The correct assessment of cloud conditions in any site is important for determining the characteristics of solar resource availability. The criterion proposed by Iqbal has been widely used for this purpose, through the classification of the daily clearness index K. However, evidence was found to suggest that this criterion may not be applicable to data measured in high altitude sites, because partially cloudy days are incorrectly classified as clear-sky days. This paper analyzes the cloudiness state of the sky by observing the values of the clearness index K-t and the clear-sky index K-c. These indices were obtained from reliable global solar irradiance data measured over a year in three places of Argentina located at different altitudes (25, 1200 and 3700 m.a.s.l). ESRA and ARG-P models were used to estimate daily clear-sky global solar irradiation values H-c. They also reveal that the most probable value of & which corresponds to an actual clear-sky day of a particular location and time depends on the model used to estimate the clearsky global solar radiation values. Thus, it was confirmed that, unlike the index K-c, the lqbal's criterion does not correctly classify the cloud cover status from data measured in high altitude sites (>1000 m.a.s.1.). (C) 2013 Elsevier Ltd. All rights reserved.