More About Solar Ultraviolet

Forrest M. Mims III

Because those of us in the Northern Hemisphere are still in the peak of the ultraviolet season, let's wrap up last month's coverage of UV.

The waves of energy emitted by the sun and other stars are collectively known as electromagnetic (EM) radiation. These waves range from tiny x-rays to visible light to giant radio waves. Because the ultraviolet rays in sunlight are the most intense form of EM radiation to which most of us will ever be exposed, it's good to know something about them.

Figure 1. This Department of Agriculture instrument measures UV-B at Texas Lutheran University in Seguin. Photograph by Forrest M. Mims III.

Rain drops break up the white light from the sun into a rainbow of colors ranging from violet to red. The invisible light beyond the violet end of the spectrum is known as ultraviolet (UV).

The invisible light beyond the red end of the spectrum is known as infrared (IR). Most remote controls for TVs and other appliances rely on infrared at 880 or 940 nm. both of which are just beyond the red light that we can see.

The shortest wavelengths of UV are known as UV-C. They quickly zap most bacteria and viruses. Germicidal lamps that produce UV-C are used to sterilize air and water.

The sun produces plenty of these dangerous UV-C rays. Fortunately, they are absorbed by the ozone layer that envelopes the earth. Without the ozone layer, which I have measured since 1990, UV-C rays would reach the earth and cause great damage to plants and animals-–and us.

Slightly longer waves of UV are known as UV-B. While UV-B rays are absorbed by the ozone layer, some leak through to the surface. That's good, for a thin slice of UV-B wavelengths produces vitamin D in the skin of people and other vertebrates. But it's bad for people who stay too long in the sun.

The longest UV waves are known as UV-A. These rays pass right through the ozone layer. While they are less efficient at causing sun burn, the longer waves of UV-A rays allow them to penetrate more deeply in the skin than UV-B rays. This means UV-A rays can break apart the collagen in your skin. Thus, UV-A is the ideal solution for people who envy the wrinkled look of a prune.

A very hazy sky can reduce the UV that arrives directly from the sun by 95 percent! That's what I've measured in Washington, DC on a very polluted day.

But while haze greatly reduces UV arriving directly from the sun, it greatly increases UV scattered by the sky. This means haze can cause high levels of UV in the shade. So a hazy sky is no guarantee you won't receive a sun burn.

The sun's UV-B is measured at more than 30 sites around the United States by Colorado State University on behalf of the U.S. Department of Agriculture under the USDA UV-B Monitoring and Research Program . I am the site manager for one of the Texas sites in this network. You can see a graph of the UV measured at any of the network's sites by going here.

"World of Science" columns are selected and sometimes revised from columns published in the San Antonio Express-News or the Seguin Gazette-Enterprise. The columns are intended for a general audience. Forrest M. Mims III and his science are featured online at www.forrestmims.org and www.sunandsky.org .