Ultraviolet Index Forecasts Issued by the National Weather Service # C
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Ultraviolet Index Forecasts Issued by the National Weather Service # c Craig S. Long,* Alvin J. Miller,* Hai-Tien Lee,+ Jeannette D. Wiid,+ Richard C. Przywarty,* and Drusilla Hufford@ ABSTRACT The National Weather Service (NWS), in collaboration with the Environmental Protection Agency (EPA), now issues an Ultraviolet (UV) index forecast. The UV index (UVI) is a mechanism by which the American public is forewarned of the next day's noontime intensity of UV radiation at locations within the United States. The EPA's role in this effort is to alert the public of the dangerous health effects of overexposure to, and the accumulative ef- fects of, UV radiation. The EPA also provides ground-level monitoring data for use in ongoing verification of the UVI. The NWS estimates the UVI using existing atmospheric measurements, forecasts, and an advanced radiative transfer model. This paper discusses the justification for a forecasted index, the nature of UV radiation, the method- ology of producing the UVI, and results from verifying the UVI. Since the UVI is an evolving product, a short dis- cussion of necessary improvements and/or refinements is included at the end of this article. 1. Introduction war, travel to vacation spots became quicker and more affordable, and fashion permitted wearing less and The sun habits of people prior to World War II less, thus exposing more of the human skin area to were much different than the sun habits of people ultraviolet (UV) radiation. At the same time, socio- today. Leisure time was at a minimum. Transporta- economic influences made having a tan desirable. The tion to sunny places was either nonexistent, too time misconception of a tan being "healthy" began during consuming, or unaffordable. Fashion dictated that this time. Consequently, many have spent hours ly- men and women wear hats outside. Around the turn ing in the sun to get a "good" tan. of the century, women often used parasols to block As a result of this increase in exposure (and over- the sun. Pale skin was a status symbol, and a tan des- exposure) to UV radiation over a period of years to ignated one as a member of the working class. When decades, the number of diagnoses of melanoma and vacationing at the beach or just relaxing outside, fash- nonmelanoma skin cancers, as well as cataracts, has ion dictated that people clothe a major portion of the risen dramatically since 1980. This increase has also body. However, as leisure time increased after the been noted in many other countries for similar rea- sons. The American Cancer Society (ACS 1995) es- timated that over 800 000 cases of highly curable basal cell or squamous cell cancers would be diag- *NOAA/NWS/NCEP/Climate Prediction Center, Washington, nosed in 1995 in the United States alone. For 1995, D.C. +Research and Data Systems Corporation, Greenbelt, Maryland. the ACS estimates that melanoma, the most serious #NOAA/NWS/Office of Meteorology, Washington, D.C. skin cancer, will be diagnosed in about 34 100 people. @EPA/OAR/Office of Atmospheric Programs, Washington, D.C. Since 1973, the incidence rate of melanoma has in- Corresponding author address: Craig S. Long, Stratospheric creased by about 4% per year (National Cancer In- Analysis Group, National Centers for Environmental Prediction, stitute 1994). Skin cancer is not just "skin deep"—it W/NP53, World Weather Building, Room 808, Washington, DC can be lethal. The ACS further estimated 9300 deaths 20233. E-mail: [email protected] resulting from skin cancers in 1995: 7200 from ma- In final form 23 October 1995. lignant melanoma and 2100 from other skin cancers. Bulletin of the American Meteorological Society 7 729 Unauthenticated | Downloaded 10/11/21 11:14 AM UTC This number of 9300 deaths translates to greater than awareness campaigns, while the NWS would be re- one death per hour in the United States. Cataracts sponsible for creating and issuing a practical and re- have been determined to cause 53% of blindness cases sponsible UV index. A reduction in the number of worldwide. In addition, studies by DeFabo and treatments for skin cancer and cataract surgeries could Noonan (1983) show that overexposure to the sun can lead to billions of dollars in health care savings. suppress the immune system. In response to these Prevent Blindness America, a group advocating pre- health-related concerns, several countries have initi- ventative measures, cites from existing data up to ated public outreach campaigns to inform the public 1993 that about 1.35 million cataract surgeries are of the dangers of overexposure to the sun. These cam- performed annually in the United States. This costs paigns explain that simple remedial steps