In research on solar energy and climatology, 'clear sky' or 'cloudless' conditions are very important. For instance, the number of clear-sky days (number of days having an average cloud cover less than 10%) is a key parameter of solar resource assessments. The instantaneous surface irradiance is highly affected by cloud variations, based on which clear-sky detection (CSD) methods can be developed. However, a general tendency in common among all CSD methods is the detection accuracy deteriorates when aerosol loading increases.