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Introduction
The Puna of Atacama is a high altitude intertropical very dry
desert region placed on the Andes mountains in the latitude-longitude
range of about (12º - 24º S, 65º - 71º W). Since in the summer
Southern Hemisphere the Earth-Sun actual distance is the lowest
one (it pass at the minimum distance, in the beginning of January),
the corresponding extraterrestrial solar irradiance, proportional
to the inverse square of the ratio of this distance to the mean
one, is 6.4 % higher than the Northern Hemisphere one (see for
example, Iqbal, 1983). Due to all these circumstances, which determine
naturally low intertropical ozone total column and low aerosol
atmospheric content, the global as well as UV solar irradiances
are within the highest in the world (Piacentini and Herman, 1996;
Piacentini and Cede, 2000).
The detailed knowledge of the solar UV irradiance is of great importance in relation to its absolute value in each region of the Earth as well as in order to determine its variation due to ozone and aerosol changes by natural or antropogenic causes (see for example, Herman et al, 2000). Also it is related to different biological and material degradation actions (UNEP, 1998; Zerefos and Bais, 1997).
Table 1. Solar risk qualification for the different ranges of
the UV Index.
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UV Index |
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The solar risk measured though the UV index, is directly related
to the erythemal irradiance (in SI units) through the factor 40 (or 0.04 when the common
mW/m2 unit of irradiance is used). This irradiance is obtained
multiplying the spectral solar UV irradiance by the well established erythemal action spectra of McKinlay
and Diffey and integrating the product in the UV (280 nm - 400
nm) range. This action spectra measures the effectiveness of the
incident photons in reddening the normal reference skin and it
is mainly concentrated in the UVB (280 nm -320 nm) range. So,
it is an alarm signal for skin damage (Jagger, 1985). Table 1
gives the relation between the numbers related to the solar UV
Index and the qualification adopted for describing the correspondent
erythemal intensity.