DEPENDING ON WHERE YOU live, you may already contemplate humidity more than you care to. In the U.S., for instance, the hot summer air across the Southeast is thick and oppressive, whereas in the western deserts it is so dry that it greedily soaks up all moisture. The differences in the wildlife inhabiting these two regions shows that humidity has deeper consequences than just the daily weather. The water content in the atmosphere has shaped evolution every bit as much as the water content in the ground, which suggests many fascinating avenues for exploration by amateur scientists.

But whether your interests are in meteorology or biology, to undertake amateur investigations of humidity you'll need some means to measure it. This column describes a way to monitor relative humidity, which is the ratio of the amount of water actually present in the atmosphere to the maximum amount that it could support at the prevailing temperature and pressure.

One of the most sensitive gauges of relative humidity is, believe it or not, human hair. Hair stretches when water fills the tiny pores within it, and it shrinks as it dries (which explains the cause of at least some bad-hair days). The length of a hair thus tracks relative humidity, and it does so with impressive accuracy. You can take advantage of this phenomenon to build a humidity meter, or hygrometer, that is both sensitive and inexpensive. The one described here uses a single human hair to rotate a tiny mirror, which then reflects a shaft of laser light onto a scale. The final position of a laser beam indicates the relative humidity. Although this hygrometer won't work at temperatures below freezing, it is otherwise quite versatile.

To start, you'll need one human hair, preferably straight and reasonably long. Wash it thoroughly with soap and water to remove the natural oils. You will also require a housing for your instrument. This enclosure must be rigid, but it should be made from a material that is easy to cut and drill. I used a plastic pot of the kind used to hold a large houseplant.

The reflector must remain stiff yet be as lightweight as possible. A glass coverslip (the thin wafer normally placed over a specimen on a microscope slide) is ideal, except that it is not very reflective. You can remedy this situation by using aluminum foil. Rest a layer of foil on some stiff paper and punch through both sheets using a standard hole punch. Then separate a small, undamaged disk of shiny aluminum. Secure it (dull side down) to the center of the coverslip with a dab of epoxy, gently rubbing the foil smooth with a handkerchief.

Use a toothpick to draw a bead of epoxy across one diagonal on the back side of the coverslip. It helps to turn a drinking glass upside down and place the coverslip on top, with the epoxy bead facing upward. Next drape a generous length of fine thread along the line of epoxy and hang small weights from either side as the epoxy sets to pull the thread taut. Then attach the hair with a dollop of epoxy placed between the center of the coverslip and one free corner. The sensitivity of this instrument depends on exactly where you attach the hair: the closer it is to the thread, the larger the angle through which the mirror will rotate. Because some people's hair responds more to humidity than others', no one prescription will work for all hygrometers. Therefore, you will need to experiment to find the best place on the coverslip to attach the hair.

After the epoxy hardens, secure your glass reflector into the instrument housing by affixing one end of the thread to the top and the other to the bottom, using a rubber band to maintain some tension. Make sure that when you look directly at the reflector, the hair is on the back of the slide cover and left of center. That way your humidity scale will increase from left to right. Before you attach the free end of the hair to the instrument case, you must rotate the reflector clockwise around the string once. The resulting torque should cause the reflector to pull the hair taut as the thread tries to untwist.

A laser pointer makes an ideal light source because it produces an extremely bright spot that can be readily seen, even in daylight. Such pointers are available for less than $50 from many sources, including Radio Shack and Jameco Electronics (telephone: 650-592-8097). But it is vital that you rig the pointer initially so that you never have to touch it again, because the slightest inadvertent nudge will upset the calibration of your instrument. The trick is to put a switch and batteries for the pointer on the outside of the hygrometer housing.

To mount the pointer itself, trim a wooden dowel to the appropriate length and attach the laser to one end. If your laser pointer has a flat case, epoxy it directly to the end of the dowel. If the case is round, gird it with a loop of coat-hanger wire and lash the ends to the dowel as shown in the illustration. Secure this assembly with a liberal coating of epoxy.

To make a scale, use a photocopier to transfer the markings from a fine ruler onto a sheet of paper, then snip out these graduations in a strip approximately two inches (five centimeters) wide. Glue this scale behind a long, rectangular hole in some dark-colored construction paper and attach the paper to the housing so that the laser beam illuminates the scale from the back side. If you intend to use your instrument in very moist environments, you should protect the paper parts from warping with a plastic coating. Spray plastic can be purchased from any well-stocked hardware store.

Your completed hygrometer should be easy to calibrate. First, take it into your bathroom and turn on the shower. As the humidity rises, you will see the bright spot shift to the right. (If you haven't coated the paper parts with plastic, turn off the shower as soon as your bathroom mirror is covered with a thin film of condensation so as not to overload the air with moisture.) When the air is saturated, mark the location of the dot on the scale. Hair-based hygrometers respond sluggishly to changes in humidity, so wait at least 20 minutes before marking your scale. This point represents 100 percent humidity.

You can attain almost zero humidity by placing a substance that readily absorbs moisture at the base of the hygrometer and sealing the instrument inside a transparent plastic bag. Any number of things will absorb water from the air: for example, powdered gelatin, activated charcoal and those little silica gel packets often found bundled with new purchases. Cobalt chloride, which you may be able to obtain from a chemical supplier, is the desiccant of choice, because it changes from blue to pink as it absorbs moisture. If, when the dot of light stops drifting to the left, you have plenty of blue crystals remaining in the bag, you can be confident that the air inside is at near-zero humidity. Be sure to bake whatever drying material you use at 300 degrees Fahrenheit (about 150 degrees Celsius) for at least an hour to remove any water already absorbed before you place it in the bag.

In general, the humidity scale will not be linear between your zero and 100 percent marks, so you should check several intermediate points against a well-calibrated instrument. One easy way to do so (albeit indirectly) is to put the unit outside and compare your measurements with readings from the local weather department.

Once properly calibrated, this device will allow you to perform diverse experiments. You can, for example, measure the rate of evaporation from different kinds of soil into dry air at a variety of temperatures. Or you can monitor how quickly different species of plants transpire under various conditions of light and temperature. To carry out those tests more efficiently, you may want to modify the hygrometer to produce an electronic signal, which will be easier to record than the position of the laser dot. The Society for Amateur Scientists will post suggestions for such modifications on its Web site.

For information about this and other projects for amateurs, visit the Forum section of the Society for Amateur Scientists on the World Wide Web. You may also write the society at 4735 Clairemont Square, Suite 179, San Diego, CA 92117, or call 1-401-823-7800.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.