Media Contacts:

Brian Wenny, 919/515-7049 or bnwenny@unity.ncsu.edu
Tim Lucas, News Services, 919/515-3470

June 1, 1998

NC State Scientists Working to Improve Accuracy of UV Index

FOR IMMEDIATE RELEASE

Before spending a day outdoors, many people check the UV index on their newspaper's weather page to determine how much protection they'll need from the sun. A high index means strong ultraviolet (UV) radiation is forecast, so sunblock, sunglasses and hats are in order. A low index means there's less risk of burning.

The index is accurate about 76 percent of the time. Scientists at North Carolina State University are working to increase its accuracy further.

"Ultraviolet radiation has been linked in numerous studies to skin cancers such as melanoma and to cataracts. Given these risks, there would be a clear benefit in making the UV index as reliable as possible," says Brian Wenny, a research associate in NC State's Department of Marine, Earth and Atmospheric Sciences.

To improve index accuracy, Wenny and his colleagues are studying the effects of cloud cover and airborne aerosols on UV levels. Cloud cover and aerosols are two of the three factors used by forecasters to calculate the coming day's UV index, Wenny says, but scientists lack accurate methods to predict them and, in the case of aerosols, to assign correct values to them in forecast equations.

"The index forecast is calculated using one set value for aerosols, regardless of time of year or location," he says. "Yet our studies indicate that aerosols absorb more UV radiation than previously believed, and that the type and amount of aerosols -- as well as their effect on the transmission of UV radiation to the earth's surface -- can vary significantly by season and region."

Urban areas, he says, generally have higher amounts of aerosols than rural areas, while aerosol amounts overall tend to rise in summer, due in part to a seasonal increase in biogenic emissions from growing plants. Man-made aerosols created by the burning of fossil fuels affect UV transmission differently than do natural aerosols originating from open soil or farmland, which in turn have a different effect on UV transmission than the salt particles released into the air by oceans.

"We've quantified these differences and now are working to derive equations that can be incorporated into forecast calculations," Wenny says. "Our hope is that by using a more reasonable value for aerosols in UV index calculations for different regions, we can improve the accuracy of the index."

Another significant finding of the studies is that on partially cloudy days when the clouds don't block the sun, the amount of UV-B radiation striking the earth's surface can actually be as much as 30 percent greater than the amount of UV-B radiation expected on days with clear skies. Scientists theorize this increase occurs because the clouds enhance the scattering and reflection of UV-B radiation toward the earth. This makes the sky "brighter" and increases the risk of sunburn.

UV-B radiation has been linked to increased risk of skin cancers and other human health problems.

Future NC State research will focus on how the chemical and physical properties of clouds affect UV radiation passing through them. "If we can identify certain cloud characteristics associated with decreased or increased UV transmission, that would be a very useful forecast aid," Wenny says.

The UV index is a forecast of the amount of radiation that will reach the earth's surface the next day at solar noon (12:30 to 1 p.m.). Forecasters calculate it by using an equation that takes into account three factors: the columnar thickness of the ozone layer; cloud cover; and the amount of aerosols. Of the three, only ozone thickness can now be predicted accurately.

The index is intended to help people protect themselves from the harmful effects of UV radiation. It uses exposure categories which correspond to how long an average person can stay in the sun before burning. According to the Environmental Protection Agency, even if the exposure category is minimum (categories 1 or 2), a sunscreen with a skin protection factor of 15 or greater should be used. If the UV index is low or moderate (categories 3-6), wear a hat, t-shirt and UV-absorbing sunglasses. If the index is 7 or greater, avoid sun exposure, especially from 12:30 to 1 p.m.

-- ballard --