lengthen when the humidity increases and then contract again when the humidity decreases? In fact, hair strands can be used as the basis for a hygrometer, a device which measures the humidity level in the air.

Humidity is one of the important measurements that weather observers make because the amount of water vapor in the air determines whether clouds or fog are likely and whether it’s going to rain or snow.

Human (or animal) hair turns out to be a pretty good way to measure the humidity, as anyone who’s ever complained about a “bad hair day,” can tell you.
The length of a strand of human hair changes with different relative humidities.
As the relative humidity increases, hair becomes longer, and as the humidity drops it becomes shorter. On very humid days, your hair actually becomes longer and this extra length causes the frizziness that gives us bad hair days.
An instrument that uses hair to measure humidity is known as a hair hygrometer. This instrument uses strands of human or horse hair with the oils removed attached to levers that magnify a small change in hair length.
An ink pen and rotating cylinder, known as a hygrograph, can provide a record of how relative humidity varies throughout the day.
The disadvantages of the hair hygrometer and hygrograph are that they are not as accurate as other kinds of hygrometers such as the sling psychrometer. Also, a hair hygrometer needs frequent adjustment and calibration.

A hair hygrometer also tends to have large errors at very high and very low relative humidities.

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