The most precise measurements of TO3 can be obtained by direct sun observations. TO3 can
also be determined from blue or cloudy zenith sky measurements, however with lower precision. For
that purpose an empirical cloud sky chart based on quasi simultaneous direct sun and zenith sky
measurements has to be constructed [De Backer, 1998, comp. Sect. 4.2 ; Stanek et al., 1998]. At
many sites zenith sky observations under cloudy conditions are required to obtain a sample of
observations, which is representative enough for reliable long-term monitoring.