HAZE IS A VITAL INDICATOR of our atmosphere's health. Most haze is natural; it is created by water vapor, wind-borne dust, forest fires and volcanic eruptions. Unfortunately, a lot of what humans do also ends up in the atmosphere--smog is the best-known example of artificial haze. Although many cities monitor the clarity of their local atmosphere, surprisingly little is known about how the amount of haze is changing globally because no one is coordinating haze observations over widely dispersed areas.

That may change with the latest design from Forrest M. Mims III. (Mims may be familiar to readers from his columns in this section in 1990.) He has invented an atmospheric haze sensor that costs less than $20 and is so simple to construct that even the most hardened technophobe can put it together in under an hour. Mims's instrument could revolutionize this important area of study by opening the field to all comers, that is, to amateur scientists.

The Visible Haze Sensor exploits the fact that sun is an ideal probe for measuring haze. The intensity of sunlight striking the top of the earth's atmosphere is essentially constant. As a result, by measuring the intensity of the sunlight at the ground and knowing the thickness of the atmosphere it has passed through, we can determine how much light has been scattered or absorbed and hence how much haze there is.

There is one complication. Air molecules themselves scatter light, in a phenomenon known as Rayleigh scattering. Although Rayleigh scattering creates beautiful blue skies and brilliant sunsets, it also complicates measurements of haze. Because the efficiency with which air molecules scatter light depends on wavelength, scientists restrict their measurements to a narrow sliver of the solar spectrum so that the effects of Rayleigh scattering can be easily corrected for.

Most professional haze instruments employ a broad-spectrum photodetector, coupled with an expensive narrow-band filter to achieve the necessary selectivity. Mims realized that a narrow-band detector would serve the same purpose. The perfect device for the amateur scientist is a light-emitting diode (LED), which generates light of a sharply defined color when a current passes through it. But this process is easily reversed: light falling on the LED creates a small current that can be readily detected. Furthermore, just as LED emissions appear only in a narrow wavelength band, the diode generates current only when stimulated by light within a small range of colors.

Mims's device uses a green LED (Radio Shack 276-022A), which emits light at around 555 nanometers and detects light at around 525 nanometers. It costs less than a dollar. The rest of the circuit consists of one resistor, an operational amplifier, two nine-volt batteries and a voltmeter. Mims houses his instrument inside a plastic VHS videocassette case with a hole drilled in one end to admit sunlight [see illustration above].

Two angle brackets on the side of the case let you align the instrument directly with the sun. (If possible, attach them with a hot-glue gun to make a rigid connection.) Place a small piece of white tape over the bottom bracket. Open the case and point the instrument toward the sun. Move it around until the bright sun spot is centered on the LED. If the outside brackets on the side are approximately aligned, a second image of the sun should appear somewhere on the tape. Lightly mark its center in pencil.

The case may flex slightly when closed, so you may need to adjust your mark. Close the case and align the instrument to the sun using the outside brackets. Then tilt the instrument slightly in different directions while watching the voltmeter and find the orientation that produces the largest voltage. Make a permanent mark at the center of the sun's image, and you're ready to go.

Although you can begin collecting data right away, you will eventually need to calibrate your photometer. To do that, you'll have to set aside half a day (either early morning to solar noon or solar noon to early evening) when the sky is clear blue and there are few or no clouds. First, you'll need to record the dark signal, that is, the voltage produced when no light strikes the detector. Cover the sun port and record the voltage. You'll need to subtract this number from every measurement you make.

Measure the voltage every 20 minutes when the sun is high overhead and every seven minutes when it is low on the horizon. Always record the time, the sky condition near the sun, and the angle of the sun above the horizon. (Keep your data reliable by recalibrating your instrument at least once a year.)

To complete the calibration, you'll need to plot your data. First, calculate the air mass for each measurement. The air mass is the depth of the air column between you and the sun; by convention it is 1.0 when the sun is straight overhead (a sun angle of 90 degrees) and infinite as the sun sets. The formula is 1/sin(sun angle). After compensating for the dark voltage, plot the natural logarithm of your photodetector measurements against the air mass. The result is called a Langley plot and should be a straight line out to an air mass value of about 10.

If you project that line back to an air mass of zero [see upper illustration in sidebar, Measuring Haze], the result will be the logarithm of the voltage that your instrument would read if you could use it to measure the sun's brightness just above the atmosphere--the so-called extraterrestrial constant, or ET. This constant is the starting point for calculations of aerosol optical thickness, which quantifies haze.

I can't think of a better way for students and adults alike to become more aware of their environment than to get involved in monitoring it. Committing to a regular program of measurements teaches young people responsibility as well as science. You can even become part of a worldwide network of haze observers by submitting observations via the Internet. Visit the TERC site for detailed instructions.

The Visible Haze Sensor photometer was developed through a grant from the National Science Foundation to the Global Lab program at TERC in Cambridge, Mass. TERC has set up a haze monitoring site on the World Wide Web. For more information about this project, check out the Society for Amateur Scientists's World Wide Web site. The SAS can be reached at 4951D Clairemont Square, Suite 179, San Diego, CA 92117 or at 1-401-823-7800. I gratefully acknowledge informative conversations with Forrest Mims.

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.