The importance of stratospheric ozone stems from its role as a major controlling factor for UV-B radiation reaching the earth's surface. Increased UV-B radiation at ground level is a potential consequence of stratospheric ozone depletion. Although UV-B is a normal component of sunlight, a significant increase in UV-B radiation above natural levels could be harmful to the environment and human health. In addition to stratospheric ozone, several other atmospheric factors affect the amount of UV-B radiation reaching the earth's surface. Because UV-B intensity at the earth's surface cannot be predicted from ozone data alone, accurate measurements of UV-B are essential if long-term trends are to be established.
	How much UV-B is present in sunlight?          Under a clear sky at noon, up to 0.5% of the energy reaching the surface of the earth from the sun consists of biologically active UV-B radiation. The absolute intensity is primarily controlled by the sun's angle so that, during winter or in the morning and evening, it is only a small fraction of that at noon in summer.   What factors other than stratospheric ozone control UV-B radiation?          Significant additional absorption and scattering occurs by clouds, and by ground level pollutants such as sulfur dioxide, nitrogen dioxide, low-level ozone, and atmospheric aerosols. Clouds can reduce UV-B radiation at the surface by up to 90%, or even increase it by up to 20%. Furthermore, the large variability of clouds from year to year makes detection of long-term trends in UV-B difficult.   Have any long-term trends in UV-B radiation been measured directly?          Data obtained with early ground-based instruments at eight locations throughout the United States showed either no change or a slight downward trend in annual UV-B radiation over the period 1974 to 1985. Although this was initially explained as possibly due to long-term decreases in local pollution near the sites, recent studies indicate that the apparent decreases in UV-B might be due to shifts in calibration of the instruments. Short-term increases in UV-B have, however, resulted from short-term decreases in ozone, for example those which occurred due to aerosol increases after the 1991 eruption of Mt. Pinatubo, or during ozone declines associated with the Antarctic ozone hole.            Satellite observations of ozone levels and calculations of clouds from satellite measurements of reflectivity, used to produce model calculations of UV-B, have indicated statistically significant increases in UV-B poleward of 40 degrees latitude. However, satellites are not capable of measuring ground level UV-B directly.            Large scale ground-based monitoring of UV-B has begun in recent years in several countries. At the present time, the observational record is too short to allow the detection of statistically significant long-term trends.
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