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The relationship between solar ultraviolet (UV) radiation reaching the Earth's surface and geographical parameters is
helpful in estimating the spatial distribution of UV radiation, which provides useful information to evaluate the potential
impacts of enhanced UV levels on human health, agriculture, environment, and ecosystems for sustainable development.
Measurements of erythemally weighted UV radiation at the sites of the United States Department of Agriculture UV-B
Monitoring and Research Program (UVBMRP) monitoring network were analyzed to investigate the geographical distribution
and seasonal variations. Twenty nine observation sites, which had continuous measurements during the recent six
years, are selected for this study; twenty seven of them are distributed in the United States, including one in Hawaii and
one in Alaska, and two of them are located in Canada along the United States border. The measurements were taken using
the Yankee Environmental Systems Inc. (YES) UVB-1 ultraviolet pyranometer. This work focuses the data from the
recent six years of 2001-2006 and the measurements during summer months (June-August) are emphasized. For each
day, the measurements are integrated from sunrise to sunset to produce the daily UV dosage, which is then averaged for
different seasons or for the whole year over the six years to generate the average daily UV dosage. A multivariable regression
technique is exploited to characterize the dependence of UV dosages on geographical parameters, including
latitude and altitude. The results show that, although there are many factors, such as clouds, ozone, aerosols, air pollutants,
and haze, that affect the UV radiation intensity at a location, the latitude and altitude of the site are the primary factors
that regulate the average daily UV dosage. On average over the last six years in the United States, more than 95% of
the variability in averaged daily UV dosages can be explained by the latitude and altitude. Longitude is not statistically
significant in predicting UV irradiance. Nonlinear relationships can be statistically established between averaged daily
UV dosage and latitude and altitude. The effects of latitude on UV radiation are much more significant than the altitude.
The average daily UV dosages decrease exponentially with the latitude. While an increase of one degree in latitude may
lead to a decrease of more than 350 Jm-2day-1 in the averaged daily dosage in the low latitudes, the decrease is around
100 Jm-2day-1 in the mid latitudes and less than 50 Jm-2day-1 in the high latitudes. The averaged daily UV dosage increases
with altitude almost linearly until up to 1500 meters. Then it increases gradually and no significant increases can
be detected above 2600 meters. Although the regression against latitude and altitude is statistically highly significant,
notable deviations from the regression predictions are observed in the lower and mid latitudes and lower altitudes. These
discrepancies are most likely due to the intense anthropogenic activities and natural events occurring in this area, including
natural fire, industrial production, driving, and farming. These locally dependent activities will generate more UV
absorbers into the air.
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