Since the atmospheric clearness index is one of the main factors for evaluating atmospheric circumstances, it is necessary to estimate its characteristics all over the cities in Korea. This study was focused on the evaluation of atmospheric condition for each 15 cities in terms of respectively or mutually analyzed clearness factor that was predicted on the assumed clear day with the model using factors such as average global insolation, cloud amount, and duration of sunshine measured for two years between 1999 and 2000. The new clearness index data will be extensively used by atmospheric circumstances analysts as well as by solar application designers or users.