can be taken the taxpayers $3.4 billion in Medicare expenses. It is to greatly reduce the risk of overexposure, and they unknown what percentage of these cataract cases are are reinforced by the daily issuance of an index that directly attributable to overexposure to UV radiation, informs the public of the potential intensity of the but clearly there is great potential for reducing costs sun's UV radiation. by very simple measures, such as wearing broad Queensland, Australia, started the first education rimmed hats and protective sunglasses. campaigns on the prevention of skin cancer and the The focus of this paper is to examine the science hazards of overexposure to UV radiation. In the mid- behind the UV index (UVI). To accomplish this, a 1980s, the Australian Radiation Laboratory began brief review of the nature of UV radiation is pre- monitoring UV radiation and broadcasting the day's sented, followed by the methodology by which the UV dosage in "minimum erythemal dosage" units NWS makes its "clear sky" UVI forecast. Then fol- (MED) for all the states' capital cities during the lows a discussion of the seasonal variability of the evening news. Also, in 1987, New Zealand initiated clear-sky UVI for both hemispheres, along with a public awareness campaigns along with issuing "burn methodology for forecasting the effects of clouds times" reports broadcast hourly on the radio. In 1992, on the clear-sky UVI in the United States. Verifica- the Atmospheric Environment Service (AES) of tion statistics of the UVI by surface observations for Canada began issuing their own UV index (Wilson 1993 and 1994 are presented. Finally, alternative 1993), a next-day forecast of UV exposure on a scale methods and improvements to the methodology are of 0-10 (where 10 is the highest value likely in south- discussed. ern Canada). All three countries have had very good success in getting the message to the public about the dangers of being in the sun too long and possible con- 2. Nature of ultraviolet radiation sequences to the skin, eyes, and immune system over a prolonged period of time. Surveys in Australia and Ultraviolet radiation can be divided into three parts Canada (Hill et al. 1993, personal communication; of the sun's radiation spectrum. Ultraviolet C is char- Decima Research 1993) have shown that people know acterized by wavelengths of less than 280 nm. Al- of the existence of their country's index and have though highly dangerous to plants and animals, this made changes in their sun habits by either avoiding part of the UV spectrum is completely absorbed by the sun during the peak UV radiation hours of the day stratospheric ozone and does not reach the earth's or by slipping on a long sleeve shirt, slapping on a surface. Ultraviolet B ranges in wavelength from 280 hat, and slopping on sun screen lotion, as taken from to 320 nm. Ozone absorbs much of the shorter wave- the Australian SunSmart "Slip, Slap, Slop" campaign. length radiation, but this absorption weakens as In the autumn of 1992, the U.S. Environmental 320 nm is approached. Plants and animals are par- Protection Agency (EPA) approached the National ticularly affected by this part of the UV spectrum. Weather Service (NWS) to develop and generate an Ultraviolet-B effects to humans are reddening of index similar to that of Canada for the United States. the skin (erythema) and reduction of vitamin-D syn- By having an index to act as a beacon, the EPA and thesis in the short term and development of skin can- the Center for Disease Control and Prevention could cer, cataracts, and suppression of the immune system launch a massive public awareness campaign to alert in the long term. The wavelengths of UV-A radiation the American public about the dangers of UV radia- range from 320 to 400 nm. Ozone absorbs very little tion. The EPA would be responsible for interagency of this part of the UV spectrum. Ultraviolet-A radia- coordination and all facets of the public education and tion is needed by humans for the synthesis of vita- 730 Vol. 77, No. 4, April 1 996 Unauthenticated | Downloaded 10/11/21 11:14 AM UTC FIG. 1. The UV spectral irradiances (thin line) from 290 to FIG. 2. The product of spectral irradiances and the erythemal 400 nm at summer solstice, 40°N, and solar noon, with 300 DU action spectrum is plotted. Note that the peak occurs between 308 of ozone overhead. For the same wavelengths but using the and 310 nm. opposite j axis, the erythemal action spectrum (bold line) is plotted. Figure 2 shows the net product of applying the min D; however, too much UV-A causes photoaging erythemal action spectrum to the irradiance spectrum (toughening of the skin), suppression of the immune shown in Fig. 1. Note that the peak value is near system, and to a lesser degree, reddening of the skin 308 nm.