Haze-SPAN: Haze Sun Photometer Atmospheric Network
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THE PROBLEM: Why Measure Haze?
-by Forrest M. Mims III

Thomas Jefferson sketch Thomas Jefferson loved to write about the mild climate of Virginia and its azure skies. Were he to return today, he would notice barely any difference in temperature, rain and snow. We know this because Jefferson kept very detailed daily records of the weather at Monticello from 1810 to 1816. But he would notice a difference in the sky, which today is cloudier and hazier than in Jefferson's time.

When dust, smoke, pollen or tiny droplets of water float in the air, the sky becomes hazy. Outdoor photographers sometimes like haze because it diffuses sunlight, thus softening its glare. Some plants that grow in shady places like haze because it provides light they would otherwise not receive.

But haze also has other effects. Since haze scatters some sunlight back into space, many scientists think that increased haze over the Northern Hemisphere has caused a slight cooling effect. On a hazy summer day, people with light skin can receive a sunburn even when in the shade. This happens when most of the Sun's ultraviolet radiation is received from the hazy sky instead of the direct sun. In some regions haze can be so thick that shadows are faint or even invisible.

Haze also obscures visibility. Over the past few decades, scenic vistas in many regions of the United States have been obscured by haze originating from air pollution created by large power plants and increased automobile traffic.


The advantage of using amateur scientists is that there are many more of us. Forrest finds that student and adult amateur scientists can do very high quality work. The bottom line is that there is no national or global haze monitoring network. The field is wide open, and amateurs can make a valuable contribution. Of course professional scientists can also participate.

Amateur science has a long tradition of excellence, Many famous discoveries have been made by amateurs. Alexander Graham Bell was a teacher of the deaf, but he invented the first successful telephone and photophone. Michael Faraday never attended school, but he discovered fundamental principles of electricity. Some 40% of comets are discovered by amateur astronomers.


Sun photometers are ideal for monitoring natural haze and air pollution by measuring the transmission of sunlight through the atmosphere. As a result of the haze or pollution, the atmospheric optical thickness is slightly increased when the sun is scattered by molecules of air. A sun photometer measures the sunlight in a narrow spectral band that passes through the Earth's atmosphere.


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