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Relationship between solar noon UVI and the daily erythemal exposures
Daily or yearly total exposures are better measures of the overall levels of the ambient UVR, because they take into account the cloud cover variability. For example, measurements at solar noon on two consecutive days in March in Melbourne resulted in similar noon dose rates of 0.23 and 0.21 W/m2 (or 9.1 and 8.5 UVI's), but the daily total dosage was reduced by 47% from 4880 to 2160 J/m2 due to cloudiness (Gies et al., 1999). To investigate the relationship between the solar noon UVI and the daily total erythemal exposure further, we performed a linear regression analysis (Figure 5). It was found the the daily total exposures can be expressed as a function of solar noon UVI. Regressions of daily averaged data revealed a slope of 2.3-2.6 MED/UVI, with large standard deviations of the difference between fitted and original data of 15-31\%. Alice Springs data show the least variation, probably because the skies are clear for more than 90% of the year.
On a monthly mean scale the relationship improves considerably at all stations. A regression coefficient of 3.0+/-0.1MED/UVI has been found independent of the site. The standard deviations of the difference between fitted and original data was less than 10%. As the relation did not change much from one station to another, this simple approximation might prove useful for monitoring the ambient UVR.
| Figure 5: Relationship between daily erythemal exposures and UV Index at noon for three stations in Australia. The results indicate that on a monthly mean time scale, the daily total erythemal exposures can be estimated from a single measurement at solar noon. However, none of these stations show a pronounced diurnal cycle in cloud cover. Future studies will investigate whether this relation holds at stations with a strong diurnal cycle of convection. |  |
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