

How humidity is measured By Jack Williams, USATODAY.com The 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. Humidity refers to the amount of invisible water vapor in the air and various measurements are used to define it. If you don't understand terms such as "dew point," "relative humidity' and "saturation," you should read the USATODAY.com Understanding humidity page before continuing on this page. A dewpoint, or condensation, hygrometer is a common way to measure the humidity. Such an instrument has a mirror that exposed to the open air, and which can be cooled as precise track is kept of the mirror's temperature. As the mirror cools, dew (or frost if the temperature is below freezing) will form on It. The temperature at which this happens is the air's dew point (or frost point in below-freezing air). An observer could watch the mirror to see when dew or frost forms, but a more precise way is to use photoelectric cells to detect when dew fogs the mirror. Other kinds of hygrometers use materials with electrical resistance that varies with the amount of moisture absorbed. With such hygrometers, a measurement of electrical resistance can be calibrated as a humidity measurement. 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 graphic below shows how this works. Source: USA TODAY research by Chad Palmer 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.