Four radiometric models are compared to study the Angstr
?m turbidity coefficient
over Gharda
?a (Algeria). Five years of global irradiance measurements and space data recorded with MODIS are used to estimate
. The models are referenced as
for Dogniaux’s method,
for Louche’s method,
for Pinazo’s method,
for Gueymard’s method and by
for MODIS data. The results showed that
and
are very close as the couple
and
.
values are between them. Results showed also that all Angstr
?m coefficient curves have the same annual trend with maximum and minimum values respectively in summer and winter months. Annual mean values of
increased from 2005 to 2008 with a slight jump in 2007 except for
. The city environment explains it since the urban aerosols predominate over all other types during this period. The jump in 2007 is attributed to the ozone layer thickness that undergoes the same behavior. Some models are then more sensitive to this atmospheric component than others. The occurrence frequency distribution showed that
,
,
,
and
had their maximum recurrent values near 0.03, 0.07, 0.10, 0.09 and 0.02 respectively. The cumulative frequency distribution revealed also that
and
yielded maximum “clean to clear” conditions with respect to others while
and
had the minimum. The opposite was observed on the same
pairs with regard to “clear to turbid” and “turbid to very turbid” conditions. Louche’s model gave middle values of sky conditions comparing to the other models.